Rain Detector Using Transistor

  This rain detector will give you a heads-up the instant it starts to rain, hopefully giving you time to close windows and bring in possessions. 

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The battery-powered circuit draws virtually no current when the sensor is dry and the current consumption is low when the buzzer is activated so a couple of AA cells will last a long time. Alternately, a molded power supply with a simple voltage regulator to drop the voltage to 3 volts could be used. 

The circuit is basically a handy flasher circuit that operates well on only 3 volts using ordinary silicon transistors. When the circuit is triggered, the buzzer is pulsed about once per second for a very short time, giving it a "dripping water" sound which seems appropriate. A slower, longer beep may be had by increasing the 1 uF capacitor. The 10 k resistor may be increased for a longer beep time without decreasing the beep rate but at some point the circuit will cease to function properly, depending on the gain of the transistors. 

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Smoke Detector Circuit

The A5347CA is a low-current, CMOS circuit providing all of the required features for an ionization-type smoke detector. A networking capability allows as many as 125 units to be interconnected so that if any unit senses smoke, all units will sound an alarm. In addition, special features are incorporated to facilitate alignment and test of the finished smoke detector. This device is designed to comply with Underwriters Laboratories Specification UL217.

 The internal oscillator and timing circuitry keeps standby power to a minimum by powering down the device for 1.66 seconds and sensing smoke for only 10 ms. Every 24 on/off cycles, a check is made for low battery condition. By substituting other types of sensors, or a switch for the ionization detector, this very-low power device can be used in numerous other battery-operated safety/security applications.

The A5347CA is supplied in a low-cost, 16-pin dual in-line plastic package. It is rated for continuous operation over the temperature range of 0°C to +50°C.

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The A5347CA is a low-current CMOS circuit providing all of the required features for an ionization-type smoke detector.

 Oscillator. An internal oscillator operates with a period of 1.67 seconds during no-smoke conditions. Every 1.67 seconds, internal power is applied to the entire circuit and a check is made for smoke. Every 24 clock cycles (40 seconds), the LED is pulsed and a check is made for low battery by comparing VDD to an internal reference. Since very-low currents are used in the device, the oscillator capacitor at pin 12 should be a low-leakage type (PTFE, polystyrene, or polypropylene).

Detector Circuitry. When smoke is detected, the resistor divider network that sets the sensitivity (smoke trip point) is altered to increase the sensitivity set voltage (pin 13) by typically 130 mV with no external

connections to pins 3 or 13. This provides hysteresis and reduces false triggering. An active guard is provided on both pins adjacent to the detector input (pin 15). The voltage at pins 14 and 16 will be within

100 mV of the input. This will keep surface leakage currents to a minimum and provide a method of measuring the input voltage without loading the ionization chamber. The active guard amplifier is not

power strobed and thus provides constant protection from surface leakage currents. The detector input has internal diode protection against static damage.

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Heat Detector

This circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to detect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1. 

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 The second part of the circuit comprises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energised state, whereas pin 2 is grounded. A resistor (R2) connected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator. The output is available from pin 3.

  Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the sound from UM3561. Resistor R4 in series with a 3V zener is used to provide the 3V supply to UM3561 when the relay is in energised state. LED1, connected  in series with 68-ohm resistor R1 across resistor R4, glows when the siren is on.

 To test the working of the circuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts conducting. Simultaneously, transistor T2 also conducts because its base is connected to the collector of transistor T1. As a result, relay RL1 energises and switches on the siren circuit to produce loud sound of a firebrigade siren.

Lab note. We have added a table to enable readers to obtain all possiblesound effects by returning pins 1 and 2 as suggested in the table.

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Human Heraing Aid

Commercially available hearing aids are quite costly. Here is an inexpensive hearing aid circuit that uses just four transistors and a few passive components. 

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 On moving power switch S to ‘on’ position, the condenser microphone detects the sound signal, which is amplified by transistors T1 and T2. Now the amplified signal passes through coupling capacitor C3 to the base of transistor T3. The signal is further amplified by pnp transistor T4 to drive a lowimpedance earphone. Capacitors C4 and C5 are the power supply decoupling capacitors.

 The circuit can be easily assembled on a small, general-purpose PCB or a Vero board. It operates off a 3V DC supply. For this, you may use two small 1.5V cells. Keep switch S to ‘off’ state when the circuit is not in use. To increase the sensitivity of the condenser microphone, house it inside a small tube.

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Sensitive Temprature Switch

This temperature switch has a high sensitivity and is quite reliable. Here, in place of a single transistor, a Darlington pair has been used for switching. 

