Tachometer mod

[oldnwrinkly]

I don't know if this is of any interest but .................

 

I have a cheap hand-held optical (non-contact) tachometer which I thought might be modified to work as a induction tacho to use for tuning saws.

In it's original state the unit has a low power laser which illuminates a piece of reflective tape that is applied to a shaft or flywheel etc. and a photo-diode which receives the reflected light pulses. My plan was to disable the photo-diode and replace it with a small induction coil which, when held close to the spark plug/HT lead (or the flywheel magnet) would pick up pulses and these would be de-coded by the circuitry to give a read out of RPM.

I tested the principle by resting a coil on the engine cover and measuring the output on a DMM set to read frequency. I also compared the accuracy of the readings by fixing a magnet to the chuck on my minilathe, which has a tacho, and after converting the frequency reading to RPM (x60) the readings were identical.

So, today I knocked up a rough and ready, very simple circuit, soldered it into the tachometer and tested it against the lathe taco. It matched the readings exactly up to 1,000 RPM - I didn't take it any higher as although I had added a few bands of tape over the magnet, the thought of what could happen should the magnet let go is scary.

I am fairly confident that it will be just as linear at higher revs as although the unit is a cheap one the chipset looks reasonable - the microcontroller is an 8051 type (Atmel AT89S52). As soon as I have made a proper job of adding the bits and pieces into the case I will try to test it against a commercially available 'saw' tacho.

 

If anyone is interested the add-on bits are - a small, fine wire, coil wound on a thin ferrite core, a 'snubber' diode wired directly across the coil and a series diode, for polarity purposes, on one end of the coil. The positive lead of the photodiode was snipped and the cathode ( ->l----) side of the series diode connected to the circuit board end of it. A short length of wire was added to the other side of the coil and this was soldered to the negative lead of the photo diode. The polarity of the photo-diode leads are marked on the circuit board.

The laser will still light up but this will not matter.

I intend to add a switch so the unit can be used in either the optical or induction mode.

I bought the tachometer on evilbay for very little money - it is identical to this one < LCD Digital Laser Photo Tachometer Non Contact RPM Tach Last Max Min Value Top | eBay >

 

 

Dave.

[blazer]

I haven't read all the post but I had an old tacho at work which used a piece of reflective sticky tape - only problem, the tape flew off when the shaft got up to speed:thumbdown:

 

buy a tiny tach - brill:thumbup:

[Ricbob]

Sorry to drag up an old thread but i have been thinking along similar lines. I have pretty much the same cheapo optical tach and think it would make a nice little project to turn it into a inductive tach.

what gauge wire and how many turns did you use? if i understand it right, the number of turns will alter the induced voltage across it?