Current Sensor

High-wattage appliances like electric irons, ovens and heaters result in unnecessary power loss if left ‘on’ for hours unnoticed. Here is a circuit that senses the flow of current through the  Appliances and gives audible beeps every fifteen minutes to remind you of power-’on’ status.

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 This is a non-contact version of current monitor and can sense the flow of current in high-current appliances from a distance of up to 30 cm . It uses a standard step-down transformer (0- 9V, 500mA) as the current sensor. Its secondary winding is left open, while the primary winding ends are used to detect the current. The primary ends of the transformer are connected to a full-wave bridge rectifier comprising diodes D1 through D4. The rectified output is connected to the non-inverting input of IC CA3140 (IC1).

 IC CA3140 is a 4.5MHz BIMOS operational amplifier with MOSFET input and bipolar transistor output. It has gate-protected MOSFET (PMOS) transistors in the input to provide very high input impedance (1.5 T-ohms), very low input current (10 pA) and high-speed switching performance.

 The inverting input of IC1 is preset with VR1. In the standby mode, the primary of the transformer accepts e.m.f. from the instrument or surrounding atmosphere, which results in low-voltage input to IC1. This low voltage at the non-inverting input keeps the output of IC1 low. Thus transistor T1 doesn’t conduct and pin 12 of IC2 goes high to disable IC2. As a result, the remaining part of the circuit gets inactivated.

 When a high-current appliance is switched on, there will be a current drain in the primary of the transformer to the negative rail due to an increase in the e.m.f. caused by the flow of current through the appliance. This results in voltage rise at the non-inverting input and the output of IC1 becomes high. This high output drives transistor T1 into conduction and the reset pin of IC2 becomes low, which enables IC2.

 IC CD4060 (IC2) is a 14-stage ripple counter. It is used as a 15-minute timer by feeding Q9 output to the piezobuzzer for aural alarm through the intermediate circuitry. Resistors R5 and R6 along with capacitor C1 maintain the oscillations in IC2 as indicated by blinking LED1. The high output from IC2 is used to activate a simple oscillator comprising transistors T2 and T3, resistors R8 and R10, and capacitor C2.

 When the Q9 output of IC2 becomes high, zener diode ZD1 provides 3.1 volts to the base of transitor T2. Since transistor T2 is biased by a highvalue resistor (R8), it will not conduct immediately. Capacitor C2 slowly charges and when the voltage at the base of T2 increases above 0.6 volt, it conducts. When T2 conducts, the base of T3 turns low and it also conducts. The piezobuzzer connected to the collector of T3 gives a short beep as capacitor C2 discharges. This sequence of IC2 output at Q9 becoming high and conduction of transistors T2 and T3 resulting in beep sound repeats at short intervals.

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Low-Cost Transistorised Inverter

This is an inexpensive fully transistorised inverter capable of driving medium loads of the order of 40 to 60 watts using battery of 12V, 15 Ah or higher capacity. Transistors T1 and T2 (BC548) form a 50Hz multivibrator.

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For obtaining correct frequency, the values of resistors R3 and R4 may have to be changed after testing. The complementary outputs from collectors of transistors T1 and T2 are given to PNP darlington driver stages formed by transistor pairs T3-T4 and T6-T7 (utilising transistors BD140 and 2N6107). The outputs from the drivers are fed to transistors T5 and T8 (2N3055) connected for push-pull operation.

Somewhat higher wattage can be achieved by increasing the drive to 2N3055 transistors (by lowering the value of resistors R7 and R8 while increasing their wattage). Suitable heatsinks may be used for the output stage transistors. Transformer X1 is a 230V primary to 9V-0-9V, 10A secondary used in reverse.

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Mains Manager

Very often we forget to switch off the peripherals like monitor, scanner, and printer while  witching off our PC. The problem is that there are separate power switches to turn the peripherals off. Normally, the peripherals are connected to a single of those four-way trailing sockets that are plugged into a single wall socket. If that socket is accessible, all the devices could be switched off from there and none of the equipment used will require any modification.

