# Simple Frequency Counter

Discussion in 'Electronic Basics' started by BA, Aug 3, 2003.

1. ### BAGuest

Hello all
I was hoping someone would be kind enough to help me out. I am trying to
get the frequency out of an AC induction motor drive so I can use it to
input into a micro controller. What I am wanting is a simple circuit to
change the ac 120 volts sign wave to dc 5 volts square wave. I keep getting
a little lost when it comes to the ac part. I have tried a voltage divider
to lower the voltage and then a diode to half rectify it. Then using a
zener to limit it to 5 volts and aproximate a sqare wave. Dosnt quite work
and is no where near square. Any help would be greatly appriciated here.

Thank you

2. ### EROMLIGNODGuest

Use a transformer (or two) to bring it down to 5 Vac, and feed this signal to a
Schmitt trigger (like a 74LS14) to square it.

Don

3. ### A EGuest

That's a 1W dissipation. Gotta be careful there, gotta get a big resistor, and
it will get hot.
But burns from the 15K might be a problem.

4. ### Robert MonsenGuest

AE, thanks for pointing this out.

A 15k resistor is going to dissipate about 120 * (120/15k) = 960mW, so I'll
caution the OP again to watch out for burns.

Also, mains voltage is deadly, so work with one hand in your lap. (You don't
want the current to pass through your heart.)

OTOH, I've been running the original circuit with a 10k 7W resistor for
about 15 minutes now, and the resistor is still cool enough to touch (with
one hand . If you use one of those it'll be fine.

Regards,
Bob Monsen

5. ### GuestGuest

Hi BA

One thing to consider, AC drives don't put out a clean sine wave.
Some use pulse width modulation or crudly approximate a sine wave using
a stepped waveform.
Combining either of these with an inductive load results in ringing.
If you want to measure motor frequency you will have to include some aggresive

low pass filtering in your design to filter out the ringing.
In my experience with AC drives there is usually a DC current or voltage
aux output for external monitoring of output frequency.
Is this an option for you?