555 PWM Circuit Issues

Thread Starter

BraedenH

Joined Mar 13, 2023
2
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I have a fairly frustrating problem with my 555 PWM circuit. Above is the circuit I'm using. In my simulator the PWM function works well, I get a good range of duty cycle and the frequency is very regular.

In the actual circuit, starting the potentiometer from the most clockwise position, and rotating it counter clockwise the frequency decreases as I do so, and the duty cycle remains constant. As I approach the center of the pot's travel the frequency reaches its lowest value. As I continue rotating the pot the frequency increases again until it reaches a similarly high frequency as before. I've tried 3 different pots, different capacitor values, different resistor values, I've swapped out the 555 chip for a spare one, nothing seems to work. The result is always the same. Any ideas as to a way to fix this? The output of the circuit is connected to an AND gate so there's little load on the output I would assume.
 

crutschow

Joined Mar 14, 2008
34,841
Sounds like a wiring error.
Double-check the pot connection and the components connected to it.
Make sure the wiper is not interchanged with one of the pot ends.
 

Thread Starter

BraedenH

Joined Mar 13, 2023
2
I had the wrong diodes installed in the circuit. I was using schottkey diodes instead of general purpose diodes. I swapped in the correct components and it worked. I'm not sure why a schottkey wouldn't work though.
 

Janis59

Joined Aug 21, 2017
1,863
Should take in consideration that US 555 have notable difference from russian 555, thus in China exist as one as another types. Probably You`re hit by this inconvenience if modeled not passes to observed.
Once I made the "thermometer" style voltage indicator where each LED was ignited by 555. Wonderfully worked on russian tablets and nothing happened on US tablets. Then I realized the algorithm is another for R and S and nominal for R must be smaller.
 

Alec_t

Joined Sep 17, 2013
14,408
Instead of connecting your 30k resistor to +5V, connect it to pin 3 (the output). That should give practically no variation in the frequency as you alter the pot.
 
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