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 Here is a mains manager circuit that allows you to turn all the equipment on or off by just operating the switch on any one of the devices; for example, when you switch off your PC, the monitor as well as other equipment will get powered down automatically.  You may choose the main equipment to control other gadgets. 

The main equipment is to be directly plugged into the master socket, while all other equipment are to be connected via the slave socket. The mains supply from the wall socket is to be connected to the input of the mains manager circuit. The unit operates by sensing the current drawn by the control equipment/load from the master socket. On sensing that the control equipment is on, it powers up the other (slave) sockets. The load on the master socket can be anywhere between 20 VA and 500 VA, while the load on the slave sockets can be 60 VA to 1200 VA.

 During the positive half cycle of the mains AC supply, diodes D4, D5, and D6 have a voltage drop of about 1.8 volts when current is drawn from the master socket. Diode D7 carries the current during negative half cycles. Capacitor C3, in series with diode D3, is connected across the diode combination of D4 through D6, in addition to diode D7 as well as resistor R10. Thus current pulses during positive half-cycles, charge up the capacitor to 1.8 volts via diode D3. This voltage is sufficient to hold transistor T2 in forward biased condition for about 200 ms even after the controlling load on the master socket is switched off.

 When transistor T2 is ‘on’, transistor T1 gets forward biased and is switched on. This, in turn, triggers Triac 1, which then powers the slave loads. Capacitor C4 and resistor R9 form a snubber network to ensure that the triac turns off cleanly with an inductive load.

 LED1 indicates that the unit is operating. Capacitor C1 and zener ZD1 are effectively in series across the mains. The resulting 15V pulses across ZD1 are rectified by diode D2 and smoothened by capacitor C2 to provide the necessary DC supply for the circuit around transistors T1 and T2. Resistor R3 is used to limit the switching-on surge current, while resistor R1 serves as a bleeder for rapidly discharging capacitor C1 when the unit is unplugged. LED1 glows whenever the unit is plugged into the mains. Diode D1, in anti-parallel to LED1, carries the current during the opposite half cycles.

 Don’t plug anything into the master or slave sockets without testing the unit. If possible, plug the unit into the mains via an earth leakage circuit breaker. The mains LED1 should glow and the slave LED2 should remain off. Now connect a table lamp to the master socket and switch it ‘on’. The lamp should operate as usual. The slave LED should turn ‘on’ whenever the lamp plugged into slave socket is switched on. Both lamps should be at full brightness without any flicker. If so, the unit is working correctly and can be put into use.

 Note. 1. The device connected to the master socket must have its power switch on the primary side of the internal transformer. Some electronic equipment have the power switch on the secondary side and hence these devices continue to draw a small current from the mains even when switched off. Thus such devices, if connected as the master, will not control the

slave units correctly.

2. Though this unit removes the power from the equipment being controlled, it doesn’t provide isolation from the mains. So, before working inside any equipment connected to this unit, it must be unplugged from the socket.

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Voltage Doubler To 6v

All miniature electronic devices operate off batteries. Some of them need higher than the standard battery voltages to operate efficiently. If the battery of that specific voltage is unavailable, we are forced to connect additional cells in series to step up the DC voltage. Thus,

the true meaning of miniaturisation is lost. A simple way to overcome this problem is to employ a voltage doubler, if the device under consideration can operate at a small current.

 Here we present a low-power voltage doubler circuit that can be readily used with devices that demand higher voltage than that of a standard battery but low operating current to work with. The circuit is quite simple as it uses only a few components. Yet, the output efficiency is 75 to 85 per cent along its operating voltage range. The available battery voltage is almost doubled at the output of the circuit. Here IC1 is wired as an astable multivibrator to generate rectangular pulses at around 10 kHz. This frequency and duty cycle of the pulses can be varied using preset VR1. The pulses are applied to switching transistors T1 and T2 for driving the output section, which is configured as a voltage-doubling circuit. The doubled voltage is available across capacitor C5.

