[Project Help Please] FM Signal Receiving Circuit

[kulong]

[Project Help Please] Probing For Signal Of A R/C Car

For this project I need to make a FM receiver circuit that can tune in and receive the radio signal from the remote controller of a R/C toy car. I know that the remote controller operates at 27MHz.

The plan is to have the circuit receiving the signal from the remote controller so we can read the signal with an oscilloscope, so that we can mimic the signal with our laptops and control the toy car with our laptops through a microcontroller.

Please point me in the right direction for making such a circuit!

Much appreciated.

 

[ccurtis]

Are sure it is FM? The couple of 27 MHz RC Car and Truck models I have encountered for repair use OOK modulation with the information presented in the pulse sequence (pulse coded). In that case, you can see the signal directly on most oscilloscopes, and then simulate it with your laptop.

 

[kulong]

Thank you for the quick reply!

I am not sure if its FM. It is a cheap $20 remote control car by RASTAR, and the remote controller has a sticker that says 27MHz. I have also opened up the chassis of the toy car and it has a small circuit board connected to an antenna and 2 motors (front motors for turning and rear motors for forward/back). If I were to get the signal directly from the toy car, where would I probe with the oscilloscope?

Thank you!

 

[ccurtis]

If you have an oscilloscope, you can put the scope probe on the antenna of the transmitter with the ground clip of the probe to the battery negative. You could try coiling a length of wire around the antenna and putting the probe across the resulting coil to get a viewable signal. You'll likely see pulses of RF as you manipulate the controls on the transmitter. The repeating pattern of the pulses is different for the various control positions.

Edit: You can probe on the toy car circuit board to see the pulses too, but it's going to be a bit of hunting and pecking without the schematic, and you may still not be sure the pulses you are seeing are those actually received by the antenna.

 

[kulong]

Thank you, I will try that in the lab and update the results.

Would I need battery in the toy car when probing for the signal? Also the coil of wire around the antenna is to strengthen the signal right?
Is there any special care needed when probing the signals using the oscilloscope? I would be looking in the frequency domain around the 27MHz range?
Sorry for all the questions =p

 

[ccurtis]

Would I need battery in the toy car when probing for the signal? Also the coil of wire around the antenna is to strengthen the signal right?

Obviously, if you are probing at the transmitter you won't need a battery in the toy car to understand the transmitted signal. Yes, the coil of wire would get more signal to the scope than just putting the probe on it. If you really want a big signal than you open the transmitter and find a point at the final output transistor to probe.

BTW, on the toys I repaired, a bad final output transistor in the transmitter was the cause of the malfunction in both cases.

 

[ccurtis]

If I were you, I would use the transmitter's circuit card and the I/O pins on your laptop to substitute for the mechanical switches in the transmitter. You're still going to have to generate the 27 MHz signal somehow, so why not use the transmitter you have already?

 

[kulong]

Obviously, if you are probing at the transmitter you won't need a battery in the toy car to understand the transmitted signal.

Is the transmitter the remote controller? I thought I would be probing on the antenna of the toy car, that would be the receiver right?

For the project as a whole, we already purchased bluetooth microcontrollers so we plan to mimic transmit the signal via bluetooth, with the microcontroller attached to the circuit of the toy car. Since the laptop cannot send RF signal directly, as far as I know...and we figured the easiest way would be through bluetooth.

What are the I/O pins of a laptop?

 

[ccurtis]

Bluetooth? Holy Moley, that was about as clear as mud from what you posted previously. So you have a substitute transmitter and receiver link using bluetooth, now you either need to find the point in the existing toy car receiver where the pulse patterns can be applied, or go to the points that drive the motor control points directly. That's a matter of hunting and pecking to find it. I can't really help you there without a schematic.

 

[heyadrian]

erm FYI 27 MHz is HF not FM Furthermore, OOK modulation with the information presented in the pulse sequence (pulse coded) <-- I agreed with ccurtis on that.

Probing the antenna? OK now I'm officially lost? Bluetooth is a totally different technology to standard tramsmissions in "old fashioned electronics", they're encoded binary signals which represent commands.