There are many automotive performance applications where you want to turn something on or off on the basis of road or engine speed. A shift light is a good example – you want a high intensity LED to illuminate just before the red-line, to warn you that it’s time to snatch the next gear. Or maybe you want a variable-length intake manifold to change from long to short runners at non-standard revs. Or perhaps you want to operate a device on the basis of road speed – eg, switch on an intercooler fan when the car is moving only slowly or sound an over-speed warning when you’re going too fast, for example.

This Frequency Switch can do all of those things – and more.

It also has adjustable hysteresis (that’s the difference between the switch-on and switch-off frequencies) and comes with both a LED and a relay. The relay is there so that you can switch big loads, while the LED can be mounted on the dash so that you can see the switch operation. Alternatively, the LED could be used purely as an indicator – eg, as an over-speed warning or as a shift light.

Construction

The Frequency Switch is a simple kit to build but you should make one decision before you start work. Will you be using it to detect a frequency that is rising to the trip point or falling to the trip point?

The Frequency Switch can be used to trigger a shift light - an indication that engine revs are getting close to the red-line and it's time to change up a gear.

The unit can be configured to work with either type of signal but if you know which way you’re going, you won’t have to make changes later on. The detection of a rising frequency will be the more common application – for example, triggering a shift-light when engine revs reach a high speed.

However, if you want something switched as frequency decreases to a certain level – for example turning on an intercooler fan when the car is travelling slowly – then you’d configure the Frequency Switch to detect a falling frequency.

So what are the changes made for the differing configurations? They’re simple: for a rising frequency (low to high) detection, link LK1 is placed in the (L/H) position (that is, closest to the top of the PC board when the board is orientated as shown in Fig.1). For detection of a falling frequency (high to low), link LK1 is moved to the H/L position.