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AM RC Toy car doubt

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fig. Tx

fig. Rx

can anybody tell me:

1. what is the bandwidth for the AM RC toy car normally?

2. why AM is used instead of FM in RC car?

3 Which AM they use? Ordinary AM? SSB? DSB? Vestigial Sideband?

4. for a RC car that can move forward, backward, left n right, encoder/decoder of how many bit is used?

5. (fig. TX) can u help me to determine which part is LO (local oscillator), Mixer, Amplifier, LPF, etc. by red-circle n label the picture (by paintbrush n upload or email to me)?

6. (fig. RX) how to make sure the spacing between each turn is evenly in ferrite winding(inductor)?

7. (fig. RX) can u help me to determine which part is LO, Mixer, etc by red-circle the picture?

8. (fig. TX n RX) if i want to add an encoder and decoder in Tx n Rx, which part i have to connect? (again, label for me.) thank u!

9. why the frequency the lower the better for the signal travels from air to water?

url: http://www.ajpotts.fsnet.co.uk/radiocontrol.html">http://www.ajpotts.fsnet.co.uk/radiocontrol.html
my email is [email protected]



Hi Janet,
To answer your questions:

1. Normally 12.5KHz

2. Simplicity for a cheap toy. It's much easier to switch a transmission on and off to get AM, than to frequency modulate the oscillator to get FM.

3. The simplest - DSB.

4. Two. Backward/forward is one proportional control, and left/right is the other.

5. In the Tx circuit, the BC547 and components to the left of, and below, it are the crystal controlled oscillator. The 10nF cap is the RF feed to the PA (power amp), the BC337. The NE555 is a pulse generator used to amplitude modulate the BC337. There is no mixer or LPF in the TX.

6. Normally enamelled wire close spaced. Actual spacing is not as fussy with a ferrite cored coil, than with an air-cored coil.

7. In the Rx, the BC547 and associated components make up the local oscillator. The 2p2 feeds the osc signal to the mixer, the BF244B. The IF is extracted at its drain and via IFT1 is fed to the ZN414 IF amp/AM demodulator. The 220pF feeds the resulting audio to the BC108 amp and thence to the NE567 chip for decoding.

8. In the tx you can replace the 555 circuit with your encoder, so it switches the RF circuitry on and off. Crude, but it works.

In the rx, feed the right hand side of the 470nF cap (from the BC108) into your decoder.

9. I have no idea why - but I have seen a model submarine (submerged in a lake), being controlled by a 27MHz transmitter.

Best of Luck. pebe.


Active Member
As i see:
0. This is a very simple and unreliable circuit only for toys...
1.This depend from application. In this case about 13kHz.(I think by IFT1 type)
2.See point 0. FM is better.
3.This modulation is CW. TX run only when 555 output is high.
4.Depend from encoder-decoder pair, i can`t say "bits"
5.BC547 is a crystal oscillator,BC337 is a "power stage". The HF-stage switch on-off by 555 tone-oscillator.
6.Many years ago i always made with hand,not critical.
7.LO the BC547 frequency determined by X1, the signal via 2.2pF go to FET gate. The received signal (selected by L and 39pF) also go to FET gate. This method is additive mixing, produced two signals: Xtal freq+received freq., and Xtal-received freq.Via IFT1 only one signal go to ZN414 amplifier and demodulator. On pin1 output coming demodulated signal, and via amplifier BC108 go to NE567 low freq.PLL input. When the mod. signal detected, the output go low.
8.High frequencies same as light and reflected from area.
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