Author Topic: Is it possible to synchronize frequency of two 555 timers in PWM mode?  (Read 395 times)

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Offline LinuxHataTopic starter

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I have two 555 PWM modules which are used to dim led lights. Picture of module attached.
One dims cool white; another one dims warm white.
Modules operate on 20khz, so there is no flicker on camera.
However, since they operate on slightly different frequencies, when both are on, there is visual "beating" of these frequencies, which can be observed as flicker on camera.

So is there a way to somehow synchronize these two modules, and make them work on same frequency, to avoid that flicker?

Yes I know I can use arduino/dual channel PWM modules/thousands of other options.

No I'm not interested in these. I'm only asking whenever I can sync two 555 to be operating on same frequency or not.

Offline aliarifat794

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This pdf may help you. Sorry, I could not find much. This is about  a master-slave electronic oscillatory system based on the 555-timer
IC working in the astable mode.
Here are some more 555 based projects if you're interested.
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Online brucehoult

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Oh gawd.

If that's using the obvious method of controlling the duty cycle on a 555 then it's not even running at the same frequency at different pot settings, but probably over a close to 2:1 frequency range.

I suspect your only hope with 555s would be to use three of them, the first making a sawtooth wave at a fixed frequency, and then the other two inputting that and triggering at different levels on the sawtoth based on the pot.

But the 2nd two devices are in a totally different setup than the modules you have.

Seriously, I'd use a single 10c microcontroller (e.g. CH32V003) in an 8 pin package and ten lines of code.

Offline tooki

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Honestly, the 555 sucks for PWM, for numerous reasons. What you want isn’t even possible (as brucehoult said, the PWM frequency is not even constant on a single 555 PWM), so you’ll have to choose another approach.

A really good one is to have a sawtooth generator (many ways to do this) and then have one comparator per channel to create PWM from it. And use a high PWM frequency: if it’s high enough, you simply won’t capture any flicker or beat frequencies on camera. By “high” I mean tens of kHz. I built a sawtooth-and-comparator dimmer that runs at around 26kHz. My favorite LED driver IC dims with 97kHz PWM.

Offline LinuxHataTopic starter

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Thanks everybody for the input.
Everything is already built so the question was whenever by soldering some jumper wires, capacitors, diodes, etc. between these two boards can make them sync somehow. If it is not possible, I'll use some microcontroller, with phased PWM output, dead times, etc - I have done all that before, so no issue with that, except that requires compete disassembly of already built device.

I already tried reducing the frequency of one of 555, but flicker is really still there, unless I make it something like 2khz instead of 20khz. But, 2khz causes flicker issues when camera shutter is set to high speed. I have not tried to run say one 555 at 20khz and another one at 40khz, should this work?

Offline soldar

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Not enough info.

IF they are both really running at the same fixed frequency (20 KHz or whatever) and doing PWM THEN synchronizing them is trivial.

IF they are running at different frequencies depending on the duty cycle THEN synchronizing them is impossible.
All my posts are made with 100% recycled electrons and bare traces of grey matter.

Offline ArdWar

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If they're the usual 555 PWM schematic then short the charge part of the potentiometers (making them rheostats), and connect the two module's discharge pins together.
It will synchronize the PWM frequency in a sense that f1=f2. There's a little catch however. The module is now a constant minimum ON time modulator between the two, so frequency is variable and duty cycle is not independent, a little inconvenience :D


Put it on a ceramic tile, stand back 3 meters, power it on with protected lab supply, and hope nothing blows up. I tend to be nervous when those aliexpress gadgets got an overoptimistic huge transistors.
« Last Edit: June 16, 2024, 09:43:17 am by ArdWar »

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