 During each cycle of the pulse occurance, the high level drives T1 into its saturation, keeping transistor T2 cut off. So transistor T1 charges capacitor C4 via the path formed by diodes D2 and D1 to a voltage level slightly lesser than the supply. But during the low period of the pulse, transistor T1 is cut off while transistor T2 is driven into saturation. Now, transistor T2 raises the charge on the negative pole of capacitor C4 by another step equal to the supply voltage. Therefore an equal amount of charging is built up on capacitor C5 via diode D3. This doubling action increases the total voltage across capacitor C5 to almost double the input voltage. If the output of the pulse generator is maintained with a high enough amplitude and frequency, the output voltage and current remain constant and cater to the needs of the load.

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  Even with the half-wave function, this circuit is almost free of ripple voltage. If the connected load doesn’t require a high current, the efficiency can be expected in the upper 90 per cent ranges. Since the input voltage is doubled, the current drain from the input power supply is also doubled at the input but halved at the output. One point of caution is that if the multivibrator’s frequency is fairly high, the output may suffer with the interference imposed over the DC voltage. In this case, the frequency must be set favourably by trials and actual load connection procedure. This tiny circuit can be assembled on the general-purpose PCB. If all of the components are surface-mount type, the whole module can be genuinely miniaturised.

  During testing with input of 8V and 1.25mA load current the output voltage was found to be around 13V.

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Voltage Based Controller For Switches

 Here’s a simple circuit for controlling four switches from a distance through just a pair of wires. In the circuit, the inverting inputs (pin 2) of operational amplifiers IC1 through IC4 are set to refrence voltages of +12V, +9V, +6V, and +3V, respectively, through a chain of four 1k resistors (R1 through R4).

The reference voltage (VREF) can be simply calculated by the following relationship:

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The non-inverting inputs (pin 3) of the four op-amps ( IC1 through IC4) are tied together and connected to a pair of wires that provide +3V to +12V input voltage for controlling the switches.

Four 12V, 200-ohm, singlechangeover relays are connected to four BC548 relay driver transistors (T1 through T4) via resistors R5 through R8, respectively.

 These relays energise depending on the voltage present at the controlling volt- Total applied voltage×Resistance across reference voltage input terminal; for example, relay RL4 energises when controlling voltage input of +3V is available at non-inverting pin 3 of IC4. Four electrical equipment can be connected to the terminals of the relays through the 220V AC, 50Hz mains.

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Solar Panel Charger

You can save on your electricity bills by switching to alternative sources of power. The photovoltaic module or solar panel described here is capable of delivering a power of 5 watts. At full sunlight, the solar panel outputs 16.5V.  It can deliver a current of 300-350 mA. Using it you can charge three types of batteries: lead acid, Ni-Cd and Li-ion. The lead-acid batteries are commonly used in emergency lamps and UPS. 

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 The working of the circuit is simple. The output of the solar panel is fed via diode 1N5402 (D1), which acts as a polarity guard and protects the solar panel. An ammeter is connected in series between diode D1 and fuse to measure the current flowing during charging of the batteries. As shown in Fig. 1, we have used an analogue multimeter in 500mA range. Diode D2 is used for protection against reverse polarity in case of wrong connection of the lead-acid battery. When you connect wrong polarity, the fuse will blow up.

 For charging a lead-acid battery, shift switch S1 to ‘on’ position and use connector ‘A.’ After you connect the battery, charging starts from the solar panel via diode D1, multimeter and fuse. Note that pulsating DC is the best for charging lead-acid batteries. If you use this circuit for charging a lead-acid battery, replace it with a normal pulsating DC charger once a week. Keep checking the water level of the leadacid battery. Pure DC voltage normally leads to deposition of sulphur on the plates of lead-acid batteries.

 For charging Ni-Cd cells, shift switches S1 and S3 to ‘on’ position and use connector ‘B.’ Regulator IC 7806 (IC1) is wired as a constantcurrent source and its output is taken from the middle terminal (normally grounded). Using this circuit, a constant current goes to Ni-Cd cell for charging. A total of four 1.2V cells are used here. Resistor R2 limits the charging current.

  For charging Li-ion battery (used in mobile phones), shift switches S1 and S2 to ‘on’ position and use connector ‘C.’ Regulator IC 7805 (IC2) provides 5V for charging the Li-ion battery. Using this circuit, you can charge  a 3.6V Li-ion cell very easily. Resistor R3 limits the charging current.

  Fig. 2 shows the circuit for a small LED-based lamp. It is simple and lowcost. Six 10mm white LEDs (LED2 through LED7) are used here. Just connect them in parallel and drive directly by a 3.6V DC source. You can use either pencil-type Ni-Cd batteries or rechargeable batteries as the power source.

  Assemble the circuit on a generalpurpose PCB and enclose in a small box. Mount RCA socket on the front panel of  the box and wire RCA plug with cable for connecting the battery and LEDbased lamp to the charger.

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Triple Power Supply

This low-cost, multipurpose power supply fulfils the requirements of almost all laboratory experiments. Nonetheless, it can be easily fabricated by hobbyists. 

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 A single transformer is used to build this triple power supply. Regulator IC LM317 generates variable power supply of 1.25 to 20V, 1A. The dual ±12V, 1A power supply is generated by regulators 7812 and 7912. Similarly, dual ±5V, 1A power supply is generated by regulators 7805 and 7905. ‘On’/‘off’ switches (S2 through S4) select the required power supply. Variable power supply is used to study the characteristics of devices. Fixed +5V power supply is used for all digital, microprocessor and microcontroller experiments. Dual ±12V power supply is used for op-amp-based analogue circuit experiments.

 Fig. 1 shows the circuit of the triple power supply, while Fig. 2 shows the pin configuration of the regulators used in the circuit. Transformer X1 steps down the mains power to deliver the secondary output of 18V-0-18V. The transformer output is rectified by full-wave bridge rectifier BR1, filtered by capacitors C1, C2, C3, C7 and C8, and regulated by IC1 through IC5. Regulator IC1 (LM317) provides variable voltages (1.25 to 20V), while IC2 and IC4 provide regulated +12V and –12V, respectively. The output of IC2 is fed to regulator IC3 (7805), which provides fixed +5V. Similarly, the output of IC4 is fed to regulator IC5 (7905), which provides fixed –5V. Capacitors C4 through C6, and C9 through C11, are used for further filtering of ripples in positive and negative regulated power supplies. LED1 glows to indicate that +5V is available, while LED2 indicates that –5V is available.

 Switch S1 is used for mains ‘on’/ ‘off’. Using switches S2 through S4, any of the three supplies can be  independently turned off when not required in a particular experiment. This reduces unnecessary power dissipation and increases the life and reliability of the power supply. Since the circuit uses three terminal regulators, only capacitors are required at the input and output. The use of few components makes the circuit very simple.

 The three terminal regulators have heat-sink provision to directly deliver 1A output current. To ensure the maximum output, do not forget to use heat-sinks for the regulators. The three-terminal regulators are almost non-destructible. These have inbuilt protection circuits including the thermal shutdown protection. Even if there is overload or shorting of the output, the inbuilt overload protection circuit will limit the current and slowly reduce the output voltage to zero. Similarly, if the temperature increases beyond a certain value due to excessive load and heat dissipation, the in-built thermal shutdown circuit will reduce the output current and the output voltage (gradually) to zero. Thus complete protection is provided to the circuitry.

  Assemble the circuit on a generalpurpose PCB and enclose in a box as shown in Fig. 3. The step-by-step procedure to build the triple power supply for the laboratory follows:

1. Collect all the components shown in the circuit diagram.

2. Connect switch S1, fuse, transformer and mains cord to the assembled PCB as well as the box.

3. Keep the multimeter in DC voltage range (more than 25V DC) and measure the DC voltage across capacitors C1 and C7 (1000 μF, 35V).  This voltage should be around 18V×1.41=25 to 26V DC. Check both positive and negative voltages with respect to ground.

4. It is advisable to use three-wiremains cable and plug.If you are using any metallic box, earthing wire/pin of the mains plug should be soldered to the body of the metallic box using an earthing tag.

5. If the 18V-0-18V transformer is replaced with 15V-0-15V transformer, the output voltage of the variable supply using LM317 will be correspondingly lower.

