# How to generate 1s pulse to trigger 555-timer?

#### XtinctElise

Joined Jan 21, 2012
10
I have the following 12 volt automotive circuit setup and working to trigger about 60 seconds of power output to a relay on output pin 3 of the 555 Timer chip.
This setup allows a negative 1 second input pulse to turn on a the 12V automotive relay for about 1 minute.
The 2nd requirement of the circuit is to also trigger the 60 second output of the 555 Timer to fire once the 12 Volt Ignition wire changes state from 12 volt to off/ground. While I can easily make it turn off by sensing ground, the timer needs the input to be momentary, because latching the input to ground keeps the 555 Timer's output high for the duration of the ground input.

Any suggestions how to send about a 1 second ground pulse when the ignition changes state from 12 volts to ground?

I know I could add a 2nd 555 Timer circuit to accomplish this but I am trying to keep the circuit complexity down as well as keep the standby current as low as possible.

Thank you for your feedback.

#### wayneh

Joined Sep 9, 2010
16,500
I know I could add a 2nd 555 Timer circuit to accomplish this but I am trying to keep the circuit complexity down as well as keep the standby current as low as possible.
Take a look at using a 556 (dual timer), the CMOS version. The IC itself will use very little current and so your challenge is to choose the timing components to minimize current there. A minute is a long time but should be doable.

#### crutschow

Joined Mar 14, 2008
25,991
So you want two different inputs to trigger the 555, both switch SW0 and the 12V going to ground?

#### schmitt trigger

Joined Jul 12, 2010
366
Two words; capacitive coupling.

A 0.1uF series capacitor in the line that goes to pin 2. And a 120k resistor from pin 2 to the 555's Vcc.

#### XtinctElise

Joined Jan 21, 2012
10
So you want two different inputs to trigger the 555, both switch SW0 and the 12V going to ground?
Correct, both a 1 second ground pulse and turning 12V to Ground, need to trigger the 555 Timer. The trick with the 12V to Ground is that it needs to be a pulse and not constant ground.

#### crutschow

Joined Mar 14, 2008
25,991
Below is the LTspice simulation of a single 555 circuit that will trigger from both a switch closing momentarily, and also the 12V going to ground:
The capacitor provides the momentary pulse, as ST suggested.

Last edited:

#### XtinctElise

Joined Jan 21, 2012
10
Below is the LTspice simulation of a single 555 circuit that will trigger from both a switch closing momentarily, and also the 12V going to ground.
Thank you crutschow! That circuit did work, but it only allowed the 12V to Ground In2 input to start the timer one-time. When I simulated turning the 12V back on to In2 (like turning on the ignition, and then turning it off again, there was no ground input to pin2. To get it work again, I had to turn the capacitor around 180*, I assume to discharge it.

What am I missing to allow this trigger to fire each time as it cycles from 12V to ground? I assume it needs to drain the capacitor somehow?

#### XtinctElise

Joined Jan 21, 2012
10
duplicate post

#### crutschow

Joined Mar 14, 2008
25,991
When I simulated turning the 12V back on to In2 (like turning on the ignition, and then turning it off again, there was no ground input to pin2.
That's because you didn't simulate the 12V going to ground due to load on the ignition circuit.
You can simulate that with a resistor to ground.

#### XtinctElise

Joined Jan 21, 2012
10
You can simulate that with a resistor to ground.
I just cant seem to get this working. Is the resistor to be inserted between V2 / In2 and Ground? Thank you for the help, it is appreciated.

#### crutschow

Joined Mar 14, 2008
25,991
Is the resistor to be inserted between V2 / In2 and Ground?
It should be connected from In2 to ground as shown below.
Try a value of about 1k ohm.

#### XtinctElise

Joined Jan 21, 2012
10
It should be connected from In2 to ground as shown below.
Thank you, it is working properly now. The issue was a diode I had between C3 and the pin 2 of the 555, not sure why that adversely affected it but moving it before R3 solved the issue and allowed the circuit to work as you described! Thanks for the help, I will post the new circuit next.

#### XtinctElise

Joined Jan 21, 2012
10
Here is the circuit that I have working now with both the ground pulsed input from the AUX alarm and also triggering from the ignition switching off.

I am powering a latching relay, so the output will also need to pulse ground to relay pin 1 for about 200ms to turn on the relay when:
1) the start of the delayed output, OR
2) when ignition (12V is turned on)

Then when the 60 second 555 Timer output ends it will have to pulse ground to relay pin 2 for about 200ms to turn off the relay.

I have the inputs working correctly. Now just need assitance in getting the outputs to fire the relay contacts for about 200ms. Currently the output signal to the Set (pin 1) contact is working but when I try to use that to power a transistor as in the example to the Reset (pin 2) it does not turn the transistor on.

I assume the capacitor is not allowing enough current thru to trigger the transistor.
How does this circuit need to change to power on the transistor for 200ms?

Last edited: