Converting switched 12V signal to ground switched input

Thread Starter

reciprocity

Joined Feb 6, 2022
13
Hi everyone,

I'm working on installing an aftermarket ECU in my car and need to connect a 12v input signal to it. However, the ECU can only recognize a grounded switch.

Here's a really poor diagram of how it needs to be wired:

ECU--(wire)--switch--(wire)--ground

The switched 12v is not PWM, and does not carry high amperage. I thought of using a relay, using the 12v signal to close a grounded switch but room is limited and I want to keep things as simple as possible.

Does anyone have an idea of how this could be easily done without the use of a relay? Thank you!!
 

djsfantasi

Joined Apr 11, 2010
8,323
Depending on the ratings (Voltage rated to 20VDC and current ratings of the ECU input), an N channel MOSFET with pull up resistor on the output and pull down resistor on the input should do it.

Automotive Experts?
 

crutschow

Joined Mar 14, 2008
29,779
I third the use of an N-MOSFET for the job.

An NPN bipolar transistor would also work, which would require a series resistor (e.g. 1-10kΩ) to the base input for the 12V signal.
 

Thread Starter

reciprocity

Joined Feb 6, 2022
13
Thanks everyone,

Looks like I have a lot to learn about mosfets! I assume they are solid state, so reliability would not be a problem?

@MaxHeadRoom there is no switch at the ecu, the 12v signal is controlled by a manual dial that turns on/off air conditioning for the car.
 

crutschow

Joined Mar 14, 2008
29,779
I assume they are solid state, so reliability would not be a problem?
Yes, they are solid-state so have long life, but the input gate is sensitive to spikes, typically above about 20V.
So, if the gate signal is from the 12V vehicle battery power in the post #7 circuit, I would add a 10kΩ resistor between the 10kΩ resistor to ground and the gate, with a 10uF capacitor from the gate to ground, to protect against any spikes from the 12V line.
 

Thread Starter

reciprocity

Joined Feb 6, 2022
13
Yes, they are solid-state so have long life, but the input gate is sensitive to spikes, typically above about 20V.
So, if the gate signal is from the 12V vehicle battery power in the post #7 circuit, I would add a 10kΩ resistor between the 10kΩ resistor to ground and the gate, with a 10uF capacitor from the gate to ground, to protect against any spikes from the 12V line.
This is very helpful, thank you. I assume that this would require a small PCB? I wouldn't want resistors and caps just hanging in the harness itself.
 

crutschow

Joined Mar 14, 2008
29,779
A 2N7000 is fully on at well below 6 volts.
OK. let's quibble. :rolleyes:

To be fully on at higher currents a Vgs of 10V is used (below).
It's easy to get that here by reversing the positions of the two resistors so that it's fully on for any current.

But if the drain current is low, then 6V should be fine.

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