6. If proper voltages are available, go to step

7. Otherwise, check the connections.

7. Connect variable regulator

LM317 to the circuit and check 1.25V to 20V output by varying the 2.2-kiloohm linear potentiometers.

8. Now connect ICs 7812, 7912, 7805 and 7905 to the circuit and check their output voltage.

9. Connect terminals, potmeter, switches and indicator LED on the front panel of the box and complete the connections. Close the box by using screws.

Precaution. At the primary side of the transformer, 230V AC could give lethal shocks. So be careful not to touch this part.

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PC Power Manager

Very often we forget to switch off the connected peripherals  like monitor, scanner and printer while switching off our PC. This leads to needless energy consumption and possible shortening of the life of the peripheral. PCs with an ATX switch-mode power supply (SMPS) unit are not provided with a mains switch outlet. It is therefore not possible to achieve automatic switching (on/off) of peripheral units with the computer power switch. 

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 Here is a simple circuit that turns the connected peripherals on/off along with your PC. It consists of a regulated power supply, a simple USB interface and two electromagnetic relays used as power switches.

 The power supply for the circuit is derived from the AC mains via transformer X1. The 15V AC available at the secondary winding of transformer X1 is first rectified by a bridge rectifier comprising diodes D1 through D4, smoothed by capacitors C1 and C2, and regulated by IC LM7812 (IC1). The regulated 12V DC is used to energise relay RL1. LED1 works as a power ‘active’ indicator.

 To set up the circuit, first connect the input socket (SOC1) of the circuit to a proper AC mains wall outlet using a three-core power cable. Now connect one end of a standard USB cable to the B-type USB input socket and the other end of the cable to any vacant USB port (A-type) of the PC. Finally, plug one standard four-way  switchboard (extension cord) into the supply output socket (SOC2) of the circuit and take power from this switchboard to activate all loads like monitor, scanner, printer and even your PC.

 To activate the PC manager circuit, proceed as follows: Press ‘start’ switch S1 and hold it in this position for a few minutes. When power-‘active’ indicator LED1 lights up, relay RL1 energises and the 230V mains power supply from SOC1 is fed to output socket SOC2 through the contacts of relay RL1 .Now start your computer as usual, by pressing the power button on the front panel.When the PC runs, there will be 5V DC at the USB interface socket. As a result, relay RL2 energises via diode D6. The contacts of relay RL2 close switch S1 permanently, and LED2 glows continuously.  

 Release ‘start’ switch S1. Now your PC manager is ready to use. When you switch off your PC, relay  RL2 de-energises. As a result, electric power from the switchboard (to which all peripherals are connected) is cut off. Switch S2 works here as an emergency bypass switch. Assemble the circuit on a generalpurpose PCB and enclose in a suitable cabinet. Connect SOC1, SOC2 and USB socket along with switches S1 and S2 and LEDs (LED1 and LED2) on the front panel of the cabinet. Refer Fig. 2 for connections.

note. Take care during fabrication and testing, as the circuit is at mains potential and may give you lethal shock.

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UNIVERSAL HIGH RESISTANCE VOLTMETER

The full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure depends on the function switch position as follows:

(a) 5V DC on position 1

(b) 5V AC rms in position 2

(c) 5V peak AC in position 3

(d) 5V AC peak-to-peak in position 4

  The circuit is basically a voltage-tocurrent converter. The design procedure is as follows:

Calculate RI according to the application from one of the following equations:

(a) DC voltmeter: RIA = full-scale EDC/IFS

(b) RMS AC voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS

(c) Peak reading voltmeter

(sine wave only): RIC = 0.636 fullscale EPK/IFS

(d) Peak-to-peak AC voltmeter

(sine wave only): RID = 0.318

full-scale EPK-TO-PK / IFS

The term IFS in the above equations refers to meter’s fullscale deflection current rating in amperes.

 It must be noted that neither meter resistance nor diode voltage drops affects meter current.

 Note: The results obtained during practical testing of the circuit in lab are tabulated in Tables I

through IV.

