RC receiver and transmitter

[Makaram]

I know theres a million of these post and ive had a honest attempt at reading them!
First question is A) with RC transmitters (say to control a robotic arm) how do the receivers differentiate the signals and transmit them into digital bits .
B) if i were looking for a circuit to transmit and receive different commands where would i look? (everything i see about transmitters is transmitting from mics).

Audioguru ur answers are always great i would love what ever sage advise you have to offer

 

[trash]

If you're talking about an old style 27MHz Radio Control, say 4 channels with servos. Then the signal needs to be broken down into different stages.

The first is encoding the information. A simple example is PWM. A joystick (potentiometer) varies the resistance which controls the timing on a 555 timer generating a PWM signal.

Next we might have four such 555 timers each generating the same pulse. We need to get all four of these channels over one channel (the radio).
Multiplexing is what is required. There are a number of methods of do this, but for this example we will just use a 4 pole switch.

So, we set the switch to position 1 and let a pulse from the first 555 timer through.
Next we change the switch to position 2 and let the pulse from the second 555 timer through.
and so on for the 3rd and the 4th.

When we have done all four, we return to the first and send the sequence over and over again.

At the receive end we use another 4 pole switch and send the PWM from each 555 timer to it's respective servo.

The problem with this is what happens if we lose synchronisation of which channel is being sent.
So we might have 5 channels. Lets call this extra channel, channel zero. It might be a pulse which is different from the other four PWM pulse making it easily recognisable.
If our RX circuit sees this special pulse, it knows the next pulse will be ch1 and 2,3,4 etc.

This is a time division multiplex system using PWM. But there is no reason why you could not use any variety of modes, codes, modulation etc.

The above circuit description is has more redundancy in its description for simplicity. In the real world we would only use say one 555 timer for all the channels and just switch which servo or joystick control it is looking at.

PWM modulation isn't like Audio from a microphone. It's basically digital. It is either on or off. The length of that time (is inherently analog) and is representative of the value of the servo.
There is no reason why I could not have chosen a system which mimics the audio from a microphone.
The Amplitude of the signal could be used to represent the position of the servo. There are some disadvantages with doing that, but your television (analog) works using this principle for it's brightness (Amplitude Modulation)

You could send a tone instead. The frequency of the tone is representative of the position of the servo. This is Frequency Modulation and your television uses this for it's audio.

Trying to use audio tones to control a robot is clumbsy. OOK (on off keying) which is a type of AM can be used, but use of this mode is conditional on the audio circuit.

One could use DTMF to control a robot or servos over an audio channel. It works fine. I know somebody who controls a steerable camera and other relays using this method. Sending DTMF tones over UHF CB. Provided your not looking for real time control (like flying an RC plane) they work just fine. The position of my friend's camera is not critical and if the camera over steers it doesn't matter. But a robot arm moving too far... the lack of precision may be a problem.

If you're looking to play with something simple. Buy a LIPD TX and RX. 434MHz units are perfect for the task. They use OOK and can transmit data up to rates of about 10000 bits per second. How you organise those bits is entirely up to you. All you need to do is give the transmitter 0/+5V and the receiver will give you the same respective voltage at the other end.

 

[Makaram]

absolutely awesome answer!! thanks

 

[kly73]

thanks