Video Wireless Transmitter

To design and build a wireless transmitter that works over the FM frequency and allows the transfer of a video/audio signal over a certain distance to an FM tuner. 

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  In this fast-paced world, there is little time for inconveniences and a greater need for portability and adaptability. The idea for an Audio/Video transmitter stems from this need. There may have been times when you've wanted to hook up your VCR from one room to another television set in another room. But that would have entailed that you first unhook all kinds of wires and plugs from the primary TV set; carry the VCR to the next TV set; and then finally re-wire everything together. An Audio/Video transmitter will let you do just about the same thing. But it would offer other conveniences as well. For example, it would allow you to set up security cameras around your home which would send video signals directly to a TV or VCR. And, there are no cumbersome wires and cables to line throughout the intended area.

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Tube Li Amplifier

The unit is powered directly from the 120 volt AC line, with no power transformers. Filaments are wired in series, with the total adding up to 117 volts (35 + 35 + 35 + 12). The 35W4 forms a half-wave rectifier, which is filtered by a three-stage RC network. The B+ for the output stage plates and screens are taken from the second capacitor, and the B+ for the preamp and phase inverter from the third capacitor in the filter. 


The input signal to the amplifier is applied directly to the volume control pot, from whence it passes through a variable high-pass filter (the "Treble" control). When the wiper is set to minimum, response is approximately flat (though actual frequency response will depend somewhat on volume control setting). When it's turned to maximum, higher frequencies are favored, with the lower 3 dB corner at around 1500 Hz. and the higher pole (plateau) around 4000 Hz. For testing purposes, the volume control was set to maximum and the treble control to minimum, to minimise the effect of this control. 

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The output of the volume/treble control circuit is applied directly to the grid of the first section of a 12AX7 twin triode. A partially bypassed cathode resistor supplies grid bias, while providing a modest amount of local negative feedback to help linearise the stage's response. A 100k resistor provides the plate load for the preamp stage. The cathode also has a 10k resistor to the "Bass" control connected to it; we'll discuss the function of these components a little later, as they are part of the global feedback network. 

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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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