 A high-input-resistance op-amp, a bridge rectifier, a microammeter, and a few other discrete components are all that are required to realise this versatile circuit. This circuit can be used for measurement of DC, AC RMS, AC peak, or AC peak-to-peak voltage by simply changing the value of the resistor connected between the inverting input terminal of the op-amp and ground. The voltage to be measured is connected to non-inverting input of the op-amp.

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Mobile Phone Battery Charger

Mobile phone chargers available in the market are quite expensive. The circuit presented here comes as a low-cost alternative to charge mobile telephones/battery packs with a rating of 7.2 volts, such as Nokia 6110/6150.

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The 220-240V AC mains supply is downconverted to 9V AC by transformer X1. The transformer output is rectified by diodes D1 through D4 wired in bridge configuration and the positive DC supply is directly connected to the charger’s output contact, while the negative terminal is connected through current limiting resistor R2.

LED2 works as a power indicator with resistor R1 serving as the current limiter and LED3 indicates the charging status. During the charging period, about 3 volts drop occurs across resistor R2, which turns on LED3 through resistor R3.

An external DC supply source (for instance, from a vehicle battery) can also be used to energise the charger, where resistor R4, after polarity protection diode D5, limits the input current to a safe value. The 3-terminal positive voltage regulator LM7806 (IC1) provides a constant voltage output of 7.8V DC since LED1 connected between the common terminal (pin 2) and ground rail of IC1 raises the output voltage to 7.8V DC. LED1 also serves as a power indicator for the external DC supply.

 After constructing the circuit on a veroboard, enclose it in a suitable cabinet. A small heat sink is recommended for IC1.

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MOBILE CELLPHONE CHARGER

Charging of the cellphone battery is a big problem while travelling as power supply source is not generally accessible. If you keep your cellphone switched on continuously, its battery will go flat within five to six hours, making the cellphone useless. A fully charged battery becomes necessary especially when your distance from the nearest relay station increases. Here’s a simple charger that replenishes the cellphone battery within two to three hours. 

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 Basically, the charger is a current-limited voltage source. Generally, cellphone battery packs require 3.6-6V DC and 180- 200mA current for charging. These usually contain three NiCd cells, each having 1.2V rating. Current of 100mA is sufficient for charging the cellphone battery at a slow rate. A 12V battery containing eight pen cells gives sufficient current (1.8A) to charge the battery connected across the output terminals. The circuit also monitors the voltage level of the battery. It automatically cuts off the charging process when its output terminal voltage increases above the predetermined voltage level.

 Timer IC NE555 is used to charge and monitor the voltage level in the battery. Control voltage pin 5 of IC1 is provided with a reference voltage of 5.6V by zener diode ZD1. Threshold pin 6 is supplied with a voltage set by VR1 and trigger pin 2 is supplied with a voltage set by VR2. When the discharged cellphone battery is connected to the circuit, the voltage given to trigger pin 2 of IC1 is below 1/3Vcc and hence the flip-flop in the IC is switched on to take output pin 3 high.

 When the battery is fully charged, the output terminal voltage increases the voltage at pin 2 of IC1 above the trigger point threshold. This switches off the flip-flop and the output goes low to terminate the charging process. Threshold pin 6 of IC1 is referenced at 2/3Vcc set by VR1. Transistor T1 is used to enhance the charging current. Value of R3 is critical in providing the required current for charging. With the value of 39-ohm the charging current is around 180 mA.   

  The circuit can be constructed on a small general-purpose PCB. For calibration of cut-off voltage level, use a variable DC power source. Connect the output terminals of the circuit to the variable power  supply set at 7V. Adjust VR1 in the middle position and slowly adjust VR2 until LED1 goes off, indicating low output. LED1 should turn on when the voltage of the variable power supply reduces below 5V. Enclose the circuit in a small plastic case and use suitable connector for connecting to the cellphone battery.

Note., the circuit was tested with a Motorola make cellphone battery rated at 3.6V, 320 mAH. In place of 5.6V zener, a 3.3V zener diode was used. The charging current measured was about 200 mA.The status of LED1 is shown in the table.

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Power supply with audio Amplifier Circuit

The circuit in Figure 1 can help if you must transfer dc power and audio over a pair of copper wires. One application for such a circuit is a low-cost door-opening system with speech input. 

