Electronics > Beginners
Help diagnosing this dual 555 timer circuit please (PWM + Blinking)
GerryR:
Not sure how you are making out on this, but a simple solution is to send the PWM output to the wiper of a relay and connect your LED's, one to the NO contact and one to the NC contact. Then connect the coil of the relay to the flasher. This keeps both circuits isolated from each other. I haven't bread-boarded your flasher circuit, but I believe you are getting in trouble with the flasher output being tied back to your charging circuit for the flasher and trying to drive the LED circuits as well. Also, it is always a good idea to use a divider circuit when driving a FET or a BJT input. (On the FET, it lowers the input impedance so it is not as susceptible to noise, and on the BJT it gives a place for the base charge to go giving faster off times. If I get time, I will bread-board the flasher / output circuit to see what is going on. Keep having fun!!
doublec4:
--- Quote from: StillTrying on June 29, 2019, 02:11:39 pm ---"If I add the decoupling caps shouldn't the flash rate slow back down? Instead it speeds up."
Where are you adding the decoupling caps.
Bipolar 555s draw current spikes of >350mA when they switch, which can cause voltage spikes on the supply, without a 100nF very close to their 1,8 supply pins strange things can happen. Having decoupling on the pin 5 but not the 555's supply can sometimes make things worse.
--- End quote ---
I didn't have 100µF but had 50µF that I put across the 12V/GND rails near the circuit. Then the 100nF caps I put close to pin 8 on both circuits. Perhaps the 50µF wasn't enough...
doublec4:
--- Quote from: GerryR on June 29, 2019, 03:47:32 pm ---Not sure how you are making out on this, but a simple solution is to send the PWM output to the wiper of a relay and connect your LED's, one to the NO contact and one to the NC contact. Then connect the coil of the relay to the flasher. This keeps both circuits isolated from each other. I haven't bread-boarded your flasher circuit, but I believe you are getting in trouble with the flasher output being tied back to your charging circuit for the flasher and trying to drive the LED circuits as well. Also, it is always a good idea to use a divider circuit when driving a FET or a BJT input. (On the FET, it lowers the input impedance so it is not as susceptible to noise, and on the BJT it gives a place for the base charge to go giving faster off times. If I get time, I will bread-board the flasher / output circuit to see what is going on. Keep having fun!!
--- End quote ---
Interesting! If you get the chance to bread board this, I'd like to see what your results are.
Thanks!
GerryR:
Attached is what I did so far; I'm still playing. The main differences from your original schematic are the way I set up the flasher and the way I biased the FETs. It appears to work OK. I don't like the way the PWM switching is done in conjunction with the flasher. I prefer doing it with a relay as suggested above. I will try that later. Outside of seeing the PWM output on the supply rail (the way I set it up while bread-boarding), everything appears to work independently and the timing is as calculated.
doublec4:
I am certainly going to give this a try this week, thanks! :)
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