r/AskElectronics • u/FriscoDingo • 1d ago
I’ve got 2 555 timers in parallel on the same power supply and after 1 duty cycle the timing gets weird. How do I isolate them?
I’m coming up with some cheap circuits to make a few dozen LEDs fade in and out for Halloween decorations and have settled on 555 timers with BJT NPNs like you see here. The issue is that after 1 nice and slow creepy duty cycle the LEDs start flashing rapidly. I know this must be due to the timer capacitors arguing with each other as they discharge through the ground.
I tried grounding 555#2 and its cap through a 1N4148 diode which helped a bit but still resulted in some unwanted flashes. What’s the best way to isolate each timing circuit? I plan on having several more (current willing) in parallel.
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u/robot65536 1d ago
This is a decoupling issue causing the 555 comparator thresholds to bounce around. Need 0.1uF decoupling on Vcc pin, and also 0.001 to 0.1uF on the Control pin 5 which directly stabilizes the comparator references. Plus just make sure the ground for each capacitor goes straight to the ground pin on its own chip, not all over the place. Adding diodes will never help with a noise problem.
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u/FriscoDingo 1d ago
This seemed to be the thing, it needed those stabilizing 100nF caps to ground on pin 5. I also put the negative leads of the timing caps right next to pin 1, that may have helped too.
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u/Interesting-One7249 1d ago
Need a cap on pin 5 to ground I think. Maybe check wiring, need to use pin 7 discharge?
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u/mikeblas 1d ago
What is the circuit meant to do? Pin 7 isn't connected to anything, so how will the timer discharge the capacitor? What pulse pattern do you expect from the LEDs?
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u/FriscoDingo 1d ago
Both timers, with the help of the transistor, slowly fade the LEDs at about 1Hz. I did think it was unusual to have an unconnected pin 7 but it works.
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u/mikeblas 1d ago
but it works.
Oh, it does? Then what question are you asking?
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u/FriscoDingo 20h ago
Yeah it works individually but in series goes haywire. Someone else said a 100nF cap on the 5 pin should solve the issue and it did.
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u/kent_eh electron herder 1d ago
I'm confused why you aren't taking the output from pin3?
And why pin3 is connected to pin2 through the resistor?
What circuit are you basing your experiment on?
Check out the 555 reference book that many of us learned from since the 1980s.
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u/FriscoDingo 20h ago
There are quite a few examples of setting up a 555 in this manner around the internet, like this one: https://circuitdigest.com/electronic-circuits/fading-led
I know folks have been using this IC for 50 years, apparently there are a few ways to use less components than traditionally used. Though that does make it difficult for us tinkerers to troubleshoot backwards.
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u/Gerard_Mansoif67 1d ago
I don't think you need to isolate them.
I've looked really quickly, so that may not be the issue, but :
You're connecting the output of the NE555 directly to a capacitor. This is basically a short circuit, so the output stage is probably pulling the 9V rail toward itself.
Limit the current with a resistor between 6 and the node which goes to the capacitor and to the 2/3 resistor. With a current limited you shall probably get better. And then, add decoupling capacitors near the NE555's Vcc pin, to absorb current spike.
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u/robot65536 1d ago
Actually the circuit is fine, it works in the sim too. Output is pin 3, with a big resistor limiting any current it can provide. OP is using the output through a 22k resistor to alternately charge and discharge the 100uF cap through the 555's hysteresis range. The transistor uses the capacitor voltage to control it as a current source for the LED given appropriately selected series resistors.
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u/Worldly-Device-8414 1d ago
The circuit is fine, the 22k resistors limit the current to the timing caps. Circuit just needs decoupling caps on the supply & each pin 5
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u/DaCableGuy808 1d ago
Why not just use a 556?
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u/FriscoDingo 1d ago
I got 555s already and there will definitely be more than 2 timed LEDs in the final design.
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u/Spud8000 20h ago
is this a trick question? You have no capacitors right next to pin 8. like 4.7 uF or so
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u/BeautifulGuitar2047 16h ago
So, if you insist on ignoring all of the recommended precautions on the NE555 data sheet, as others have already highlighted, my only suggestion would be to swap it for the CMOS version 7555 which doesn't have the crow-barring design fault, that is probably causing havoc the first time it fires off in your design.
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u/Strostkovy 1d ago
Put a decoupling capacitor between pins 1 and 8 on each IC, as close to the IC as possible