How to create analogue signal?

Can you get the original analogue signal from the PWM signal?

• I'm making a radio controlled car simulation for some electronics coursework and I've stumbled across a problem... I'm going to control the speed of the motors using PWM and sending it across using an r.f transmitter and receiver, I was just wondering if there is any way of me, after receiving the signal, converting it back to the original analogue signal so I can stick it into a comparator to determine which way the motor will spin? Or is there an easier way to go about this? Thanks in advance.

• Answer:

  Yes you can. The R/C signal is itself a form of PWM, where the pulse repeats every 20 ms and the width varies from 1 ms to 2 ms (and in some cases from 0.5 ms to 2.5 ms). The midpoint of this range, 1.5 ms, corresponds to neutral - pulse widths (PW) less than 1.5 ms correspond to one direction, and those above 1.5 ms are for the other direction. I'm not an R/C guy, but as far as I can tell, the reason for the small PW range (i.e. 2 ms instead of 20 ms) is so that multiple control functions (e.g. steering, throttle, ...) can be accommodated on one radio channel. So, depending on how you are decoding the signal, digitally or with an analog low-pass filter, there are different ways of determining the direction. Digitally, you would program a microcontroller to time the precise width of each pulse and compare the result with a value stored in memory, corresponding to 1.5 ms (plus or minus a dead zone to prevent jittering). With an analog low-pass filter, you would run the resulting voltage though a pair comparators that compare it with upper and lower voltages corresponding to each direction. Again, you'll want to have a dead zone so the you never have both comparators turning on at the same time (as when the voltage is near the neutral value).