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555 timer help. Fixed frequency, adjustable duty cycle.

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Hello!

I'm currently using a PWM circuit built from the schematic shown above. It works very well and does what exactly I wanted to do except for one thing, it does BUZZ since the circuit design is around 1.5KHz (very much audible range) with an adjustable duty cycle.

Can someone help me modify this circuit so that I could have around 62Hz, with still the same adjustable duty cycle (5%-96% minimum).

Thank you so much!

Notes:
1. POT is 10K
2. Pin 3 (switch) is driving an N-Channel Mosfet (this is the one that's actually buzzing)
3. Suppy Voltage is 3 x 18650 series (12.6V on full charge)
 
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MikeMl

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What is the end-to-end resistance of the pot?

For starters: new timing capacitor = old timing capacitor * 1500/67
 
What is the end-to-end resistance of the pot?

For starters: new timing capacitor = old timing capacitor * 1500/67
Resistance at 0% on the Pot dial is around 3ohms, and at 100% dial is around 9.8K Ohms. At the current circuit gives me around 0.98V at 0% dial and 12.1V at 100% dial. Which is fine, I just wanted to lower the frequency to non audible range and 62Hz is good and makes the mosfet cooler.

The problem with caps is that im on a very outback place where caps with common values are the ones available, like 0.01, 0.47 etc. So if I get non-common values that would be nearly impossible to acquire.

Edited OP to 62Hz.
 

MikeMl

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Ok, so how is the switch wired? The switch somehow has to get back to the wiper of the pot?
 
Ok, so how is the switch wired? The switch somehow has to get back to the wiper of the pot?
It isn't actually wired to the switch and is only directly tied to the N-channel mosfet's gate. The 2 diodes on PIN 7 is actually for the outer pins of the POT, while PIN6 is wired to the wiper of the POT.
 

MikeMl

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So you either have to change the timing cap value from 0.1uF to 2.2uf, or you have to change the pot resistance from 10K to ~200KΩ.

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Or find a Rpot*C(Farads) = 0.2
 
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alec_t

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There's something wrong with the FET or its mounting if it buzzes.
I think you'll find 62Hz is equally annoying. Why not raise the frequency to >20kHz? That way you'll only annoy stray dogs and bats :).
 

MikeMl

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I'll bet that he is hearing a ground-loop in his audio, not an acoustic sound created mechanically.

I tried to create a PWM lamp dimmer for my airplane (to replace a brute-force rheostat), and I could hear it in my headphones (and it contaminated my transmitter's audio as well). The coupling was caused by common-mode currents (ground-loops, lack of single-point-ground) in the airframe.
 
There's something wrong with the FET or its mounting if it buzzes.
I think you'll find 62Hz is equally annoying. Why not raise the frequency to >20kHz? That way you'll only annoy stray dogs and bats :).
There's nothing wrong with the FET. The circuit I used is very common and known to buzz due it being at an audible frequency. And I'm not using this for an audio application. Heater coil control actually.

So you either have to change the timing cap value from 0.1uF to 2.2uf, or you have to change the pot resistance from 10K to ~200KΩ.

Or find a Rpot*C(Farads) = 0.2
Thanks so much MikeMl, the problem was solved using 200K Ohm pot. But I'll try changing the cap from 0.1uF to 2.2uF because a 200K trimpot is not really common here (I tested using a 200K ohm large pot).
 
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