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Incredibly cheap voltage switch

Switch on and off devices using the standard car sensors

by Julian Edgar

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Over the years we have covered a number of different voltage switches. But this one is different – it’s incredibly cheap and doesn’t need to be built from a kit. 

But first, of what use is a voltage switch is car modification?

The answer is simple: any voltage outputting sensor in a car can be used to activate this switch. 

So let’s say you want to switch on an additional electric radiator fan. You could add a new sensor (many people use a new sensor that fits under a radiator hose, often causing leaks) or you could simply use the output of the existing ECU coolant temp sensor to additionally trigger the voltage switch and so the fan. You can set the temp that the fan comes on at just by adjusting the pot on the switch module. 

Because the voltage switch draws only a tiny amount of current, it won’t change the normal function of the coolant temp sensor. 

But the radiator fan switching is just one example. What if you want to trigger something that switches on above a certain throttle angle – for example, turn on an intercooler water spray or fan at high throttle angles? In that case, just tap into the throttle position sensor output!

You can turn on a low fuel warning light in older cars (just monitor the fuel gauge sender) or turn on something when lateral acceleration exceeds a pre-set level (monitor the torque control system accelerometer in an active all-wheel drive car).

Here are some of the other potential uses:

Triggers from... 

To achieve... 

Throttle position sensor

Nitrous activation

Auto trans kickdown

Airflow meter

Intercooler water spray trigger

Intercooler fan trigger

Engine management modification trigger

Turbo wastegate anti-creep control

Fuel level sender

Low fuel warning light

Oil pressure sender

Low oil pressure alarm

Oxygen sensor

Engine management modification trigger

Closed loop indication

Coolant temperature sensor

Trigger radiator fans

Intake temperature sensor

Engine management modification trigger


Active four wheel drive modification trigger

MAP sensor

Intercooler water spray trigger

Intercooler fan trigger

Engine management modification trigger

Boost light

So how cheap is this thing? Through eBay, at the time of writing, it will cost you AUD$8.66 delivered to your letter box! Available from a number of eBay sellers, the device is called “DC 12V Dual Wire Actuation Type Photoswitch Sensor Relay Module”. 

The module

As indicated in the above description, the device is sold as a photo-switch – a device that switches a relay on the basis of different light levels. The module comes with a light sensor that can be easily remote mounted. In fact, it’s a perfect module for switching on lights when it gets dark. 

But hold on, what’s this got to do with a voltage switch suitable for car use?

The trick is this: if you unplug the light sensor and instead feed a variable voltage to one of the exposed pins, the module then becomes a universal voltage switch, suitable for monitoring voltages in the range of about 0.5 - 5 volts. 

So to change the functionality from being a light sensing module to a voltage sensing module, you simply unplug the light sensor – no difficult wiring changes are needed!


We suggest that you first test the module as a light switch – just to make sure everything is working as it should. 

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Here is the module as it comes from the supplier. 

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Remove the link (arrowed) and place it in a secure place – it’s easy to lose it.

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Connect +12V and ground wires as shown here. It is easiest to solder these wires to the pins, but if you don’t have a soldering iron, you may be able to make a small connector. Switch on power and the red LED should light, indicating that the module is powered-up.

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Shade the light sensor and the green LED (arrowed) should light and the relay click. Adjust the pot and you will be able to adjust the light level at which the relay operates. 

Voltage switch

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Carefully unplug the light sensor. Cut away part of the connector housing for the light sensor so that you can solder directly to the pins. Solder your input signal wire to the pin furthest from the power connections, as shown here by the arrowed brown wire. This is the wire that connects to the output of the sensor. 

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Rotating the pot anti-clockwise increases the voltage at which the module switches. When the pot is adjusted correctly for the application, a dab of nail varnish can be used to hold it in the chosen position.

Loading-down sensors?

The maximum current drawn from the sensor by the module is only 150 micro-amps – this is very, very small. We’d expect that with such a low current draw, you could monitor any voltage-outputting sensor on the car without changing its output or affecting its operation.

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The relay terminals are configured (from, top to bottom): Normally Open, Common, Normally Closed. If the link is placed as arrowed (that’s the link you put aside earlier), 12V is fed to the Common relay terminal. This makes wiring much simpler when you want to operate a low current load like a buzzer or lamp. 

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Here is how the module is configured to turn on a light when the monitored voltage exceeds the pre-set level. 

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Here is how the module is configured to turn on a light when the monitored voltage is below the pre-set level. (Note how the green LED is now off.)


If the input voltage alters only very slowly, you’ll find that the relay will chatter at the switch-on point. That is, the hysteresis (difference between switch-on and switch-off values) isn’t sufficient large. 

If you are simply monitoring the output of the sensor (e.g. to run a dash LED) just unsolder the relay and remote mount the green LED. For example, if you want to monitor the output of a narrow band oxygen sensor, remove the relay and mount the LED on the dash. (This would allow you to see if the system is in closed loop [LED flashing] , open loop [LED on] or over-run fuel cut-off [LED off] ). In this situation the low hysteresis doesn’t matter.

But if you are monitoring a sensor whose voltage changes only slowly, you can prevent relay chatter by soldering a capacitor across the relay coil terminals. These terminals are easily accessed under the board. 

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Here a 100uF 63V capacitor has been soldered in place to prevent relay chatter. Note that the negative terminal of the capacitor (indicated by a line of negative symbols on its body) connects to the solder pad closest to the edge of the board. If the relay still chatters too much, use a higher value capacitor. 

If you are driving a really high current load, like a radiator fan, use the on-board relay to drive a heavy duty automotive relay. 

If you are confident with soldering, you can easily replace the on-board pot with a multi-turn external one of the same value, allowing you to adjust the switch-on voltage even more finely. 


This is a great building block – useful in the simplest modification to the most complex. By using the existing car sensors, the job becomes easier, neater and cheaper! Under for under ten bucks, well…

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