Hello, I have 2 different circuits below, both claim they are 555 PWM circuits but looking at the circuits they seem to be hooked up differently. Can someone explain the basic differences in an easy to understand way? This link here is info on one of the circuits and how it works, which makes sense but it confuses me on why they are hooked up differently?
Well, firstly, it is not recommended to use pin 3 as a reference, as the load can influence the voltage value, which is why you created the chip with pin 7.
Basically, circuits that do not need much precision use pin 3 as return only to facilitate the life of the assembler.
When I see a circuit like this using pin 3, I already imagine that it must have been elaborated by some amateur.
Reading the various datasheets available on the internet should help you understand all the secrets of the old LM555.
Both are an a 555 astable configuration with diodes to increase the duty cycle range (which is normally limited to between 50% and 100%)
I also prefer the first circuit. I'd suggest pulling pin 4 to Vcc and a 100nF decoupling capacitor to ground on pin 5.
The second drawing/schematic is just crap. Internet crap. Look at the proto board circuit that is the source of that drawing, they don’t match. It’s wrong in so many ways, don’t waste your time with it.