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The Simple Voltage Switch

An indispensable building block

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At a glance...

  • Operate radiator cooling fans, nitrous, intercooler water sprays, low oil pressure warning alarms, low fuel warning alarms, closed loop monotors - you can do nearly anything with this module!
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The newly published book High Performance Electronics for Cars contains a number of ‘building block’ projects. The electronic kits, which are all available from the AutoSpeed shop or Jaycar Electronics, include a Frequency Switch, Temperature Switch and the one covered here – a Voltage Switch.

The Simple Voltage Switch (SVS) is a brilliant device because it 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, temperature senders (both for the dashboard gauge and ECU), oil pressure senders and others.

Don’t see any uses?

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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. So you can trigger radiator cooling fans from the ECU or dashboard temp sensor, turn on intercooler water sprays at high loads, switch on intercooler fans when the engine load is very low (ie the engine is idling), sound a low oil pressure alarm 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!

The SVS uses an on-board double pole, double throw (DPDT) relay so you can operate most devices directly from the module. The use of the DPDT relay also allows you to operate two completely separate circuits, even letting you turn off one device as the other turns on! And it gets even better. The switch-on voltage 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.

Whew! You can see why – especially at a kit price of AUD$29.95 – we think that this is a damn good thing!

The Background Story

So how did this kit come about?

The Simple Voltage Switch was developed and designed by Silicon Chip ( electronics magazine. The kit, along with many others, is covered in the Silicon Chip publication, High Performance Electronics for Cars. The book is a must-have for DIY modifiers. The kit for the Simple Voltage Switch is available from Jaycar Electronics ( ) or through the AutoSpeed shop.

The electronics design and development of the Simple Voltage Switch was carried out by electronics engineer John Clarke, while I came up with the concept and did all the on-car development. (During this period I wore a different hat to an AutoSpeed contributor, working for Silicon Chip Publications as a freelance contributor to High Performance Electronics for Cars.)

So by no means should the Simple Voltage Switch be seen as an AutoSpeed-developed project, but at the same time I am happy that AutoSpeed endorses it and promotes it.

Julian Edgar

Making the Kit

The kit uses a screen-printed Printed Circuit Board (PCB) which shows which components go where. This makes it easy to construct – that is, if you have experience with making electronic kits. If you’re a beginner or don’t particularly want to learn how to assemble kits, the SVS is available completely built and tested from the AutoSpeed shop. However, you’ll pay quite a lot more so keep that in mind when making your constructional decision.

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Most of the construction is quite easy – the only tricky bits are the diodes which require a magnifying glass to read their part numbers. Of course, you must get the polarity right on the diodes, electrolytic capacitors, the IC and the transistor. Also don’t forget when soldering the connecting terminals in place to have their openings facing out from the centre of the board – otherwise, getting wires into them will be tricky...

The kit can be used to sense voltages that rise to the trip-point (called low/high, as in "low voltage to high voltage switching") or the opposite. In most applications you’ll want low-to-high switching - for example, when the airflow meter voltage rises to a certain level, an intercooler water spray switches on. However, when sensing the output of a temperature sensor, it’s likely you’ll want a high-to-low setting, as the voltage output of these sensors usually falls with rising temp.

So it’s best to use a multimeter to measure the output of the sensor in question so you’ll know which way to configure the kit.

Testing the Kit

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Always test the kit on the bench before installing it in a car. The relevant chapter in the book (a B&W photocopy is included with the kit) shows how to use a 10K pot as a variable voltage source, allowing you to test the operation of the SVS. Make sure that the when the setpoint is exceeded, the relay clicks over and the LED comes on. Try adjusting the hysteresis (the single turn pot) and checking that you can vary how much the signal input voltage has to drop before the switch clicks off.

The SVS can be adjusted to operate when the input signal voltage is anywhere in the range from 0-16V. In addition, the device uses a high input impedance so it won’t load sensors down – not even oxygen sensors. This means the sensor can keep on performing its original function and at the same time also work with the SVS.

Kit Uses

So what are some uses? They’re almost unlimited but here’s a selection:

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


Low battery voltage warning

MAP sensor

Intercooler water spray trigger

Intercooler fan trigger

Engine management modification trigger

Boost light

We’ll be using the SVS in lots of projects, so expect to see a lot more of this versatile module!

Contact: AutoSpeed Shop

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