Wireless doorbell 300MHz

Posted on Apr 12, 2012

The operation of a 303MHz circuit has been covered in our project WIRELESS DOORBELL. We are not going over how the circuit works but explain the importance of some of the components and how they effect the range. The most critical component is the transistor. A quality transistor is important in the RF section and Japanese transistors are by far the best for this purpose. The transistor used in the 303MHz oscillator has a maximum frequency of operation of 1,000MHz and this is where its gain is equal to `1,` so we want a transistor to have a good gain at 300MHz. A BC 547 transistor will not operate at this frequency so we have used a 2N 3563 which is low cost and will operate up to 1,000MHz.

Wireless doorbell 300MHz
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Wireless doorbell 300MHz - image 1
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The following circuit uses a CD 4069 IC to produce the 32kHz tone and 4 gates in parallel to turn the oscillator transistor on and off at the tone-rate. A single gate will not have enough output to pull the emitter to ground, however 4 gates will bring the emitter close to 0v rail. It must not be at exactly 0v as the 6p will not have an effect in maintaining oscillation. The chip has 6 gates and when an input is just above mid rail, the output goes LOW. When the input is just below mid rail the output goes HIGH. The gap between detecting a low and a high is not very large and the gate will detect signals called "analogue signals." This type of circuit makes an ideal basis for experimentation. You can experiment with the RF side of the circuit as well as the high impedance sections. Each gate is capable of providing a very high gain and by putting a 1M from output to input the gate is kept in a state of excitement, oscillating at approx 500kHz, if no other components surround the gate to control the frequency. This is designed to keep the gate active so that the smallest signal will be processed. In the case of the gate between pins 13 and 12, the 1n capacitor between the input and ground will reduce the frequency enormously, as well as the effect of the 2n2 and 5k6 resistor. The second and third gates simply increase the amplitude of the signal and do not provide any form of removal of unwanted signals. The end...

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