momentary activation of relay upon voltage detection

Thread Starter

bar891

Joined Aug 7, 2018
6
I'm looking for some help on a circuit for my vehicle. i Need a circuit that when it senses 12v from the ignition switch it then triggers a relay to activate momentarily . Maybe it could be done without relay open to your thoughts.I need to pull a signal to ground momentarily.

Many Thanks
 

Thread Starter

bar891

Joined Aug 7, 2018
6
It likely could be done with a resistor, a capacitor, and a MOSFET.
What's the signal you want to pull to ground?
It for a steering wheel switch that pulls a 5v signal to ground. I want it to happen automatically when the ign is turned on.
 

crutschow

Joined Mar 14, 2008
27,945
Below is a circuit that should do what you want.
It holds the output low about 1.4 seconds for the R1 and C1 values shown.
These values can be changed if you need a different time

RTest and V2 are just for simulation purposes and are not needed in the real circuit.

upload_2018-8-8_0-33-9.png
 

Thread Starter

bar891

Joined Aug 7, 2018
6
So when V1 is ign switch on it will pull V2 to ground for 1.4 seconds then release because V1 will still be on?
 

crutschow

Joined Mar 14, 2008
27,945
So when V1 is ign switch on it will pull V2 to ground for 1.4 seconds then release because V1 will still be on?
Not sure I understand the question.
V1 has to stay high during the delay time for the circuit to work, which I assume the ignition voltage does.
The MOSFET Gate voltage initially goes to 12V through capacitor C1 when the input goes to 12V.
This V(gate) voltage decays towards zero through R1 until it goes below the MOSFET input threshold voltage (≈3v), at which point the MOSFET turns off, causing the output voltage to return to high, as shown in the simulation below.

upload_2018-8-8_11-13-31.png
 

Thread Starter

bar891

Joined Aug 7, 2018
6
Not sure I understand the question.
V1 has to stay high during the delay time for the circuit to work, which I assume the ignition voltage does.
The MOSFET Gate voltage initially goes to 12V through capacitor C1 when the input goes to 12V.
This V(gate) voltage decays towards zero through R1 until it goes below the MOSFET input threshold voltage (≈3v), at which point the MOSFET turns off, causing the output voltage to return to high, as shown in the simulation below.

View attachment 157766
What i am asking is that if V1 is the ignition voltage (12v) after it is turned on it stays on, will that affect the working of the gate?
 

dl324

Joined Mar 30, 2015
13,314
What i am asking is that if V1 is the ignition voltage (12v) after it is turned on it stays on, will that affect the working of the gate?
That's in post #8. When 12V is applied to C1, it will eventually discharge below the threshold voltage of the MOSFET and it'll turn off.

On time depends on the threshold voltage which isn't tightly controled. So values need to be adjusted for the specific MOSFET used.
 

crutschow

Joined Mar 14, 2008
27,945
What i am asking is that if V1 is the ignition voltage (12v) after it is turned on it stays on, will that affect the working of the gate?
No, it was designed to have the 12V stay on.
On time depends on the threshold voltage which isn't tightly controled. So values need to be adjusted for the specific MOSFET used.
Only if the on-time is critical and it doesn't sound like it is in this application, as long as it's longer than an unknown minimum.
 
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