Hi all,
With the help of a few people here I made a PWM circuit that has two discrete duty cycles. It switches between them using a DPDT switch (breadboard prototype uses mechanical switch... eventually might use analog switch on PCB). I tested this circuit to PWM some LED modules at bright and dim... works great!
Then, I found this "rail road crossing flashing" circuit:
http://www.555-timer-circuits.com/flashing-railroad-lights.htmlI used the output to drive the gates of a P channel and N channel MOSFETs instead of using the diode set up in the circuit since the 555 can't sink/supply enough current for my LED modules. Again, when bread boarded alone this circuit worked well although I had to modify the resistor and capacitor to slow the flash rate down considerably.
Next plan was to combine the two circuits to be able to flash the two LED modules back and forth and have a bright/dim mode. See attached circuit.
Now this circuit does work.. but what I can't seem to figure out is when I first used it with the 70% PWM circuit going, the flash rate of the second 555 timer increased quite a bit (quick strobe). I had to increase C2 and R7 to the values you see in the attached circuit to get ~2 second period (more of what I am looking for). Then, when I switch to the dim mode (20% PWM) the flash rate is affected again... it slows down again even more. This time it is a slight change, but it is noticeable.
Can anyone shed some light as to why this would happen and how I can prevent it? I would like a consistent blinking rate between the two PWM duty cycles if possible.
Thanks!
Edit: You will notice there are no current limiting resistors for the LEDs in my circuit attached. They are LED "modules"... they have the resistors built into them so I left them out of the circuit diagram.