Failed to use a low pass filter in order to convert PWM input to digital output

Question

I want to convert a servo control signal to a digital 5V or 0V output.

This is the format of my PWM signal:

The different thing is my input pulse width will be either 1ms (minimum) or 2ms (maximum.) There can be a slight noise but there are no values like 1.5 ms. Only 1 or 2.

I tried to create a circuit that does this:

When I measure Vout:

• 1 ms pulse width: 170 mV
• 2 ms pulse width: 295 mV

I really do not know why it did not work. I was expecting this:

• 1 ms pulse width: 0V
• 2ms pulse width: 5V

Do you guys know why didn't it work and what I can do to improve it?

Answer 1

From the link in the diagram, I assume you are using a microcontroller, possibly an Arduino?

In that case, I would connect the PWM input to an interrupt pin and use the pin-change interrupt to trigger an Interrupt Service Routine (ISR) to measure the on-time of the pulse. The ISR would set/reset a flag to indicate whether it was 1 ms or 2 ms.

Answer 2

Do you guys know why didn't it worked and what can I do to improve?

The filter can only average your signal into a DC value. If you directly connected a DC voltage multimeter to your pulsing signal, you'd get exactly the same DC value.

$$\boxed{\text{In other words, the filter isn't bringing any magic to the party}}$$

If you want to do it more effectively you could use a monostable circuit. The monostable would be triggered by the positive rising edge of your digital servo control signal. It would produce a pulse of duration of around 1.5 ms (i.e. it would time-out half way between the two logic level times).

And, when that pulse times out you would use that event to sample the digital servo control signal. You would likely use a D type flip-flop for this and the output would resolve to a 0 or a 1 representing the logic-level behind the digital servo control signal output.

That's how I'd do it anyway.

You could use a comparator after your filtered signal make the logic output but then there are other things to consider such as how much hysteresis is needed, how much RC filtering is needed and how much delay time can be tolerated due to the filter. The monostable/flip-flop will resolve the digital value in one cycle (20 ms) maximum.

Answer 3

One Rube Goldberg (UK: Heath Robinson) approach would be to dissect an RC servo and replace the motor with an DC-input optocoupler + series resistor eg. PC817 + 330 ohms. Set the feedback pot to approximately 50% of travel or (more reliable) replace it with two 2.49K resistors.

When the arriving pulses are less than ~1.5ms the servo ASIC will attempt to drive the motor in one direction and the optocoupler LED will conduct. When they are greater than 1.5ms the ASIC will attempt to drive the motor in the reverse direction and the LED will block.

On the receiving end you just need a pullup resistor such as 10K.

You might be able to find the servo ASICs as components, but I don't know the part numbers offhand and most are apparently made by obscure Chinese suppliers.