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.
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
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!
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
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.
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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