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.
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
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 –
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
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
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
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 (www.siliconchip.com.au)
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 www.jaycar.com.au
or through the AutoSpeed
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