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The circuit uses only one IC, the wellknown LM317, a low-cost powersupply regulator. Using this chip, you can modulate the adjustment-pin input with the audio signal from an electrets condenser microphone, connected between the output and the adjustment terminals of the IC. The LM317 regulates the output in such a way that the voltage on the microphone is always 1.25V dc. This application uses a WM34 electret microphone, which comes in a standard 10-mm capsule from Panasonic and is common in low-cost equipment.You can use nearly any electret capsule, because the well-regulated voltage on the microphone never exceeds 1.25V.Every electrets capsule contains an integrated JFETbased impedance converter that translates speech into a current flowing from the source to the drain terminal. This current through the microphone modulates the voltage on the variable resistor, RP. Because the output of the LM317 must follow the voltage on RP, you obtain a low-impedance audio signal riding on the output dc voltage.

 The microphone directly modulates the adjustment pin, so a smoothing capacitor, such as C1, for noise and hum does not influence the level of the audio signal. C1 shunts some of the audio signal to ground, but the LM317 compensates for the loss with internal gain. To avoid excessive losses in the LM317, use a capacitor with as low a value as possible. The circuit works well without a capacitor, but values as high as 47 _F do not present a problem.Using RP, you can adjust the dc output voltage and the gain for the microphone signal. For proper operation, the LM317 needs to deliver a minimum current of 4 mA from its output terminal. If your design uses no loudspeaker, you can connect a load resistor to sink this 4 mA. Designs using low-impedance loudspeakers must also have load resistors. You must add the ac current in the audio signal to the minimum current requirement of 4 mA. For an 8_ loudspeaker, you need a minimum resistive load of 470_ to avoid distortion.

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CONTACTLESS AC MAINS VOLTAGE DETECTOR

This is a CMOS IC (CD4033) based circuit which can be used to detect presence of AC mains voltage without any electrical contact with the conductor carrying AC current/voltage.Thus it can be used to detect mains AC voltage without removing the insulation from the conductor. Just take it in the vicinity of the conductor and it would detect presence of AC voltage.

  If AC voltage is not present, the display would randomly show any digit (0 through 9) permanently. If mains supply is available in the conductor, the electric field would be induced into the sensing probe. Since IC used is CMOS type, its input impedance is extremely high and thus the induced voltage is sufficient to clock the counter IC. Thus display count advances rapidly from 0 to 9 and then repeats itself. This is the indication for presence of mains supply. Display stops advancing when the unit is taken away from the mains carrying conductor.

 For compactness, a 9-volt PP3 battery may be used for supply to the gadget.

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CHARGE MONITOR FOR 12V RECHARGEABLE LEAD-ACID BATTERY

A battery is a vital element of any battery-backed system. In many cases the battery is more expensive than the system it is backing up. Hence we need to adopt all practical measures to conserve battery life.

 As per manufacturer’s data sheets, a 12V rechargeable lead-acid battery should be operated within 10.1V and 13.8V. When the battery charges higher than 13.8V it is said to be overcharged, and when it discharges below 10.1V it can be deeply discharged. A single event of overcharge or deep discharge can bring down the charge-holding capacity of a battery by 15 to 20 per cent. 

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 It is therefore necessary for all concerned to monitor the charge level of their batteries continuously. But, in practice, many of the  battery users are unable to do so because of non-availability of reasonably-priced monitoring equipment. The circuit idea presented here will fill this void by providing a circuit for monitoring the charge level of lead-acid batteries continuously. The circuit possesses two vital features:

• First, it reduces the requirement of human attention by about 85 per cent.

 • Second, it is a highly accurate and sophisticated method.

  Input from the battery under test is applied to LM3914 IC. This applied voltage is ranked anywhere between 0 and 10, depending upon its  agnitude. The lower reference voltage of 10.1V is ranked  ‘0’ and the upper voltage of 13.8V is ranked as ‘10.’ (Outputs 9 and 10 are logically ORed in this circuit.) This calibration of reference voltages is explained later.

