Simple Frequency Counter

[BA]

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

 

[EROMLIGNOD]

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

 

[A E]

Robert Monsen wrote:

Buy an H11AA1 optoisolator, which can be driven by the 120v source. Use a
15k resistor in series with its internal LED emitter diodes, which will

That's a 1W dissipation. Gotta be careful there, gotta get a big resistor, and
it will get hot.

limit the current to about 10ma. The opto detector drives the base of an NPN
transistor. You can use a 5v power supply and a 1k resistor between the
transistor collector and 5V, and tie the emitter to ground. That gives you a
nice 5V swing at the transistor emitter. Also, its isolated, so you don't
need to worry about electrocuting yourself by touching the 5vdc electronics.

But burns from the 15K might be a problem.

Note that the output will be double the frequency, so you'll need to divide
it in half.

Regards,
Bob Monsen

 

[Robert Monsen]

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

"A E" <aeisenhut@videotron.ca> wrote in message
news:3F2E897F.62A6380@videotron.ca...

Robert Monsen wrote:

Buy an H11AA1 optoisolator, which can be driven by the 120v source. Use
a
15k resistor in series with its internal LED emitter diodes, which will

That's a 1W dissipation. Gotta be careful there, gotta get a big resistor,
and
it will get hot.


limit the current to about 10ma. The opto detector drives the base of an
NPN
transistor. You can use a 5v power supply and a 1k resistor between the
transistor collector and 5V, and tie the emitter to ground. That gives
you a
nice 5V swing at the transistor emitter. Also, its isolated, so you
don't
need to worry about electrocuting yourself by touching the 5vdc
electronics.

But burns from the 15K might be a problem.


Note that the output will be double the frequency, so you'll need to
divide
it in half.

Regards,
Bob Monsen

 

"BA" <ba109@home.com> wrote:

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

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?