I have the following circuit built from the 'choppy' pwm controller kit.

After testing it with an LED strip, I realized that the 1N4148 diodes were actually in reverse...after switching them around to the correct direction. Nothing changed visually with respect to circuit operation?

What is the purpose of these diodes and why did the incorrect polarity not affect the circuit?

 

The diodes and VR1 allow duty cycle to be set. Reversing polarity of the diodes does not matter as both configurations are electrically equivalent if the pot is at the half way point. Set duty cycle to 75% then reverse the diodes. You should see a new duty cycle of 25%. That's a hint of what's happening.

 

Reversing D1 and D2 absolutely makes no difference, except for the position of VR1 (clockwise vs counter-clockwise rotation).
All you are doing is swapping D1 and D2.

 

hi Max,
Ref this image.
E

 

Hello,

@ericgibbs , The circuit does not use the discharge pin 7.
It is a hysteretic oscillator.
Here is a circuit without the PWM:
https://www.allaboutcircuits.com/textbook/experiments/chpt-8/555-hysteretic-oscillator/
The PWM version is shown in figure 8 of the page:
https://sound-au.com/articles/555-timer.htm

Bertus

 

Here is a 555 configuration making use of the same principle for setting duty cycle. Add a transistor to pin 3 for a quick and easy PWM driver.