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The eLabtronics Voltage Switch, Part 1

Useful in literally hundreds of car modifications

by Julian Edgar

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At a glance...

  • Fully built, cheap electronic module
  • Let's you control:
  • radiator cooling fans...
  • nitrous...
  • intercooler water spray...
  • low oil pressure warning alarms...
  • low fuel warning alarms...
  • ...and a whole lot more!
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The eLabtronics Voltage Switch is a brilliant device - no matter what the car or its modifications.

And why is it brilliant? Because it allows you to trigger devices using any car sensor that outputs a voltage. Those sensors include airflow meters, throttle position sensors, fuel level senders, temperature senders (both for the dashboard gauge and ECU), oil pressure senders and a whole lot more.

Don’t see any uses?

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

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So you can trigger radiator cooling fans from the existing ECU or dashboard temp sensor, turn on an intercooler water spray at high load, switch on an intercooler fan when the engine load is very low (ie the engine is idling), sound a low oil pressure warning if the oil pressure drops, and so on.

Straight away, you can forget using add-on pressure switches, throttle position micro-switches or temp switches. No more trying to fit a sensor that was never designed to be there – you just make use of the existing factory sensor! And the original function of the sensor is unchanged – connecting the module won’t change the way the original system works.

And the eLabtronics Voltage Switch has additional, in-built functions.

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Take the situation where you want an audible and visual low fuel warning. You can configure the module so that when the fuel level (or water level in a water injection tank, or....) falls to the required level, a LED comes on and a buzzer sounds two warning beeps and then goes quiet. The buzzer alerts you and the LED keeps on reminding you...

In other applications you can configure the output to pulse on and off (eg to flash a light) or to hold steady (eg to run a radiator cooling fan).

A heavy duty output transistor (called a MOSFET) is fitted so that loads of up to 10 amps continuous can be directly driven. That means that with a suitable heatsink fitted to the transistor, horns, sirens, high power lights, pumps and other current-hungry devices can be directly driven without a relay.

For even bigger loads, just add a conventional or solid state relay (covered later in this story).

And it gets even better. The switch-on level is set by a multi-turn pot, allowing very fine adjustment. Furthermore, the hysteresis (the difference between the turn-on and turn-off levels) can be adjusted so you’ll never have a problem with on/off chattering.

By just altering a switch position, the eLabtronics Voltage Switch can be set to trigger when the monitored signal level is falling, or when the level is rising.

Finally, by adding a few low-cost components, you can have a standalone temperature or light switch. We’ll cover this aspect in Part 2.

The module is available fully built and tested for just AUD$59 – see eLabtronics Voltage Switch - Prebuilt.

Using the eLabtronics Voltage Switch

That might all sound pretty complicated, so let’s take a look at an application, exploring the unit as we go. That application is automatically turning on an intercooler fan at low loads (eg at idle).

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The eLabtronics Voltage Switch has just four wiring connections. These are:

  • Power - marked on the board as ‘+’. Power is nominally 12V and you’d usually use an ignition-switched source.

  • Ground – marked as ‘-‘. You’d normally connect this to chassis earth or the negative terminal of battery.

  • Input – marked as ‘in’.

  • Output – marked as ‘out’.

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When the Voltage Switch output MOSFET is turned on, battery power is available at the output terminal. So all you need to do is to wire your load (lights, buzzers, horns, solenoid, fans, pumps, etc) between the output terminal and chassis ground. If the load has a polarity, the positive terminal goes to the Voltage Switch. (Note that as with all MOSFETs, there is a slight voltage drop across it, so at high loads, a little less than full battery voltage will be available at the output at high loads.)

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So if we wanted to wire up an intercooler fan so that it came on at idle (stopping ‘cooler heat-soak), we’d do as shown in this circuit diagram.

The fan is wired between the Output and ground, and the Input is connected to the airflow meter signal. (Even without workshop manual it’s easy to find the airflow meter signal – just back-probe with a multimeter until you find a wire with a voltage signal on it that varies with engine load – free-revving the engine will help.) Note that the airflow meter signal won’t be loaded down when the Voltage Switch is connected.

