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Help diagnosing this dual 555 timer circuit please (PWM + Blinking)
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doublec4:
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.html

I 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.
GerryR:
First I think you need a .01uf cap from pin 5 to gnd on both of the 555's.  Then, if you are trying to make a 1 sec on, 1 sec off flasher, you need to add a resistor from pin 7 to pin 6.  I calculate the resistor to be about 1 M with a 1.5 uf cap, and change the existing 1 M resistor to 1 K.  Thi = .693 (RA + RB) C;  Tlo = .693 (RB) C.  Therefore to get Thi = Tlo (or close), RB has to be much greater than RA.  (I think I did this right!)  Have fun!!
doublec4:

--- Quote from: GerryR on June 27, 2019, 11:41:54 pm ---First I think you need a .01uf cap from pin 5 to gnd on both of the 555's.  Then, if you are trying to make a 1 sec on, 1 sec off flasher, you need to add a resistor from pin 7 to pin 6.  I calculate the resistor to be about 1 M with a 1.5 uf cap, and change the existing 1 M resistor to 1 K.  Thi = .693 (RA + RB) C;  Tlo = .693 (RB) C.  Therefore to get Thi = Tlo (or close), RB has to be much greater than RA.  (I think I did this right!)  Have fun!!

--- End quote ---

Ah yes, I did have the 0.01uF cap on the PWM 555 timer on the breadboard, I must have missed that when I made the circuit diagram. However, I did not have a 0.01uF cap on the Flasher 555 timer because the diagram I based it off of did not. I will try and add one on the breadboard and see what happens.

I'll also give the resistor/caps a try and see what happens :) Thanks!
doublec4:
Okay, as mentioned in my last post, I have the 0.01uF cap on the PWM 555timer pin 5 to GND, but I did not have it on the Flasher 555timer pin 5 to GND. When I add it to the Flasher 555 Timer, in bright mode it slows down the flash rate, and in dim mode it speeds the flash rate up, all other things left unchanged. Obviously one circuit is affecting the other in some way that I am not seeing.

I will go ahead and try the different circuit for the flasher... something similar is posted here:

https://www.electronics-tutorials.ws/waveforms/555_oscillator.html

doublec4:
I tried the changes in the resistor/capacitor combination and placement you suggested GerryR, but the flash rate actually increased dramatically. Same thing happens with similar circuits from that link I posted.

When tested on it's own, the circuit flashes at the rate you calculated, same as the circuit in the link I posted. However, as soon as I combine it with the PWM circuit as shown in the circuit I posted, they are interacting in some undesirable way to produce a much faster blinking pattern. I just cannot seem to understand how/why

Since the behavior changes when I switch between PWM duty cycles, I feel that it has something to do with that portion of the circuit controlling the duty cycle on the PWM 555 timer. Something about that portion is not "isolated" and it is affecting the flash rate of the Flasher 555 circuit:


* Higher duty cycle PWM results in higher frequency of blinking in the flasher circuit. Can be compensated for by adding 0.01uF cap on pin 5 of flasher555 and flash rate slows a bit
* Lower PWM duty cycle results in lower frequency of blinking in the flasher circuit. Adding 0.01uF cap on pin 5 of flasher 555timer speeds it up.
As always, any more help would be greatly appreciated!

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