555 Circuit Peculiarities

Thread Starter

damnyankee36

Joined Dec 19, 2015
13
I am using a 555 chip for a monostable time delay circuit. I am using a trimmer to adjust the delay, which is around 10 mS -200 mS. It works quite well at the longer delays and within tolerable limits the lower I adjust it. However, when I get below 20 mS, the delay occasionally swings down 10 mS or more. It happens every 5-20 triggers, very random.

The circuit is below. I am using a relay to power and trigger the 555 simultaneously at the + and – inputs with a regulated 12v. Could contact bounce be the cause? The relay is producing about 5 mS worth of bounce. I’m also wondering; when does the 555 decide to recognize the input, at the beginning of the bounce pulses or at the last one? I am also using .01uF cap at terminal 5 for possible noise control.

I am using 2.2 uF and 4k values for the 10 mS setting.

555.jpg
 

Alec_t

Joined Sep 17, 2013
10,699
Pins 4 and 5 of the 555 are floating. Interference picked up on those pins might be causing erratic triggering/timing.
I am using a relay to power and trigger the 555 simultaneously at the + and – inputs
Why are you using this rather unconventional triggering method involving switching both power rails of the 555?
 

AnalogKid

Joined Aug 1, 2013
8,251
The appnote suggests a 10nF from pin 5 to GND - but I've seen a few people get away with not bothering.
That would be me. In the original 555 the three resistors that set up the two comparators were 5K each, so the circuit impedance at the control input is less than 5K. A little noise here does affect the circuit timing, but not greatly. If the application can stand a 5% variation in timing, I skip the noise filter cap.10 cents is 10 cents.

ak
 

Thread Starter

damnyankee36

Joined Dec 19, 2015
13
Thanks for all your suggestions! This is my first attempt at using a 555. So I was totally dependent on finding a circuit that was already proven.

The appnote suggests a 10nF from pin 5 to GND - but I've seen a few people get away with not bothering.
Because the diagram didn't show it, I mentioned in my post that I used a .01 uF on pin 5. I should have informed you all that I found this elsewhere and used it because it did what I wanted. I needed it to sink the load, not supply it as most diagrams show.

Pins 4 and 5 of the 555 are floating. Interference picked up on those pins might be causing erratic triggering/timing.
Would I put another cap on pin 4? What size?

Why are you using this rather unconventional triggering method involving switching both power rails of the 555?
I think I am following you. Are you saying I should have it already powered before I trigger it? Not sure how I would do that...

Is pin 4 connected to pin8, as its not showing on your drawing...also i would put a 100uF capacitor across pins 1,8
I connected both those pins together and didn't notice any difference on the scope. However, when I placed the cap across 1 and 8, the input and output timing went all to hell!
.
 
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Thread Starter

damnyankee36

Joined Dec 19, 2015
13
So, I just used the micro switch that was previously triggering the input relay. The switch by itself without the relay was much cleaner and made the <20mS much more consistent. Now I am looking at a simple debounce circuit with an RC circuit since the final design has to include the input relay. Looked at solid state relays but DIP ones are only powered with 1.2v. I need to keep the relay coil side of the circuit at 24-30 volts.

I only have so much room for the PC board. I need to keep the component numbers down and relatively small.
 

Thread Starter

damnyankee36

Joined Dec 19, 2015
13
I've been doing a lot of searching on 555's with phrases that included your suggestion. It seemed like everything I saw had the output sourcing the load instead of sinking it. I wasn't concerned with the rest of the circuit at that time. I'll give it another shot.

Thanks!
 

ian field

Joined Oct 27, 2012
6,539
Thanks for all your suggestions! This is my first attempt at using a 555. So I was totally dependent on finding a circuit that was already proven.


Because the diagram didn't show it, I mentioned in my post that I used a .01 uF on pin 5. I should have informed you all that I found this elsewhere and used it because it did what I wanted. I needed it to sink the load, not supply it as most diagrams show.


Would I put another cap on pin 4? What size?


I think I am following you. Are you saying I should have it already powered before I trigger it? Not sure how I would do that...


.

You have a trig input to switch the output on, a threshold pin to switch the output off and a reset pin to disable the whole chip.
 

Dodgydave

Joined Jun 22, 2012
8,671
I've been doing a lot of searching on 555's with phrases that included your suggestion. It seemed like everything I saw had the output sourcing the load instead of sinking it. I wasn't concerned with the rest of the circuit at that time. I'll give it another shot.

Thanks!
What is it you're trying to do, fire a relay after a delay, or a delay first then fire it?
 

Dodgydave

Joined Jun 22, 2012
8,671
So power on, delay, then relay fires and stays on till power removed,
The circuit you have will do that, as well as this one, omit Q1, R2, and connect the relay to pin 3,..
 
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ian field

Joined Oct 27, 2012
6,539
I've been doing a lot of searching on 555's with phrases that included your suggestion. It seemed like everything I saw had the output sourcing the load instead of sinking it.

Thanks!
The bipolar 555 can source or sink 200mA - I'm not up to date with the CMOS version, but I know the current capability is less.
 
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