Tachometers (Wiki) measure the rotational speed of something.
Using these to measure RPM on small DC PM Motors. There are two ways to get the RPM into the meter, one is a direct mechanical connection and the other is with a photocell. In order for the photo tachometer to work I have made flywheels where half the face is black and the other half white so one pulse per revolution.
It is also called an Ametek C-891 tachometer. This was on eBay for a good price maybe because the lamp in the sensing probe did not light up.
There are two batteries inside (remove the four bottom screws to access the batteries) and the Battery Test function only checks the 9V battery. To test the "D" cell you just look into the probe to see if there's light. Note the lamp in the probe is a 222 standard penlight lamp and it's being run from a 1.5 Volt battery instead of two batteries. This means it will last a very long time and that it will not be bright, but easy to see.
In my case the negative terminal of the battery holder was eaten away be a leaking battery. The meter passed the battery check and the florescent lamp calibration, but did not work on reflective targets. I added a negative terminal contact and now it's working fine.
Reads about 4,700 on the 10,000 RPM range with the test motor at 12.0 Volts. Need to check the calibration.
On the back there appears to be four small metal screws holding plastic feet. But in reality the plastic part is an 8-32 screw and the small metal heads do nothing. You need pliers with teeth to grab the plastic feet in order to remove them to access the batteries. I plan to replace these with some 8-32 plastic screws.
The probe uses a 222 flashlight lamp with built-in lens for illumination and by means of a 45 degree half silvered mirror the reflected light is sent to a photo transistor.
This is golden era construction that can be repaired.
The On/Off button failed. It is a SPDT switch, not the SPST
I was expecting. Why the second pole?
Guess: memory keep alive when not on????
A very small red plastic part jumped out of the swtich
never to be seen again.
Replaced the broken push button
with a SPDT switch. Yellow = common.
This is actually an improvement since you don't
need to hold down the prior button, like when
your hands are holding the motor.
This is a Made in China photo type digital tachometer. It has a built inRed diode source so you see the light on the flywheel.
Reads about 5,000 RPM on the test motor.
It was made for a left handed person. The button you need to press and hold down is on the right side.
This is a Made in China mechanical type digital tachometer. The body is made the same as the DT-2234A+ except it has a nose with a mechanical shaft. I expect that inside it's really a photo tachometer. The problem it that for small DC PM motors it loads down the motor and so does not give an accurate reading. (about 3600 RPM on the test motor).
It comes with three rubber tips and a wheel. The wheel has a known diameter so that you can get a feet per minute readout.
DC PM Motors
Testing DC Motors
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Page created on 29 June 2015