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Frequency Switch, Part 1

by Julian Edgar

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At a glance...

  • DIY frequency switch kit
  • Control shiftlight, manifold length change, intercooler fan
  • Fully adjustable
  • Available pre-built
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Barely a day goes by when someone on a web discussion group doesn’t lament the absence of an adjustable, cheap frequency switch. Well, they don’t put it quite that way but instead ask for an ‘rpm switch’ or a ‘speed switch’. The Silicon Chip Frequency Switch is the answer to their prayers. It’s a simple to put together electronic kit that monitors speed and clicks over a relay when the speed reaches a preset point.

The relay can be used to operate a dashboard speed alarm, to turn on an rpm shiftlight, to switch the length of a changeover inlet manifold, or to control an active spoiler. Not only is the click-over point finely adjustable, but the difference between switch-on and switch-off values can also be set. The Frequency Switch can get its input signal from the injectors, ignition, tacho, speed sensor – you name it! It is an incredibly useful building block.

The kit is available for DIY assembly for just AUD$35.95 or pre-built and tested for AUD$108.

The Kit

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While reasonably straightforward to make, the Frequency Switch kit isn’t one for an absolute beginner. As with all electronic kits, if you get a diode in the wrong way around, or the Integrated Circuit has its pins bent when being inserted into its socket, or you put a resistor into the wrong spot - or lots of other things like that - it’s likely that the kit won’t work... and you won’t ever be able to make it work!

However, if you’re handy with a soldering iron, can recognize what the different electronic components look like, and are a careful worker, there shouldn’t be any problems. The printed circuit board (PCB) has an overlay on it showing where the components go and during the construction this can be used in addition to the provided instructions.

The kit can be constructed so that it switches as the frequency (ie speed) rises to the trip point, or as the frequency falls to the trip-point. In most cases you’ll want the click-over to occur as the frequency rises to the trip point (eg for a shiftlight or changeover manifold) but in some applications this will not be the case. For example, if you want a certain thing to happen when road speed drops to idle (eg an intercooler fan to come on) you’d configure the Frequency Switch to click-over as the frequency falls to the trip-point.

Click for larger image

The different configurations are achieved by the placing of a wire link in one position or the other, and by the orientation of a diode. The Frequency Switch shown here is set for the more common low-to-high click-over configuration. That is, the bottom wire link is in the upper position and the diode band is across to the right.

The Frequency Switch also needs to be configured for the frequency range over which it will be working. It can be set to two ranges: 10 – 100Hz or 50 – 500Hz. So what does this mean? ‘Hz’ stands for Hertz and means how many times per second the signal changes.

For example, if the engine has a sensor that outputs one pulse per revolution, at 5000 rpm it will be outputting a frequency of 83Hz (5000 revs per minute divided by 60 = 83 per second). So in this case you’d use the 10 – 100Hz range. But what if you’re using a signal that outputs 5 pulses per engine revolution? In that case, at 5000 rpm the output signal will be 417Hz, so you’d use the 50 – 500Hz range.

The best way of working out what range to use is to measure the signal with a frequency measuring multimeter. (They’re now really cheap and make things a lot easier in this sort of work.) However, you can do it by trial and error – you won’t break anything if you get it wrong... the frequency switch just won’t click over.

To set the frequency range, connect power and ground to the Frequency Switch. Then connect a multimeter between Test Point 1 (TP 1 – you may need to put a stake in the PCB hole) and ground. Adjust VR2 (the multi-turn pot) until you gain a reading of 1.5V (lower frequency range) or 6V (upper frequency range).

Connecting the Relay

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The relay allows the operation of heavy duty loads – up to 5 amps. If you want to trigger a light, buzzer or solenoid, the Frequency Switch is wired-up as shown here. Power and earth are connected to the module, along with the signal input from the injectors, speedo input signal, or whatever. Twelve volts is also fed to the Normally Open connection of the relay and the adjacent Common is fed to one side of the light or solenoid. The other side is connected to ground. This way, when the Frequency Switch clicks over, the light will come on (or solenoid will be switched, etc).


The Frequency Switch is adjustable for hysteresis, that is, the difference between switch-on and switch-off values. This is set by turning VR3. Normally you’d set this so that there’s no chattering of the relay at switch-on and switch-off.

Next week we’ll fit the Frequency Switch to a car to control a shift buzzer. (No, not a shift-light but a buzzer!)

Frequency Switch preassembled and tested - AutoSpeed Shop

Frequency Switch DIY kit -AutoSpeed Shop

The Background Story

So how did this kit come about? It’s worth briefly backgrounding its genesis, if only so that you know what has gone into it.

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

The electronics design of the Frequency 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 Performance Electronics for Cars.)

So by no means should the Frequency 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

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