# Frequency counter as gas engine tach?

Discussion in 'Electronic Basics' started by Chevy454, Jan 26, 2008.

1. ### Chevy454Guest

I have a Radio Shack frequency counter (#22-305), with two frequency
ranges and a digital pulse mode. Its input is a 50 ohm BNC connector.

Can someone suggest a suitable inductive pickup for this to wrap
around a spark plug wire that will provide a signal that is not too
strong?

My first impulse was to use a a few feet of RG-58 with a BNC at one
and and a foot of insulated wire on the other end soldered to the
center conductor, fitted with an alligator clip to clip to the
exposed cable shield after wrapping the spark plug wire a few times.

Ken C

2. ### Tom BiasiGuest

There are many ways to pick up the impulse from the spark plug. Your way
will grab some signal.
You will then need to condition the signal for your counter.
You will need a scope to look at what you are getting and shape it to the

Tom

3. ### JamieGuest

Use 1 loop of wire in a very small radius. Use a pair of diodes
like 1N914's to clamp the input at the 2 connections. Each diode
goes the opposite direction across the connection..
You may also want to use a 47 ohm resistor in series with the loop
feeding the center connections. the diodes should be clamped at the
center connection.
this will give you an average of 0.6 volt +/- pulse to the input of
the counter..

all you should need to do is just lay the loop next to the wire.

It's a crude method but I've done a coupling like that before to
detect RPM.

4. ### John O'FlahertyGuest

The frequency of an engine at highway speed, maybe 2500 rpm, is 41.66
Hz. The frequency of firing of a single cylinder is 1/2 of that
(four-cycle engine), or about 20.83 Hz. If you can pick up a signal
for all cylinders, as on the center of the distributor cap on an older
car, you would get 125 Hz for a six-cylinder engine.
According to some web references, the frequency counter you have has a
lower frequency limit of 1 MHz, so it probably won't work.
Even if your frequency counter could see it, for such low frequencies
a period measurement might be more practical, if you invert and scale
it for rpm.

5. ### Clint SharpGuest

Actually, it might not be. Depends on the ignition system in use, if
it's a Distributorless Ignition System it could be using wasted spark
where the coils are double ended and fire two cylinders (one firing, one
exhaust stroke) at a time, hence it fires twice per revolution. It could
also be multi-spark in which case you're screwed. Of course it might not
even use an ignition coil if it's a lawn-mower engine or similar!
On a DIS coil pack you can get a very usable signal from the cables back
to the ECU from the coil-pack, I've used 12 turns of roughly 26 SWG ECW
on a toroidal ferrite core using a couple of diodes and resistors to
clamp the signal as 'Jamie' describes and had excellent results.

The best results I've had were using a hall effect device (get one from
a dead DC brushless fan) on one of the injector bodies feeding a simple
comparator (LM393). Of course this will also work on the body of an
ignition coil if the car doesn't have fuel injection or you just can't
get at the injectors (single point injection)
Period measurement with averaging and smoothing (you have to reach a
compromise though if the engine can change speed quickly, think bike
engine) is the best way to go, even if the frequency counter can measure
that low the gate time will be a killer unless there's a PLL to lock in
(then you have problems with jitter because internal combustion engines
are rarely stable from one revolution to the next for a myriad of
reasons)

If the OP is just trying to measure RPM it doesn't have to be a spark
plug signal, there may well be a top dead centre signal from the
camshaft or crankshaft or at least a crank position sensor that outputs
some kind of pulse train (usually an inductive sensor and a pattern
formed into the flywheel with a missing pulse to indicate some period
before TDC)