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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