Help, PWM

[Chunkymonkey94]

I am trying to make a PWM circuit but its not going as well as I would like it. I'm not sure if im doing it right. Here is a image of what I tried to make.
View attachment 63505
The only variable resistors I have are 2K, 1M and the transistor I am useing is a normal mps2222a.

 

[alec_t]

its not going as well as I would like it.

Can't see any obvious problem with the circuit in principle. Values of components will need tweaking to suit the load. What isn't going right?

 

[Chunkymonkey94]

I tried to use a led to if it would get dimmer but it doesn't seem to do anything when I turn the variable resestor's dial either direction.

 

[House0Fwax]

There is no ground connection for the 555.

 

[alec_t]

If you're using a 2k variable resistor in conjunction with the 10k resistor to control the PWM duty cycle then there will only be a small change in duty cycle (10:24 going to 12:24) so you won't see much change in LED brightness. Use the 1Meg variable instead (but that may slow down the pulse rate to an unacceptable level: your call).

 

[Chunkymonkey94]

What if I connect the variable resistor to the discharge input and not the output?

 

[alec_t]

You need to clarify that arrangement. How will you be using all 3 terminals of the 1meg? What happens to the diodes and 10k resistor?

 

[Chunkymonkey94]

It would stay the same, the variable resistor would still be attached the two diodes and the 10k resistor. The wire following the 10k resistor would be connected to discharge, not the output.

 

[alec_t]

In that case the circuit won't oscillate. The output will stay at 5V.