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Digital programmable voltage switch

Subtitle: Brilliantly versatile and cheap

by Julian Edgar

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At a glance...

  • Operate fans, warning lights and other devices
  • Uses existing voltage outputting sensors
  • Cheap and well built
  • Fully programmable with digital voltage display
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Over the years we’ve covered many voltage switches – devices that are incredibly useful in modified cars. But we’ve never covered one like this – a fully digital, programmable voltage switch that not only has different selectable modes of operation, but even has an on-board display of the monitored voltage.

And best of all, it is cheap, cheap, cheap – under AUD$14, delivered to your letter box!

You can find it on eBay by doing a search under “Voltage Tester Monitor Charge / Discharge Over / Under Voltage Overload Protect”.

So why would you want a voltage switch in a modified car?


A voltage switch allows you to trigger outcomes from any voltage outputting sensor on the car. That includes throttle position sensors, most airflow meters, most oxygen sensors, fuel level senders, intake air and coolant temperature senders, oil pressure senders and others. No more trying to fit a sensor that was never designed to be there – you just make use of the existing factory sensor!

So whatever variable is already being sensed (whether that’s engine load, engine temperature, oil pressure, fuel level, etc) can now be used to additionally switch something on and off.

So you can trigger radiator cooling fans (use the coolant temp sensor), turn on intercooler water sprays at high intake air temps (use intake air temp sensor), switch on intercooler fans at high engine loads (use throttle position sensor), sound a low oil pressure alarm if the oil pressure drops (use factory oil pressure sensor), and so on.

You don’t need to disconnect the ECU or dash - just tap into the signal with the voltage switch!

Loading-down sensors?

The maximum current drawn from the sensor by the module is only 50 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.

Unfortunately, the instructions that come with the module are not very clear, but we’ve addressed that problem in this article.

The module

The module is fully built, and is sized at 67 x 44 x 20mm. It has three on-board pushbuttons:

  • Set

  • Sw1

  • (+)

  • (-)

It has a 3-digit LED display that shows the monitored voltage, and two on-board LEDs - one red (power on) and one blue (relay tripped). The relay is a single pole, double throw unit rated at 10 amps at 30V DC. The electronics on the board are surface mounted.

At one end of the board is a 4-terminal screw-type connector. The connections are:


positive supply (battery 12V)


negative supply (chassis or ground)


the positive of the monitored voltage (eg to sensor output signal)


the negative of the monitored voltage (chassis or ground)

At the other end is a three terminal strip for the relay – C (common), NO (normally open) and NC (normally closed). When the relay closes, the C and NO terminals are connected.


The functionality of the module is programmable. There are five different functions available – F1 through to F5.

Also programmable are two voltage levels – P1 and P2.

The functions are shown below:




Voltage display only. No relay operation.


Relay on when voltage below P1. Does not go off until voltage rises above P2.


Relay on when voltage above P2. Does not go off until voltage falls below P1.


Relay on when voltage between P1 and P2. Off at other voltages.


Relay off when voltage between P1 and P2. On at other voltages.

So what are the implications of these functions? Here are some examples:

If you wanted an intercooler water spray to come on at high throttle angles, you’d select Function 3 and use the output of the throttle position sensor.

If you wanted a light to come on when the engine was either cold or hot (i.e. is outside of normal operating range) you’d use the output of the coolant temp sensor and Function 5.

Programming the board

To select Function:

  1. Long press Set until P-0 shows

  2. Short press Set

  3. Use + and – buttons to select correct Function (F-1 to F-5)

  4. Long press Set until voltage display resumes

To set P1 (lower voltage):

  1. Long press Set until P-0 shows

  2. Press Sw1 until P-1 shows

  3. Short press Set

  4. Short press Sw1 until the correct digit flashes

  5. Use (+) and (-) keys to change displayed digit

  6. Repeat steps 4 and 5 until the correct voltage has been set

  7. Long press Set until voltage display resumes

To set P2 (higher voltage):

  1. Long press Set until P-0 shows

  2. Press Sw1 multiple times until P-2 shows

  3. Short press Set

  4. Short press Sw1 until the correct digit flashes

  5. Use (+) and (-) keys to change displayed digit

  6. Repeat steps 4 and 5 until the correct voltage has been set

  7. Long press Set until voltage display resumes

Two other functions

  • Calibration

P3 (accessed in the same way as P1 and P2) allows the voltage display to be calibrated. Double presses of the (+) and (-) keys are needed to change tenths of a volt.

  • Display on/off

Short press of Set switches off LED. Short press of Set turns it back on.


Click for larger image

Here is an example of the switch wired to light a dashboard warning light. If you are operating high currents (e.g. a radiator fan), you’d need to use an additional relay as the current draw of the fan is too high for the on-board relay. However, the on-board relay is fine for powering warning lights, a buzzer, intercooler water spray pump, etc.


The minimum hysteresis (i.e. the difference between switch on and switch off voltages) is 0.2V – that’s the case even with P1 and P2 set to the same number. This hysteresis prevents the relay from chattering. Of course, by setting P1 and P2 to voltages more than 0.2V apart, you can set the hysteresis to be much greater than 0.2V.

This adjustable hysteresis is useful in many real-world applications. For example, if you turn on an intercooler water spray when the engine load is high, you don’t want it going off as soon as engine load drops just a bit. Instead, you’d want it to turn off when engine load drops quite a lot. By setting a high hysteresis and using Function 3, you’d achieve this.


It’s only a few years ago that a module like this would have been $50 or more – if in fact you could even get it. This is a bargain – and one that is very useful in a whole range of car modification. In fact, it’s probably worth buying a few just to put on the shelf…

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