# Measuring negative-switching coils for a tachometer

I'm somewhat of a noob when it comes to more advanced electrical systems, so excuse my ignorance.
I'm trying to build a tachometer for my motorcycle, a 1982 Yamaha Virago 920 (V-twin, two ignition coils). On one cylinder, two fires happen per engine revolution (wasted spark ignition). To trigger the spark, the TCI switches (disconnects) ground on the appropriate cylinder's ignition coil, then reconnects it.

I'm sure there are terms I'm missing that's making this really difficult for me to research, but how would I check the frequency of these fires? What kind of complications might I run into when doing so?

Any help is appreciated!

Sounds pretty simple.

A coil operates by running current through the secondary building up magnetic energy in the core. Then this current is interrupted the magnetic energy will search for a place to go by creating a large voltage on both the secondary and primary.

The secondary is connected to the ignition switch
The primary is connect to the spark plug.

When the current in the secondary is interrupted:

1. the Secondary goes about 300V negative for a short time.
2. the primary goes to 40k volts, resulting in a spark.

Since the pickup triggered every time the piston is a top dead center, there will be one pulse per revolution.

You will have to search the internet of an already tested circuit to pickup the coil firing. Perhaps a Yamaha forum. Whatever circuit you fine, you must be careful to put almost no electrical load on the coil secondary, else you will adversely effect the sparkplug voltage.

Old cars used contacts for the secondary circuit, these contacts were called "points". You might try searching for some combination of tach, points, automotive, coil, RPM

Sorry I couldn't provide any circuit help, there are too many variables to just take a stab at what would work.

If that the case it can be detected by a contactless nearby Hall effect sensor?
I used it to sense the revolution of the "generators" an other kind of "tachometer' to measure slow revolution from a few RPM and up.
You may not need to sense both coils, 1 then time 2?

What is a "secondary" and "primary" in the coil?
Also, since it's a 4 stroke, the piston is at top dead center twice per revolution. While it's not necessary, a second spark occurs when the piston is at TDC on the exhaust stroke as well as TDC on the compression stroke.

I thought it would be something somewhat simple, the reason I say that is because I've seen (and used) several cheap generic unbranded moto gauge units (like \$30) that accurately measure RPM just from the "signal" wire of the coil between the TCI and the coil.

The primary of the coil is the high voltage windings
The secondary of the coil is the coil you connect to 12V

It is somewhat simple. Only a few resistors, capacitors and maybe a transistor. In production quantities these components likely cost 10 to 15 cents.

The cylinder is at TDC one time on each revolution of the crankshaft. So one count = one revolution. So 10,000 RPM = 167 Hz. Or one pulse every 6 milliseconds.

I was hoping you would find a suitable circuit to sense the negative pulse. Here is a circuit I drew off the top of my head. I can't guarantee it will work but I'm sure it will be close.

I would look at the wiring for your bike , there may already be connections for a tacho in place

There is, it's the signal wire for the ignition coil. Unless you mean a dedicated wire for tachs specifically, then not really.

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