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!
Unfortunately, the instructions that come with the module are not very clear, but we’ve addressed that problem in this article.
The module is fully built, and is sized at 67 x 44 x 20mm. It has three on-board pushbuttons:
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:
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:
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:
To set P1 (lower voltage):
To set P2 (higher voltage):
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…