So that’s the wiring done! Next, we’ll take a look at the switch options.

Switch Options

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The eLabtronics Voltage Switch has a four-position DIP option switch. Position the board so that the terminal strip is on the right and then the following switch positions give the listed behaviour. Note that the position of the last switch doesn’t alter.

  1. Output Switches on as Voltage RISES ABOVE Set-point





Switches on as voltage rises above set-point, then on-board LED illuminates and output stays fully on. LED and output switch off when input voltage falls below set-point. This mode will be one of the most often used.





Switches on as voltage rises above set-point, then on-board LED illuminates and output constantly pulses. LED and output switch off when input voltage falls below set-point.





Switches on as voltage rises above set-point, then on-board LED illuminates and output gives two pulses. LED and output switch off when input voltage falls below set-point.

In addition, there is another that holds the output constantly on, irrespective of the input signal. This mode can be used in testing eg to check that the load has been wired correctly.





Output constantly on (useful during set up only)

  1. Output Switches on as Voltage FALLS BELOW Set-point





Switches on as voltage falls below set-point, then on-board LED illuminates and output constantly on. LED and output switch off when input voltage rises above set-point.





Switches on as voltage falls below set-point, then on-board LED illuminates and output constantly pulsed. LED and output switch off when input voltage rises above set-point.





Switches on as voltage falls below set-point, then on-board LED illuminates and output gives two pulses. LED and output switch off when input voltage rises above set-point.





Again there is another test mode; this one holds the output constantly off. This is useful during set-up only.

In the box above we’ve said that the output switches off when the voltage moves back past the set-point. In fact, the amount of hysteresis that’s been set will determine exactly when the output turns off. More on this in a minute.

Back to the Intercooler Fan Example

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OK, so back to our example application, where we want an intercooler fan to be automatically turned on at idle.

Since we want the intercooler fan to come on when the engine load is low, and the airflow output signal falls as load is reduced, we need configure the Voltage Switch to turn on when the voltage falls below the set-point. Next, we don’t want the fan to pulse on and off; we just want it to run continuously while the airflow meter signal is low.

To achieve that outcome, we set the switches like this:





Remember, the switches are viewed with the board orientated so that the terminal strip is on the right.


By this stage you’d have wired the unit in place, putting it in a box (Performance Modules Box - Bulkhead - Black) so that the bare underside tracks can’t touch anything.

The next step is to set the point at which the intercooler fan starts.

There are two adjustment pots on the board. These pots are multi-turn so don’t expect to make only one rotation when setting them. Multi-turn pots also don’t have clear end-stops (although they can sometimes be heard clicking when they’ve reached the end of their adjustment).

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In this pic, the pot nearest the bottom adjusts the trip-point. Rotating this pot clockwise increases the input voltage level at which the switch activates its output.

Because the voltage trip point range is from 0.1 to 50V and the airflow meter output at idle is likely to be around 1V, the pot will need to be rotated anti-clockwise a fair amount. In fact, the easiest way to set the system up is to have the car idling and then rotate the pot anti-clockwise until the intercooler fan just comes on. The red LED on the module will also light to show that the output has been activated.

The other pot sets the difference between the switch-on and switch off values. (This is called hysteresis.) Rotating the hysteresis pot clockwise increases the hysteresis.

Being able to set hysteresis is very valuable – especially in an application like we’re describing. With the hysteresis set to a low value (pot a long way anti-clockwise), the intercooler fan will switch off as soon as engine load rises even a small amount. But in an intercooler fan application, it makes sense to have a high hysteresis – that way, the fan will keep on aiding outside airflow through the intercooler until you’re well under way and the airflow meter voltage has risen much higher than its idle value. So in this application you’d set the hysteresis fairly high by turning the pot clockwise.

Triggering an intercooler fan from the airflow meter output is just one example of the module’s use – in a moment we’ll cover some more.

