556 Dual timer. Whats different between using 5V and 9V - Page 1
Forum section: electronics / beginners

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 05:49:40 pm
content: I have made this circuit.At 5V supply, if the base of T2 is made high momentarily, then bought low, the LED flashes at a rate of about 4 times a second for a period of 11 seconds or so, then stops until the base is bought high again. Which is what I would expect to happen.However, using a 9V supply, having changed the LED resistor value to one appropriate for a 9V supply, if the base of T2 is bought high the LED begins to flash at a rate of about 4 flashes per second, but stops flashing the moment it goes low.Can anyone think of a reason why it behaves differently at 5V and 9V ?

author: jwet
author_rank: Frequent Contributor
timestamp: October 09, 2025, 06:24:50 pm
content: I suspect that the big 10 sec cap doesn't get fully discharged well in the 9v case and bounces back up after a brief trigger. 330 uF is pretty enormous, perhaps lower it to 33 uF and up the R to give you the 10s. You could also add a helper transistor across the cap, perhaps a little nmos 2N7000 that turns on when trigger is low (tie to transitor trigger input). The will help flatten the cap. If you afford it, consider using a CMOS variant of the 555, they're better in a lot of ways and would let you push this C down another decade and raise the R another decade (input currents are tiny).Good luck.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 06:32:36 pm
content: Quote from: jwet on October 09, 2025, 06:24:50 pmThis.I would go for 1M and 10µF. The current through the resistor will still be higher than the tiny bias currents and capacitor leakage, not to make any practical difference.

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 08:27:39 pm
content: I have gone with the suggestion of 10uf and 1M.Slightly different issue now. 5V it turns on but doesn't stop at all.9V same as previously.I will experiment with differing values. I'm not sure why the difference in capacitors values (and associated resistor values) make a difference. I was happily bumbling along thinking that as long as T=1.1R1C1 came out with the correct T it was good to go.Clearly there is more to this than meets the initial eye.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 08:30:50 pm
content: What's the supply decoupling like?I notice there's no capacitor on CV1.

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 08:34:29 pm
content: Thanks for this.I have tried the suggestion below (1M 10uf) but that has a different issue at 5V.I will give your suggestion a run. I think I understand the helper transistor suggestion, I have a good number of 2n2222A's courtesy of Aliexpress that I will use. If I pop a few, it won't matter much. [Edit. 47uf and 220K has got it back working on 5V, 9v still just runs while base is high, Transistors next. ][Edit. Additional 2n2222a fitted, its emitter to negative rail, and its collector to U3 THR2 pin (so effectively across C4) base to T2 base. Hasn't made any difference that I can detect, but then I'm not sure what difference it would make.]As I say below, I thought as long as the T came out OK the values of R and C to get it didn't much matter

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 08:37:17 pm
content: Not sure what that means.I see what you say about CV1.I will pop a duplicate of the capacitor on CV2 to see how if that helps. [Edit. It didnt.]

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 09:20:08 pm
content: I mean, the chip needs a 10µF capacitor across it, as close to the power pins as possible.Have you tried a different IC?What does the physical construction look like? Is it built on a solderless breadboard, stripboard or PCB?

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 09:42:04 pm
content: Quote from: Zero999 on October 09, 2025, 09:20:08 pmAh. I will decouple. [Edit. Decoupling seems to have sorted it..... Possibly the Transistor thing as well...]I haven't but I will.Picture attached. Not pretty, but the frequencies are really low.

author: Nick0268
author_rank: Contributor
timestamp: October 09, 2025, 10:08:15 pm
content: This works at 9 Volts!!!!Thanks to all for the suggestions.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2025, 08:59:42 am
content: My bet is it's the 10µF capacitor which did the trick.Connecting transistor bases in parallel like that is generally a bad idea, because both transistors will not saturate. The correct way to do it is to use separate base resistor for each transistor, but in this case I doubt T3 is making any difference. Try removing it, to see if it makes any difference.It's not low frequency, the 555 timer is notorious for drawing, short high curent spikes, when its output changes state. These have high frequency components. Contrary to popular opinion, the 555 timer is not a beginner friendly IC. It requires good layout and decoupling to work properly and is prone to glitches.

author: Seekonk
author_rank: Super Contributor
timestamp: October 10, 2025, 05:02:43 pm
content: I was around when the 555 first came out. It should have been named 666, the spawn of the devil. Recently there have been a number of articles about using the 555 as a FET driver using the reset pin as input. While I have sworn to never use them except to blink a LED, thought occurred to me maybe I had held a grudge too long. First thing I noticed was the reset pin had no hysteresis. That rose some red flags. I wired up the circuit that should have been low powered since no output was connected. Power supply was totally dragged down to 6V from 15V. When reset pin voltage is near the trip point the chip goes into a lockup condition drawing close to 40ma. Remember, this is with no load. Circuit works with a high current power supply but not with a zener regulated power supply designed for 25ma. I gave it a chance. The grudge is still on for another 50 years.

author: ledtester
author_rank: Super Contributor
timestamp: October 10, 2025, 08:59:12 pm
content: Quote from: Seekonk on October 10, 2025, 05:02:43 pmLike this one...