View Full Version : Digital Tachometer?

10-17-2011, 12:12 AM
Once again I find myself challenged by electronics questions. Perhaps someone here can put some light on this for me.

The question is, can a Digital Frequency Counter like this one:

be used with either a magnetic or optical sensor to count revolutions of a motor or spindle? In other words, can the above unit be purchased, a power supply and sensor be attached so you end up with a tachometer or is this a gross oversimplification?

10-17-2011, 12:19 AM
That doesn't appear to be scalable or need other electronics to make it usefull?
I picked up this digital tach up on Amazon for $16.00.
Same as this ebay one #190587561380
Amazon http://www.amazon.com/Digital-Photo-Laser-Tachometer-Contact/dp/B001N4QY66/ref=sr_1_1?ie=UTF8&qid=1318825432&sr=8-1
Check out the feedback.

10-17-2011, 12:19 AM
The simple answer is that you can. You just need a circuit that creates a pulse that the meter can detect. It looks like it will count pulses of up to 16 volts.


10-17-2011, 12:22 AM
Sure, it will work. It doesn't say what the gate time is but it is most likely one second. Regardless, all you need is a periodic input that is related to the rotation of the item being measured. A reed switch has a pretty low maximum switching frequency in the tens of closures per second at best so something like an optical sensor is a much better bet. Depending on the gate time and the number of pulses generated per revolution you will need to do some simple math to arrive at the RPM.

10-17-2011, 12:35 AM
It will work, but you will have to scale the pulsed time 60 to get a meaningful reading. That will give you a reading with 60 rpm resolution or so.

10-17-2011, 01:09 AM
Like THIS (http://littlemachineshop.com/products/product_view.php?ProductID=2658&category=)?

10-17-2011, 01:48 AM
It lists:

Input Waveform: Regular Periodic Waves

Kind of vague, but it implies a sine wave would be the required input. A hall effect sensor or optical sensor would be a pulse, not a wave, I'm not sure if it would work.

I was in need of a tach, I bought the Harbor Freight non contact digital tach, I put a reflective sticker on my lathe and mill, one tachometer for both.

J Tiers
10-17-2011, 09:02 AM
It may need "zero crossings" to work..... in which case capacitor coupling a magnetic inductive pickup will work OK.

Even pulses can be made into a "through zero" wave by capacitor coupling. But for a switch such as Evan mentions, you probably need a resistor to "plus" as a "pull-up" to make pulses if you are going to capacitor-couple them.

Some analog units need that too, ones based on the national Semi Frequency to Voltage chip do, I have used that unit.

10-17-2011, 09:16 AM
Bicycle speedometers are a common low cost substitute for tachometers on mini lathes and should work fine for most any shop machine. Some bicycle speedometers include a rev counter mode, others can be used by setting the wheel diameter to an appropriate value.

Here's an example:

It's worthwhile to poke around on Mike's site, lots of innovative ideas there.


10-17-2011, 02:27 PM
I used this:


and this:


as a tach for my variable speed lathe. The mag pickup reads the teeth on a gear, I needed the amplified pickup to read the lower speeds when I'm in backgear. It works very well.

10-17-2011, 02:43 PM
Get a gear tooth timing sensor from a local auto wrecker, Hall effect no magnet needed.
Looks like this by Honeywell.

10-17-2011, 03:55 PM
The device operates on plus 5vdc only so the input is ground referenced. It doesn't need a zero crossing. The input will be a schmidt trigger to allow for slowly/continuously varying waveforms.

I use a frequency counter when I need an accurate tach although my counter is a little bit higher quality with twin crystal oven timebases calibrated using the local triple atomic clock stack at the nearby Loran C station. :D

10-17-2011, 06:49 PM
I have one these and it works quite fine for as cheap as they are.


10-17-2011, 06:52 PM
I have one these and it works quite fine for as cheap as they are.

That the one in post #2.