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  At normal room temperature germinum diode D1 (0A79 or equivalent) has a back resistance value of about 10 kiloohms.As a result Darlington pair, comprising transistors T1 and T2. Conducts and keeps the anode terminal of diode D2 at ground potential. Consequently, transistor T3 does not get base bias and thus relay RL1 is not activated.

 But when temperature increases. The back resistance of diode D1 decreases sharply, which results in cutting off of Darlington pair and forward biasing of transistor T3 via resistor R2 and diode D2. As a result, relay RL1 energises and switches on the alarm. Potmeter VR1 may be adjusted for required sensitivity.

  This simple circuit can be used as an overheat indicator, fire alarm, or it can be used in a constant temperature circuit for switching on a fan etc. The circuit can be easily assembled on a piece of veroboard. Diode sensor D1 must be of germanium type and not silicon.

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Invisible Broken Wire Detector

Portable loads such as video cameras, halogen flood lights, electrical irons, hand drillers,  drinders, and cutters are powered by connecting long 2- or 3-core cables to the mains plug. Due to prolonged usage, the power cord wires are subjected to mechanical strain and stress, which can lead to internal snapping of wires at any point. In such a case most people go for replacing the core/cable, as finding the exact location of a broken wire is difficult. In 3-core cables, it appears almost impossible to detect a broken wire and the point of break without physically disturbing all the three wires that are concealed in a PVC jacket. 

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 The circuit presented here can easily and quickly detect a broken/faulty wire and its breakage point in 1-core, 2-core, and 3-core cables without physically disturbing wires. It is built using hex inverter CMOS CD4069. Gates N3 and N4 are used as a pulse generator that oscillates at around 1000 Hz in audio range. The frequency is determined by timing components comprising resistors R3 and R4, and capacitor C1. Gates N1 and N2 are used to sense the presence of 230V AC field around the live wire and buffer weak AC voltage picked from the test probe. The voltage at output pin 10 of gate N2 can enable or inhibit the oscillator circuit. When the test probe is away from any high-voltage AC field, output pin 10 of gate N2 remains low. As a result, diode D3 conducts and inhibits the oscillator circuit from oscillating. Simultaneously, the output of gate N3 at pin 6 goes ‘low’ to cut off transistor T1. As a result, LED1 goes off.

  When the test probe is moved closer to 230V AC, 50Hz mains live wire, during every positive halfcycle, output pin 10 of gate N2 goes high.

 Thus during every positive half-cycle of the mains frequency, the oscillator circuit is allowed to oscillate at around 1 kHz, making red LED (LED1) to blink. (Due to the persistence of vision, the LED appears to be glowing continuously.) This type of blinking reduces consumption of the current from button cells used for power supply.

 A 3V DC supply is sufficient for powering the whole circuit. AG13 or LR44 type button cells, which are also used inside laser pointers or in LED-based continuity testers, can be used for the circuit. The circuit consumes 3 mA during the sensing of AC mains voltage.

 For audio-visual indication, one may use a small buzzer (usually built inside quartz alarm time pieces) in parallel with one small (3mm) LCD in place of LED1 and resistor R5. In such a case, the current consumption of the circuit will be around 7 mA. Alternatively, one may use two 1.5V R6- or AA-type batteries. Using this gadget, one can also quickly detect fused small filament bulbs in serial loops powered by 230V AC mains.

 The whole circuit can be accommodated in a small PVC pipe and used as a handy broken-wire detector. Before detecting broken faulty wires, take out any connected load and find out the faulty wire first by continuity method using any multimeter or continuity tester. Then connect 230V AC mains live wire at one end of the faulty wire, leaving the other end free. Connect neutral terminal of the mains AC to the remaining wires at one end. However, if any of the remaining wires is also found to be faulty, then both ends of these wires are connected to neutral. For single-wire testing, connecting neutral only to the live wire at one end is sufficient to detect the breakage point.

 In this circuit, a 5cm (2-inch) long, thick, single-strand wire is used as the test probe. To detect the breakage point, turn on switch S1 and slowly move the test probe closer to the faulty wire, beginning with the input point of the live wire and proceeding towards its other end. LED1 starts glowing during the presence of AC voltage in faulty wire. When the breakage point is reached, LED1 immediately extinguishes due to the non-availability of mains AC voltage. The point where LED1 is turned off is the exact broken-wire point.

 While testing a broken 3-core rounded cable wire, bend the probe’s edge in the form of ‘J’ to increase its sensitivity and move the bent edge of the test probe closer over the cable. During testing avoid any strong electric field close to the circuit to avoid false detection.

