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firbikrhd1
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:
http://www.ebay.com/itm/9999Hz-LED-Digital-Frequency-Hertz-Panel-Meter-Counter-/330549316696?pt=AU_B_I_Electrical_Test_Equipment&hash=item4cf6464c58

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?

MaxHeadRoom
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.
Max.

danlb
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.

Dan

Evan
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.

macona
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.

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

Iraiam
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.

GadgetBuilder
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:
http://mikesworkshop.weebly.com/low-cost-tachometer.html

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

John

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

http://www.ebay.com/itm/Red-Lion-Controls-DT3A-Digital-Speed-Indicator-UOS-/200643249143?pt=LH_DefaultDomain_0&hash=item2eb74503f7

and this:

http://www.ebay.com/itm/Red-Lion-Controls-LMPC0000-Logic-Magnetic-Pickup-/330576793262?pt=LH_DefaultDomain_0&hash=item4cf7e98eae

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.

MaxHeadRoom
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.
http://www.kosmodrom.com.ua/pdf/1GT101DC.pdf
Max.

Evan
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

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

http://s.dealextreme.com/search/tachometer.html?category=405

MaxHeadRoom
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.
Max.