555 timer getting triggered on power up

Thread Starter

robnoper

Joined Mar 5, 2019
24
I have got this circuit working with some of your help and now have a small issue on repeatability.

12v is supplied and the timer module of a mains on off timer is powered with 1.5v. when the timer is set to ON, this then powers up the 555 timer circuit and a receiver board on JP2. pin 4 on JP2 is connected to pin 2 of the 555 timer and starts it. All of this works well.

HOWEVER!!!

when I power on the initial 12v to the 555 sometimes triggers and sets off the sounder circuit. once powered on when I cycle the mains timer module from off, auto and on, when It gets to on, the 555 timer triggers then also. It should only trigger when J2 pin 4 gives it a negative signal.

If I disconnect the timer board...all works well powering on and off so when no load on the 555 timers it all is fine.

Any thoughts.

Capture.PNG
 

BR-549

Joined Sep 22, 2013
4,938
May I ask, what is the end goal? What are you trying to do?

What is a timer module and what are the mains?
 

Thread Starter

robnoper

Joined Mar 5, 2019
24
May I ask, what is the end goal? What are you trying to do?

What is a timer module and what are the mains?
the end goal is to have the whole thing powered by 12v dc

1.5 is provided from the 12v supply to power up the LCD clock module. This clock module provides 1.5v when in the on position. When in the on position, it powers up the 555 timer, ready to be triggered by JP2 pin4. When the 555 timer is started it powers a sounder board that has built in LED's

The LCD clock should be able to power up all the sub circuits without triggering the 555 timer. This should only ever get triggered by JP2 pin4, but it tends to get triggered when the 12v power is provided and when the LCD clock is cycled.
 

AlbertHall

Joined Jun 4, 2014
8,540
You need to hold the '555 reset pin 4 low for a short time when the '555 is powered up. You can do this by connecting a capacitor from pin 4 to ground and a resistor from pin 4 to the supply, pin 8. Make the capacitor 100nF and the resistor 22k.
 

danadak

Joined Mar 10, 2018
3,620
In mono mode that apparently is a problem with 555.

Many micros these days are designed such that they do not generate
false GPIO transients when they power up. Note their GPIO powers
up in Tristate which requires a pull down to insure no false triggers.

And parts like an ATTINY85 can function like a 555 and a side effect
is much more accurate timing, over T and V, generated. a well as more
complex triggering.

Example using mBlock to do pulse generation -

https://forum.allaboutcircuits.com/threads/visual-programming.157949/#post-1370895


Regards, Dana.
 

Thread Starter

robnoper

Joined Mar 5, 2019
24
You need to hold the '555 reset pin 4 low for a short time when the '555 is powered up. You can do this by connecting a capacitor from pin 4 to ground and a resistor from pin 4 to the supply, pin 8. Make the capacitor 100nF and the resistor 22k.

Nice....sort of come to that conclusion myself...know I could trust you guys to come up with a solution. will test tonight
 

Thread Starter

robnoper

Joined Mar 5, 2019
24
You need to hold the '555 reset pin 4 low for a short time when the '555 is powered up. You can do this by connecting a capacitor from pin 4 to ground and a resistor from pin 4 to the supply, pin 8. Make the capacitor 100nF and the resistor 22k.
Any particular type of capacitor?? Ceramic, Electrolytic??
 

crutschow

Joined Mar 14, 2008
23,526
You can do this by connecting a capacitor from pin 4 to ground and a resistor from pin 4 to the supply, pin 8. Make the capacitor 100nF and the resistor 22k.
Because the Reset threshold voltage is only 0.5V (basically one Vbe drop), a much larger capacitor is required to hold the voltage below 0.5V during power up.
My simulation showed you need over 4μF.
I would use 10-100μF (electrolytic).
 

AlbertHall

Joined Jun 4, 2014
8,540
Because the Reset threshold voltage is only 0.5V (basically one Vbe drop), a much larger capacitor is required to hold the voltage below 0.5V during power up.
My simulation showed you need over 4μF.
I would use 10-100μF (electrolytic).
How long does reset need to be held low?
 

Thread Starter

robnoper

Joined Mar 5, 2019
24
Ok I have done some more testing and J2 pin 4 is upsetting the 555 some how on power on.
If I disconnect J2 pin4 then power the circuits on, then plug in J2 pin4 to pin 2 of the 555 all works well and reliably..
So I need some way electronically doint the same with a transistor or something. just need to delay presenting J2 Pin4 to the 555
 

crutschow

Joined Mar 14, 2008
23,526
Ok I have done some more testing and J2 pin 4 is upsetting the 555 some how on power on.
If I disconnect J2 pin4 then power the circuits on, then plug in J2 pin4 to pin 2 of the 555 all works well and reliably..
So I need some way electronically doint the same with a transistor or something. just need to delay presenting J2 Pin4 to the 555
That's what the added RC circuit we are talking about, does.
 

ElectricSpidey

Joined Dec 2, 2017
739
According to my 555 timer cookbook, connecting a large value capacitor between pin 5 (control voltage) and zero volts will stop power-up triggering.
This works in LTSpice and stops a relatively long trigger pulse, and doesn’t even need a very large cap. (depending on startup pulse length)

Didn’t test it in real life.


A_Trigger_Inhibit.JPG
 
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