VOM to measure speed?

[dll932]

Just got a new VOM that has freq. and duty cycle functions. I wondered-any way to use/adapt it to work as a tach? Just a thought.

[Dan Baldwin]

Sure. You'll need to make a photo transistor circuit to have something to feed to your VOM. Set the VOM to frequency, and multiply the reading in HZ times 30 to get RPM if you use a 2 bladed prop. Multiply times 20 if you use a 3 bladed prop, or 15 for a 4 bladed prop.

Dan

[dll932]

Sure. You'll need to make a photo transistor circuit to have something to feed to your VOM. Set the VOM to frequency, and multiply the reading in HZ times 30 to get RPM if you use a 2 bladed prop. Multiply times 20 if you use a 3 bladed prop, or 15 for a 4 bladed prop.

Dan
Cool-any ideas where I can find a circuit diagram for that?

Just had a thought...I believe it measures up to 400hz-not sure what the sample rate is, either.

[Dan Baldwin]

I was afraid you were going to ask that. I don't have any photo transistors with me or I would try to work something up for you. I did find a couple of circuits that should work for you. I haven't built either one of them, so I can't tell you how well they will work.

Dan

[slipstick]

The last one I built consisted of an NPN phototransistor, a resistor (10K I think) and a PP3 battery.

+9V to resistor to collector of transistor, emitter to ground, take the feed to the meter from collector. It worked pretty well.

Steve

[dll932]

Sounds good guys! Still wondering how I can read 2 to 3000 rpm is max freq on the meter is 400hz, and what effect meter sampling rate might have.

[Cirip]

dll932,
Don't worry.
400Hz=400s^-1=60*400rpm=24000rpm
Conversely, 3000rpm=3000/60 Hz=50Hz.

Cirip

[slipstick]

400Hz is actually 24000 pulses per minute = 12000 rpm for a 2 blade prop (2 pulses per revolution), 8000 rpm for a 3-blade etc.

BTW the frequency reading DMM I have doesn't actually "sample", it basically just counts pulses.

Steve

[dll932]

That's why I come here, smart people!

Thanks!

[Dan Baldwin]

My Radio Shack DVM that has a frequency counter function that reads down to .1 HZ. If it was actually counting pulses, it would have to count for 10 seconds, but it updates several times a second. I assume that it actually reads the period of a cycle(maybe of several cycles) and calculates the frequency.

Yes, 400 hz max would give you a max of 12,000 RPM with a 2 bladed prop.

Dan

[dll932]

My Radio Shack DVM that has a frequency counter function that reads down to .1 HZ. If it was actually counting pulses, it would have to count for 10 seconds, but it updates several times a second. I assume that it actually reads the period of a cycle(maybe of several cycles) and calculates the frequency.

Yes, 400 hz max would give you a max of 12,000 RPM with a 2 bladed prop.

Dan
I can live with that. :) Now I just have to figure how to hang it together.