Small engine tach from DMM

[6PTsocket]

I was going to buy a small engine tach. All the engines are either  1 cyl, 2cycle or wasted spark 4 cycle. That is one pulse per revolution. I set my DMM to frequency and just grounding the negative lead to the engine and stray pickup to the positive lead (antenna) was enough to get pretty stable readings. The problem is I am reading pulses per second, not per minute. It is not practical to keep dividing by 60 in my head. I see there are any number of divide by N chips but I have no familiarity with them. Also the divider might not be as sensitive to stray pickup as the DMM so maybe I would need some amplification on the front end. Any suggesrions appreciated. Thanks.

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[xani]

It is not practical to keep dividing by 60 in my head.

I think you've meant multiplying ? Dividing digitally is easy, multiplying is a bit of work.

Easiest option: frequency counter with RPM mode. Or a "car" multimeter with similar option.

Cheapest option: An Arduino (or any other tiny micro), any display and a bit of circuitry (could probably be done with schmitt trigger and few diodes)

Loopiest option: Get acquainted with PLLs and design x60 multiplier. Or cheat, and program a micro to measure a frequency and then generate one that's x60 of that

Wonkiest option: Go to aliexpress and look for chinese tachometers

[John B]

I built a tacho for my own car using arduino. The standard arduino library isn't suitable for measuring high frequency, use these instead: https://www.pjrc.com/teensy/td_libs_FreqCount.html

Using a micro gives you more options for smoothing the signal and types of output. You might want to drive an LED display, in my case I had to drive an air core tachometer so it involved RPM -> PWM -> 2-6V DC level

[6PTsocket]

Yeah multiply. My bad. It seems it may not pay to try and use the DMM frequency feature even though it is picking up so well.

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[Brumby]

It seems it may not pay to try and use the DMM frequency feature even though it is picking up so well.

It's cheap (zero extra cost) and it works.  It's also a single piece of kit - which is less to carry and less to go wrong.

Keep using it and you'll soon be able to multiply a whole range of values by 60 in your head.

The idea isn't new - there have been scale doubler ranges on some analogue meters in days gone by.  Sure, dividing by 2 is easier, but it is a mathematical process nonetheless.

[JS]

A phone app... no, not a calculator to multiply by 60, but to use as a tachometer with the microphone, there are a few and they work reasonable, you configure the typo of motor, #cylinders, strokes, it does the rest.

JS