Tachometer

Tested a Radio Shack (Archer) cat no. 49-201 Infrared Photoelectric Relay - Intrusion detection unit (out of date, originally sold for $69.95). Attached the Radio shack Infrared Photoelectric Relay with a .1 uf capacitor on the output to filter the 60/120-cycle nose that was showing up on the frequency meter. The result was workable up to about 11 Hz before the circuit stopped switching. This is equivalent to about 660 RPM. Does work but is hard to get it to line up and reflect off the tape on the rotating shaft. This is because of the invisible infrared beam. Not really that portable or easy to use. Currently runs off 115V AC but with some modification could run off 12V DC. I didn't think it was worth going any further with this.

Next Attempt: Tried an old hand held cassette tape recorder that had a good audio amp with a mic and speaker jack. A small solar cell was attached to a microphone coax cable and plugged into the microphone jack on the recorder. A 200-ohm variable resistor was attached across the solar cell. Hand held laser pointer to reflect off the aluminum tape on a rotating shaft. The output of the speaker wire went to the DM 645 frequency meter and an oscilloscope for testing. The pot was adjusted to allow the frequency meter to select different amplitudes in its calculation. All AC lights (florescent and incandescent) near by needed to be turned off to cut down on the 60/120 cycle picked up by the solar cell. By adjusting the pot sometimes one can get the correct frequency. Most of the time the frequency measured was way too high. The frequency response of the audio amplifier in this typical small tape recorder was not high enough to work. Was taking the pulse and changing it into a decay sign wave. Which was to be expected. This introduced lots of higher frequencies. Thus this approach is not recommended.

The best approach so far has been to use a solar cell attached to an oscilloscope with a laser pointer bouncing off aluminum tape stuck to the shaft. The timing is then done by counting centimeters on the scope screen and multiplying by the sweep rate to get fraction of a second between pulses, or for one revolution. 60 sec/min is then divided by the fraction of a sec or number of sec for one revolution. This then gives RPM.

After much research: The most reasonably priced off the shelf unit that I can find that will measure low RPM is Photo/contact Tachometer ($155.00 part no. 01DT2236) from Electronix Express (800) 972 2225. This unit measures between 5 to 100,000 RPM using the photo approach and .5 to 19,999 RPM using contact. Note: The frequency meter used above was a DM 645 ($38.95 part no. 01DM645) from the same place. Being able to measure to one Hz is really not enough. 5 Hz measured is 300 RPM, 1 Hz is 60 RPM. As you can see this is not very good accuracy at low RPM. Thus all in all I think it better to purchase the above unit.

My next action is to purchase one of these, for portable use unless I have missed something easy to try. I don't think I want to spend the time designing or finding a circuit for a solar cell that will amplify the square wave pulse keeping the original shape rejecting 120/60 Hz noise.

Offered by Mike.