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555 frequency control another 555 controlling a servo motor.? Answered

Hey guys, I have another question. I'm in the planning stages of a project, It's basically supposed to be an astable 555 sending out a frequency that's supposed to control the mark space ratio of another 555 that's going to send a signal to a servo motor, causing it to oscillate back and forth.

Tl;dr, I want to make a servo wiggle around with two 555s. I have used the output of a 555 to modulate another one before, but I understand that using a 555 to control a servo is slightly different than just audio output.

So is there anything I should be aware of? Is there anything I can read or look at to point me in the right direction? Thanks.

EDIT: I just had an idea. What if I took the output of the first 555 (thus forth referred to as IC1) and used it to turn on a transistor, which connects a resistor in parallel to some part of the IC2 which decreases mark space and turns the servo.


This one uses some pots to control rotation, if a resistor is connected in parallel using a transistor... something will happen. It moves.


Its going to take you three 555 timers, but here's the circuit for the hard bit.

You need to sort out your position signal, a 50Hz square wave reference and the scaling for the whole shebang, but....

The UN modulated pulse width = 1.1 x R x C.

You'll need the usual supply reservoir caps, and a 100nF across that.


Three timers? Yikes. Ok then, how would that work?

Read the circuit.....

1.) As shown,
2.) one to make a 50Hz 90% pulse train.
3.) One to provide your 0...3V modulation signal


if you haven't found your solution yet. there is another post on here for a single servo walking robot that uses a different type of logic chip that seems to accomplish what you are looking for. search this site for "one motor walker", write-up by robomaniac

What kind of frequency do you want to wiggle it at ?

I want it to go back and forth at maybe one or 1.5 hertz, but I want to put a trimpot so I can fine-tune it. The first 555 is the easy bit, I'm not worried about that, it's how do I get the output of the first one to modulate the second one.

Understand how the servo works - I assume your talking an RC servo

If so assuming an analogue servo not digital:

They are operated by a pulse width modulation - changing the pulse width changes the position so it is the mark space ratio (off on ratio) you need to change not the frequency.

Right, yeah. I was somewhere else when I wrote that. But you understand what I want to do right? I want to use one 555 to alter the duty cycle of another. Can I do that?

Duty cycle. Shut up Mr. Merino, you have no idea what you're talking about. Let the nice man explain how to modulate the mark space.

To be honest I am not too sure - The M/S of the 555 is set by the Resistor and Capacitor connected to Pins 6 and 7 trig and threshold.


shows a way to alter the M/S ratio by altering the resistor R1 R2 values. I can see you doing this with another 555

To be honest I would use a PIC micro - My choice would be from www.PICAXE.com

They interface direct to the servo and program in basic which is way easier than the A'duino and battling with C

I kind of need this to be as simple as possible, since I need this done in a few days. I also don't have quick access to a microcontroller, so I figured that 555s would be easy (if I clean my desk I could probably find 30 of them).

Again, thanks.


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