| Electronics > Beginners |
| Help diagnosing this dual 555 timer circuit please (PWM + Blinking) |
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| Doctorandus_P:
The PWM circuit has an RC time constant of roughly,, half a meg times 10nF. >> 5e5*10e-9 ans = 0.0050000 While the right section has a time constant of 1M * 20uF = 20s None of these are close to the 2s that OP is observing. If the circuit starts behaving very differently when decoupling & buffer caps are added, then it is probably because those capacitors are doing their work. Also: Go buy an oscilloscope, they are indispensable for debugging electronic circuits. |
| mikerj:
--- Quote from: doublec4 on June 28, 2019, 02:05:11 pm --- --- Quote from: mikerj on June 28, 2019, 01:33:50 pm ---The relatively high LED current is almost certainly putting significant noise onto the supply rails and I don't see any supply rail decoupling. Add some bulk capacitance (e.g. 100uF or so) and a 100n ceramic cap per 555, and put them as physically close as possible to the supply and ground pins. --- End quote --- Just tried what you suggested and it drastically increases the flashing rate :-\ --- End quote --- If adding decoupling to the supply rails significantly changes the flashing rate this is telling you there was a bad noise problem. Leave the decoupling caps in place and adjust the timing components to get the flash rate you require. |
| doublec4:
That is the part I am having a hard time understanding though. If I add the decoupling caps shouldn't the flash rate slow back down? Instead it speeds up. I had to have the calculated RC time of the flash circuit at 20s to achieve appx 1Hz flash rate when the two circuits were combined. If I use a RC time of 2s on the flash circuit and test it independently, then of course it flashes as designed. As soon as I combine it with the PWM circuit, even with the decoupling caps, it speeds way up. |
| StillTrying:
"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. |
| Arjunan M R:
--- Quote from: doublec4 on June 28, 2019, 03:06:18 am --- I did not have a 0.01uF cap on the Flasher 555 timer because the diagram I based it off of did not. --- End quote --- If you don't know, to type ยต hold down Alt+2+3+0. ;) :) |
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