 IC 74LS147 is a decimal-to-BCD priority encoder which converts the output of LM3914 into its BCD complement. The true BCD is obtained by using the hex inverter 74LS04. This BCD output is displayed as a decimal digit after conversion using IC5 (74LS247), which is a BCD-to-seven-segment decoder/driver. The seven-segment LED display (LTS- 542) is used because it is easy to read compared to a bar graph or, for that matter, an analogue meter. The charge status of the battery can be quickly calculated from the display. For instance, if the display shows 4, it means that the battery is charged to 40 per cent of its maximum value of 13.8V.

  The use of digital principles enables us to employ a buzzer that sounds whenever there is an overcharge or deep discharge, or there is a need to conserve battery charge. A buzzer is wired in the circuit such that it sounds whenever battery- charge falls to ten per cent. At this point it is recommended that unnecessary load be switched off and the remaining charge be conserved for more important purposes.

 Another simple combinational logic circuit can also be designed that will sound the buzzer when the display shows 9. Further charging should be stopped at this point in order to prevent overcharge. The circuit is powered by the battery under test, via a voltage regulator IC. The circuit takes about 100 mA for its operation.

 For calibrating the upper and lower reference levels, a digital multimeter and a variable regulated power supply source are required. For calibrating the lower reference voltage, follow the stepsg iven below:

• Set the output of power supply source to 10.1V.

• Connect the power supply source in place of the battery.

• Now the display will show some reading. At this point vary preset VR2 until the reading on the display just changes from 1 to 0.

 The higher reference voltage is calibrated similarly by setting the power supply to 13.8V and varying preset VR1 until reading on the display just changes from 8 to 9.

 After the calibration is completed, the circuit may be housed in a suitable enclosure.

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15-STEP DIGITAL POWER SUPPLY

Here is a simple circuit to obtain variable DC voltage from 1.25V to 15.19V in reasonably   smallsteps as shown in the table. The input voltage may lie anywhere between 20V and 35V.

  The first section of the circuit comprises a digital up-down counter built around IC1— a quad 2-input NAND schmitt trigger (4093), followed by IC2— a binary up-down counter (4029).

Two gates of IC 4093 are used to generate up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029). The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1.

   IC2 receives clock pulses from the oscillator and produces a sequential binary output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1.

   Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts down by closing switch S2. The output of counter IC2 is used to realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for their operation. The switching action is performed using BC548 transistors. External resistors are connected in parallel with thereed relay contacts. If particular relay contacts are opened by the control input at the base of a transistor, the corresponding resistor across the relay contacts gets connected to the circuit.

  The table shows the theoretical output for various digital input combinations. The measured output is nearly equal to the theoretically calculated output across regulator IC3 (LM317). The output voltage is governed by the following relationship as long as the input-to-output differential is greater than or equal to 2.5V:

 Vout = 1.25(1+R2'/R1')

Where, R1' = R15 = 270 ohms (fixed) and

R2' = R11 + R12 + R13 + R14

= 220 + 470 + 820 +1500 ohms

= 3,010 ohms (with all relays energised)

   One can use either the binary weighted LED display as indicated by LED1 through LED4 in the circuit or a 74LS154 IC in conjunction with LED5 through LED20 to indicate one of the 16

selected voltage steps of Table I. The input for IC4 is to be tapped from points marked ‘A’ through ‘D’ in the figure. This arrangement can be used to replace the LED arrangement at points A, B, C, and D. This 74LS154 IC is a decoder/ demultiplexer that senses the output of

IC2 and accordingly activates only one of its 16 outputs in accordance with the count value. LEDs at the output of this IC can be arranged in a circular way along side the corresponding voltages.

Working

    When the power is switched on, IC2 resets itself, and hence the output at pins 6, 11, 14, and 12 is equivalent to binary zero, i.e. ‘0000’. The corresponding DC output of the circuit is minimum (1.25V). As count-up switch S1 is pressed, the binary count of IC2 increases and the output starts increasing too. At the highest count output of 1111, the output voltage is 15.19V (assuming the in-circuit resistance of preset VR2 as zero). Preset VR2 can be used for trimming the output voltage as desired. To decrease the output voltage within the range of 1.25V to 15.2V, count-down switch S2 is to be depressed.

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