# Trigger 12v relay with 3v source

#### Newmember

Oct 29, 2023
4
Hi, I have a little project on the go but I am stuck and hoping to find some help, to try and keep things simple I have a circuit board that watches the rpm of my gas powered motor and once the motor hits a certain rpm a 3 V LED turns on… for what I am trying to do. I need a heavy (10amp+) 12v relay to turn on when that 3 V LED turns on. I am not sure if there is a relay that can be triggered by the very little power the 3 V LED is getting and switch a 12 V source to then switch the heavy 12 V relay or if there is something else out there that can do what I am trying to do?

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#### Delta Prime

Jul 29, 2020
1,792
Welcome to Make Pro.
More information is needed. In the schematic below is to show the idea the reason why the schematic includes 3 volts.
What is powering that 3-volt LED is it 3 volts? 5 volts?
You have a schematic?
And you must realize you take one simple circuit and combine it with another simple circuit; that becomes a complex circuit. Also that relay can be mechanical or solid state.

#### crutschow

May 7, 2021
832
if there is a relay that can be triggered by the very little power the 3 V LED is getting
What powers the LED circuit?

#### Newmember

Oct 29, 2023
4
Welcome to Make Pro.
More information is needed. In the schematic below is to show the idea the reason why the schematic includes 3 volts.
What is powering that 3-volt LED is it 3 volts? 5 volts?
You have a schematic?
And you must realize you take one simple circuit and combine it with another simple circuit; that becomes a complex circuit. Also that relay can be mechanical or solid state.
View attachment 61265
The red, black and green wires are the only ones that mean anything, the white3 go to an adjustment knob. The drawing is the best I can do to explain what I’m trying to do

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#### Newmember

Oct 29, 2023
4
What powers the LED circuit?
I just replied to another guy showing everything

#### Delta Prime

Jul 29, 2020
1,792
The drawing is the best I can do to explain what I’m trying to do
Thank you for your efforts.Make, model, of that unit and or link to where you have purchased it.
Thanks.

#### Newmember

Oct 29, 2023
4
Thank you for your efforts.Make, model, of that unit and or link to where you have purchased it.
Thanks.
Got it on Amazon.. ESUPPORT Red Led Adjustable Tachometer Rpm Tacho Gauge Shift Light 1000-11000 Universal https://a.co/d/gYmX8rM

#### Bluejets

Oct 5, 2014
6,852
Easiest way I can think of would be put an opto input straight across the existing LED(4N25 or similar), drive a relay with the open collector output.

https://www.vishay.com/docs/83725/4n25.pdf

Diagram above showing a 1k5 base resistor would be way too high a value, more like maybe 220R.

#### crutschow

May 7, 2021
832
Here's the LTspice simulation of a circuit similar to what Bluejets suggested:
It uses a Darlington output opto coupler (for isolation if the LED is low-side driven) with a high transfer gain, driving a MOSFET driver for the relay to minimize the current drain from the tach LED:
It works because the forward voltage drop of a red LED is slightly higher than the drop of the opto IR input LED.

The sim shows a maximum opto input current of <0.5mA (blue trace) from the LED, which should have no significant effect on the LED brightness.
The relay (yellow trace) activates at a low LED current (green trace) as the LED starts to turn on.

Last edited:

#### Delta Prime

Jul 29, 2020
1,792
You can place the light dependent resistor in front of the light source as above adjusting sensitivity with potentiometer.

Far more sensitivity can be
acquired with the circuit below.

But I suspect you wish to go modular pre-made off the shelf.
I would search for light triggered 12 volt relay 15 amps only because you said 10+.

#### Alec_t

Jul 7, 2015
3,581
a 3 V LED
Are you sure there is actually 3V across that LED? It looks like a normal red LED, which would have about 2V across it (as Crutschow has recognised).

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