See Performance Electronics, Part 1 for more on set-points and hysteresis. They’re both extremely important in getting set-up correct.

Potential Uses

The Voltage Switch has literally hundreds of uses. Here’s a selection:

Signal 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

Coolant temp sensor

Over-heating alert

Oil pressure sender

Low oil pressure alarm

Coolant temperature sensor – gauge or ECU

Trigger radiator fans

Intake temperature sensor

Engine management modification trigger

Over temp alarm

Intercooler water spray


Active four wheel drive modification trigger


Low battery voltage warning or shutdown

MAP sensor

Intercooler water spray trigger

Intercooler fan trigger

Engine management modification trigger

Boost light

Output Power

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The output MOSFET (transistor) on the Voltage Switch is rated to handle a continuous 10 amps – but that’s when it is fitted with a big heatsink. How hot the MOSFET (and the circuit board) get depends not only on the output current but also whether or not the output is being pulsed or held continuously on.

For short pulses, the heatsinked MOSFET will handle up to 15 amps.

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As a rule of thumb, no heat sink at all will be needed if you’re operating warning lights or LEDs – even high powered ones.

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If you are switching on a string of low power filament lamps, a small heatsink will be needed.

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If you’re turning on a pump or small fan, a medium sized heatsink will usually be needed.

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Finally, if you’re switching loads like multiple car horns or multiple headlights, a large heatsink will be needed.

The heatsink needs to be isolated from ground and positive supplies, so either mount it so it fits inside a box (and can’t touch anything metallic!) or mount the heatsink to the MOSFET using an insulating spacer and nylon nut and bolt. In either case a smear of heatsink compound will be needed between the MOSFET and the heatsink.

Don’t forget that in most uses of the Voltage Switch, no heatsink – or only a small heatsink – will be needed.

Ultra High Currents

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But what if you want to operate really big electrical loads – like multiple radiator fans, high-powered sirens or the like? There’s no problem – you’ll just need to buy a solid state DC relay. These relays are fully electronic, so have no moving parts.

In addition to being very durable, an electronic relay can switch very large currents. When equipped with a suitable heatsink, the relay shown here can handle 100 amps continuously and cope with a very short term switch-on current gulp of 240 amps.

When using an external sold state relay, the Voltage Switch doesn’t need to use a heatsink, so packaging becomes easier – the Voltage Switch can easily fit into a box and the solid state relay can mount remotely.

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This diagram shows how the relay is wired to the module. The electronic relay is available from the AutoSpeed shop for AUD$40 – see Solid State Relay.

Note: if the load is not going to be turned on and off a lot, a conventional relay can be used.


The eLabtronics Voltage Switch is a stunningly useful module. You can switch devices on and off by monitoring engine management sensor output voltages, you can monitor gauge sensors, and you can configure the output to be a short double warning pulse, to be continuous pulsing, or to be continuously ‘on’. The very fine control over the set-point and the widely adjustable hysteresis add enormous versatility to the module’s uses.

Simple to wire into place and set up, very effective, and well priced!

Next week: using standalone temp and light sensors to automatically trigger the Voltage Switch

Voltage Switch Specifications

Operating Power: 10 – 40 V DC

Output power: up to 10 amps continuous with appropriate heatsink, up to 15 amps short pulsed with appropriate heatsink, up to 100 amps with appropriately heatsinked external solid state relay

Wiring connections: power, ground, input, output

Sensing voltage range: 0.1 – 50V

Sensing direction: selectable for rising or falling

Hysteresis adjustment range: 1 – 100 per cent

Output Options: double pulse, continuous pulse, fully on. On board LED lights when Voltage Switch is tripped.

Fuse: 15 amps

The Voltage Switch is available fully built and tested from the AutoSpeed Shop - eLabtronics Voltage Switch - Prebuilt

The eLabtronics modules are engineered and manufactured by eLabtronics. The modules are based on concepts and specifications developed by Julian Edgar, with the aim being to provide cost-effective and useful modules for car modification (and also industrial and educational uses).

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