Issues with RF transmitter/receiver pair in DIY remote control car

[Robert Hill]

Hi all,
I'm using these cheapy RF transmitter and receivers:

I've hooked them up to HT12E and HT12D serial encoder/decoders and hooked the decoder outputs to a motor driver IC.

I'm having the following issue:

The circuit all works perfectly when I have the transmitter antenna almost touching the receiver antenna, I.E. I press a button on the transmitter side and the motor spins the correct direction. I release the button and the motor stops spinning.

However, if I move any distance away and press a button the motor will start to spin but will continue to spin once I release the button SO, the receiver gets the signal to start spinning the motor but doesn't seem to realise the signal has stopped being transmitted by the transmitter. I have an LED hooked up to the HT12D which shows when the IC is receiving a signal. It goes out when a signal is being received and come on again when nothing is being received. In the above situation where the motor continues to spin the LED remains off so it seems the IC thinks it is continuing to receive a signal when it isn't!

It's an odd problem since the transmitter and receiver are obviously making a connection in order to start the motor spinning in the first place but something is going wrong when the signal ceases from the transmitter and the receiver doesn't realise that. Once I move the transmitter closer to the receiver the motor will then stop spinning.

SO in summary I guess I'm asking why does the receiver pick up when the signal starts but not when it stops? (you're answer might be stop being a cheapskate and using cheap RF components.

The circuits I'm using are below. Thanks in advance.

Note in the below schematic the D0-3 pins are actually connected t the motor driver inputs.

 

[Arouse1973]

Sounds like you have some interference from the motors. Do you have any EMI supression capacitors across the motor?
Adam

 

[Robert Hill]

Thanks for the response.

Looks like you're right about interference from the motors. I disconnected the motors and tested things again and the LED on the receiver side flashed off and on in response to the button pushes on the transmitter. I also reconnected them but added a length of wire between the circuit and the motor so the motors were physically more distant from the receiver and this increased the range I could get a correct transmission.

The motors I'm using are recycled from some such device or other. They both have three ceramic disk capacitors (something like 0.01uf value) soldered onto the motors themselves (between the 4 'quarters' or the motor?) I think these must be the EMI suppression you were talking about?

Is there a way to reduce this interference? (or are there motors which would cause less interference?) or shield the receiver circuit? Ideally the motors need to be close to the circuit.

 

[Robert Hill]

A side question is, would a Infra red communication system be less prone to interference?

 

[davenn]

Is there a way to reduce this interference? (or are there motors which would cause less interference?) or shield the receiver circuit? Ideally the motors need to be close to the circuit.

start with some 0.1uF caps across each of the motor terminals

A side question is, would a Infra red communication system be less prone to interference?

in general, yes, with the proviso that it depends on how the motor interference is getting into the receiver...
if it was only via the RF input to the receiver, then yes IR would solve the problem

But I suspect it's more likely on the connecting wires from the motors to the L293D and then via power leads and signal leads between the receiver and the L293D

That will be mostly confirmed if the addition of the caps improves the situation


Dave

 

[Robert Hill]

I've added capacitors across the motor (3 in total) connecting the four terminals on the casing. I've also cut the connecting wires as short as possible and twisted them together as I read this might help. This has given me about 30cm range which is a good improvement. What might be a good next step? I've read about using LC filters as another way or reducing noise. The motors are just the very cheap ones that come out of toys so perhaps some better ones would help things.

 

[Arouse1973]

Hello Robert. Before you try the common mode choke you could try and connect another two capacitor. Leave the one you have and connect another two capacitors. One from the positive (+) terminal to the case of the motor and another one from the common terminal (-) to the case and try that.
Adam

 

[Robert Hill]

I've got capacitors on the motor as in the picture below

There is one from the case to + one from the case to - and one connecting + to -. That doesn't seem to make any difference to the interference.

I put some coils round a toroid(?)( the other item in the picture) in series with the motor and this gave me about 1m range which could be enough for what I need.

Are there other things I can try?

 

[davenn]

I put some coils round a toroid(?)( the other item in the picture) in series with the motor and this gave me about 1m range which could be enough for what I need.

good, that would have been the next step

Are there other things I can try?

other than complete shielding, probably not much other than finding less noisy motors

 

[Colin Mitchell]

Use a separate supply for the motor and add a relay for the moment to see if the range improves.