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.
Main Features
Adjustable switching level between 10Hz and 500Hz input.
Dual double-pole changeover 5A relay contacts
Switches on rising or falling frequency
Adjustable hysteresis
Easy to build
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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 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.