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ANTI-COLLISION REAR LIGHT

During poor visibility, i.e., when there is fog, or at dawn or dusk, or when your vehicle gets stalled on a lonely stretch of a highway, this flashing light will provide safety and attract the attention of people to help you out. It uses highbrightness yellow LEDs. 

  The circuit uses a dual binary counter CD4520, quadruple 2-input NAND schmitt trigger CD4093, 8-stage shift-and-store bus register CD4094 and some descrete components. An oscillator is built around gate A, whose frequency can be varied through preset VR1 when required. The output of the oscillator is fed to IC1 and IC3. When the circuit is switched on, the oscillator starts oscillating, the counter starts counting through IC1 and the data is shifted on positive-going clock through IC3. As a result, the four groups of LEDs flash one by one.

  All the LEDs will then glow for some time and switch off for some time, and the cycle will repeat. Input pins 12 and 13 of the unused gate D must be tied to ground and pin 11 left open. Preset VR1 should be of cermet type and used to change the flashing rate of each group of LEDs. The circuit works off regulated 12V. Assemble it on a general-purpose PCB and house suitably.

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DARK SENSITIVE SWITCH

  This circuit will activate a relay when light falls to a preset level. Light level can be adjusted with VR1 and the relay contacts may be used to operate an external light or buzzer.

   The light sensor used is the ORP12 photocell. In bright light the resistance of the ORP12 can be as low as 80 ohm and at 50lux (darkness) the resistance increases to over 1 Mohm. The 1M control should provide a wide range for light intensities, if not its value may be increased. The op-amp senses the voltage difference between pins 2 and 3. The control VR1 is adjusted so that the relay is off, the output of the op-amp will be around 2 Volts. When light falls, the resistance of the photocell increases and the difference in input voltage is amplified by the op-amp, the output will swing towards full supply and drive the transistor and relay. 

  The 270k resistor provides a small amount of hysteresis, so that the circuit switches on and off with slightly different light levels. This eliminates relay chatter. Take great care if you decide to wire the relay to activate a mains lamp. Make sure the relay contacts provide adequate isolation and have ample rating for the load.      The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees Celsius. A suitable bead type thermistor is used. The 100k pot allows this circuit to trigger over a wide range of temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents relay chatter when temperature is near the switching threshold of this circuit.

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DTMF Proximity Detector

 

    A DTMF-based IR transmitter and receiver pair can be used to realize a proximity detector. The circuit presented here enables you to detect any object capable of reflecting the IR beam and moving in front of the IR LED photodetector pair up to a distance of about 12 cm from it.

 The circuit uses the commonly available telephony ICs such as dial-tone generator 91214B/91215B (IC1) and DTMF decoder CM8870 (IC2) in conjunction with infrared LED (IR LED1), photodiode D1, and other components as shown in the figure. A properly regulated 5V DC power supply is required for operation of the circuit. 

   The transmitter part is configured around dialer IC1. Its row 1 (pin 15) and column 1 (pin 12) get connected together via transistor T2 after a power-on delay (determined by capacitor C1 and resistors R1 and R16 in the base circuit of the transistor) to generate DTMF tone (combination

of 697 Hz and 1209 Hz) corresponding to keypad digit “1” continuously.

   LED 2 is used to indicate the tone output from IC3. This tone output is amplified by Darlington transistor pair of T3 and T4 to drive IR LED1 via variable resistor VR1 in series with fixed 10-ohm resistor R14. Thus IR LED1 produces tone-modulated IR light. Variable resistor VR1 controls the emission level to vary the transmission range. LED 3 indicates that transmission is taking place.

  A part of modulated IR light signal transmitted by IR LED1, after reflection from an object, falls on photodetector diode D1. (The photodetector is to be shielded from direct IR light transmission path of IR LED1 by using any opaque partition so that it receives only the reflected IR light.) On detection of the signal by photodetector, it is coupled to DTMF decoder IC2 through emitter-follower transistor T1.

  When the valid tone pair is detected by the decoder, its StD pin 15 (shorted to TOE pin 10) goes ‘high’. The detection of the object in proximity of IR transmitterreceiver combination is  Indicated by LED1. The active-high logic output pulse (terminated at connector CON1, in the

figure) can be used to switch on/off any device (such as a siren via a latch and relay driver) or it can be used to clock a counter, etc.

  This DTMF proximity detector finds applications in burglar alarms, object counter and tachometers, etc.

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