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Transmitters and adjustments of rc car systems

Most rc car transmitters have their own operating frequency determined by the 10.7MHz ceramic resonator. It produces a continuous signal (commonly called CW) which is classical on shortwave worldwide.

It is worth looking at your rc car transmitter. There are adjustments to be made. The coil L3 is 15 turns of AWG26 wire on a ferrite core 8mm diameter by 1cm long. The trimmers are all equal. They consist of variable capacitors whose value can be between 3-and 5-50pF 30pF. The choke can be replaced by a resistor of 100K / 1W on which are wound 15 turns of AWG32 wire. The power supply must provide 12V with a current of around 2A.

You can proceed to add other features. A radio may have very good frequency stability, approximately 1Km range (under ideal conditions) and has excellent audio sensitivity. All this was achieved by adding an rc car RF amplifier (with 10dB gain) and an AF pre modulation reinforces its ideal point. It is very easy to build. L1 is formed by 3.25 spirals, which are part of the drawing board. The two transistors BC547 can be replaced by any general purpose NPN as the 2N2222. The final stage consists of a general purpose PNP transistor, the BC557.

The circuit consumes about 30mA, which varies slightly when you touch the line, a good sign that the system works well. You must remove the 4K7 resistor if you use a dynamic microphone. The e circuit board is 50mm x 25mm. The transmitter can be powered by a 9V battery. The output power is around +10 dBm. This, theoretically, provides 1.6km of reach. But in practice it could take more than 700 meters (open field) or 100 meters in a room in an apartment.

This circuit can listen to the conversations that airline pilots maintain the control base airports. It is a passive receiver tuned to VHF air band followed by an AM demodulator and audio amplifier. Luckily, this rc car circuit requires no passive oscillators or frequency generators, which can cause interference with aviation equipment.

As demodulator you may use a 1N34 diode type known in the crystal set. The variable capacitor is not critical and any value between 5 and 20pF may work well. The entire circuit is powered by a 9v battery. The antenna can be a piece of wire a few inches. In our tests we used a mini telescopic antenna 15cm long, which adjusts according to the tuned frequency. Sets can also be placed inside the cabinet, provided that it is not metallic. The resistance in series with the output serves as a current limiter in the receiver.

The inductor tuner stage is a small RF choke with a few turns of fine wire enamel inside. To avoid rc car frequency shifts put a knob on the shaft of variable capacitor.

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