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Frequency Switch, Part 2

Using it to trigger a shift buzzer

by Julian Edgar

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At a glance...

  • Fitting the Frequency Switch to a car
  • Shift buzzer automatically triggered at high revs
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Last week in Frequency Switch, Part 1 we introduced the universal building block for switching things on the basis of speed or rpm. As we covered in that story, the Frequency Switch can have its input signal taken from the injectors, speedo, tacho or pretty well any other signal that’s pulsed with a variable frequqncy. This week we install the Frequency Switch in an EF Falcon, using it to trigger a shift buzzer that indicates when the engine is near the redline.

Picking the Signal

A shift-light or shift-buzzer can pick up its signal from a few different sources. One easy approach is to tap into one side of an injector – in cars that don’t change in the injector firing logic (and that’s by far the majority), the frequency of injector firing goes up in direct proportion with engine rpm. Taking a feed from an injector is easy because you don’t need to know the pin-outs of the ECU, nor struggle to find just the right wire in its connectors. However, the injector signal will probably go off on the over-run when you lift your right foot, so the light or buzzer won’t sound if you give it a big over-rev by making a too early down-change. For this reason, it’s better to take the signal from the tacho – either at the instrument panel or at the ECU output that feeds the tacho.

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In the case of the EF Falcon, the dashboard pin-outs are easy to obtain and show that on Pin 7 of the right-hand connector, a pink/dark blue wire, is the tacho signal input wire. To feed the Frequency Switch, this wire doesn’t need to be cut – just the insulation bared over a short section and a new wire soldered to it. The tacho will keep working fine, even though we’re also using its signal for an additional purpose.

Power and Earth

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Power and earth (or power and ground, if you prefer) also need to be fed to the Frequency Switch module. These connections can be picked-up from the same location as the signal (eg from the ECU or from the dash) however, in the case of the Falcon, it was easier to make these connections to the cigarette lighter wiring. Why was this easier? Well, primarily because the cigarette lighter has only two main wires going to it – one is ground and one is ignition-switched 12V!

Shift Buzzer

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In this application I decided to use a buzzer rather than a light. There are a few reasons why a buzzer may be preferable to a light. Firstly, the buzzer can be located out of sight so there’s not the drama of trying to integrate a light onto the dashboard. (Course, you might wanna have a huge shiftlight on the dash and that’s fine. But I didn’t want one.) Secondly, it can be a juggle to configure a light so it’s bright enough to be seen in full sunlight but not so bright it’s blinding at night. And finally, in some situations you can miss a shiftlight (for example, if your eyes are on the apex of a corner) but it’s hard to avoid hearing a loud buzzer.

However, if you have a sweet revving engine that’s often near the redline, a shift-light may be preferable to a buzzer – simply because you’ll be hearing it so often. The Falcon, on the other hand, isn’t an engine that in standard form likes to rev, so a buzzer’s fine....


Once the tacho signal wire had been tapped into, I used a multimeter to measure the frequency on the wire (positive probe to signal, negative to ground). This measurement showed that the Falcon has a tacho output signal of about 50Hz per 1000 rpm. (It may well be exactly this value but just an approximation is needed.) If I wanted the shift buzzer to trigger at 5000 rpm, the Frequency Switch would be seeing an input frequency of 250Hz (50Hz by five lots of 1000 rpm). This puts the requirement into the upper range and so the Frequency Switch was calibrated for its 50 – 500Hz range. (More on this was covered last week; it’s also in the module’s instructions.)

The Frequency Switch was then temporarily wired into place and the engine warmed. It was then revved hard and the switch-on rpm set by adjustment of pot VR1. (Note: if VR1 doesn’t give enough adjustment range, alter VR2 as well.) When the switch was clicking over at 5000 rpm (as shown by the on-board LED lighting and the relay clicking) it was time to add the buzzer.

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A loud 12V buzzer (a suitable variable volume one can be found in Projects, Kits & Components) was wired as shown here. The board was then put into a box, the buzzer held on one side with double-sided tape.

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The box and buzzer were then placed inside the dash below the vents; with the dashboard front panel back in place, the module and buzzer are completely hidden but the buzzer is still easily heard.


The Frequency Switch has lots of uses and because it runs an onboard relay, can directly drive devices like buzzers, lights or solenoids. It’s the answer that lots of people have been looking for!

Frequency Switch preassembled and tested

Frequency Switch DIY kit

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