Help me figure out a problem with RC timing in a servo motor control design

Hi-
I am trying to get an audio signal to control a servo motor. The servo's position is controlled with pulse width modulation.
The circuit is a non-inverting amplifier -->comparator-->RC timing-->555 timer to get PWM for servo control.
When I look at the output of the LM324 (pin 8) I see what I want, a switching voltage from +5 to 0V. But without the Resist/Cap I have no control over the speed and it needs to be faster. The servo is jerky and doesn't move enough.
The circuit works OK without the RC timing (diode-10uF cap-10k resistor) but when I attach that part of the circuit, the motor doesn't drive at all.
When I look with the scope at the cathode there is only flat line at about +3.

I don't know what the diode does to attenuate the signal so much? I don't know the part number but it is a "fast" diode. I left the diode in the design from
Scary Terry's circuit. I want to build my own and understand it.
http://www.scary-terry.com/audioser...ontrol timing of the signal. Thanks! :D Nora

Re: Audio-> PWM

Most likely the diode is similar to 1N4148, which is a general purpose 100mA switching diode .. There is nothig special about it ..
Circuit build around D1, R2 and C5 acts as a loudness detector ..

Maybe you should try to separate that circuit's output from the R10 potentiometer by adding another 1N4148 (or similar) diode. In this case the 555 will be affected (modulated) only if there is an audio signal present ..

Regards,
IanP

Re: Audio-> PWM

Output of comparator is square wave with variable duty cycle and frequency depending on audio signal. Purpose of diode and RC behind it is to give average value of comparator output. RC filters out frequencies above 2Hz and thus the output voltage is lower. In this way only the frequencies below 2Hz can be used for control and they have full amplitude.
Directly applying this rectified voltage to timing part of 555 IC is wrong. Use original circuit from the link

Re: Audio-> PWM

Thanks for replies-
But why is applying rectified circuit to timer input wrong?
Isn't that what he's doing in the linked circuit?

The filter for 2Hz can't be correct, otherwise no sound would come thru at all.
My sound is at 4000Hz approx.

Thanks-
Nora

Re: Audio-> PWM

I want to build my own and understand it.

Click to expand...

First of all you must first understand it and then you can build it.
Explain what you want to acheive with this circuit.

Re: Audio-> PWM

Hi- thanks for reply-
Objective: I want to make the 555 timer put out a variable square wave for PWM of a servo motor.

I mostly understand Terry's linked circuit but not totally. I still don't understand why or how I would need to get an average output from the comparator with the diode/RC. I do understand that I need to have the RC for timing.
If my input to the 555 timer ultimately goes to pin6, it goes to an internal comparator which after internal circuitry outputs a logic 1 or 0 at pin3. I think the amount of time that the 1 stays on is proportional to the amplitude of the voltage received at pin 6. Am I correct or no?

I rewired my circuit as attached. I did not use Terry's R14, does that create the filter with C2? BTW I had always thought when a cap is shunted to ground it is a low pass filter. But I always get LP and HP filters confused.
That is really the only difference between the 2 circuits...also he uses a transister amp for the servo and I use a buffer....any explanation for why his amp is a better design is welcome. *edit: my circuitry causes a small whine in the motor, not too noticeable but not good. I thought maybe if I swtched the buffer amp to a power transister things might improve?

Thanks-
Nora