Is there a way to build a simple tachometer without a microcontroller?

I would like to measure the speed of a motor that i have, but have nothing to do it with. If there is a simple kit (cheaply), then please point me to that. I don't want to have to program a microcontroller, though.

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orksecurity8 years ago
There's also the old mechanical technique: Have the device spin a gear. Hold a card or something of that sort against the gear's teeth to produce a tone (much like the card-in-the-bike-spokes trick). Figure out what musical note that comes closest to. Convert that to cycles-per-second -- there are tables which do this, or you can start with the fact that 440Hz is A below middle C and that going up or down a half-step involves multiplying or dividing by the twelfth root of 2. Divide that cycles-per-second by the number of teeth on the gear to get rotations per second.
qwertyboy (author)  orksecurity8 years ago
i might try that, because i only need a rough measurment
qwertyboy (author)  qwertyboy8 years ago
thanks! it worked. what i did, though is record the sound and then open it up in audacity, analyzed it, and did the rest of the math.
mhkabir8 years ago

You could use this simple way to caculate RPM.Go here

Make the changes to the calculator.Now, attach a reed switch to the two wires. Attach a small magnet, on a disk, on your motor.Place the reed switch over the magnet, keeping a bit of air gap between the magnet and the switch.Start the counter.Subtract 1 from the result of the counter in 1 minute. The answer is the RPM!!!.Reset the counter again to read other RPMs, this setup is easy, a bit xcumbersome, but works nevertheless and is a cheap solution.
wii552 mhkabir7 years ago
I believe that you could do that, or simply hook up the microphone to an oscilloscope to get the frequency, and multiply that by 60
Here is a circuit we use at work for a flowmeter - it takes pulses in one side, turns them into a display voltage across skt 2 pin 1 and 3 and compares it to a reference (for an alarm signal). If you ditch everything right of and including R13 It will work as a tachometer, but you'll have to calibrate it against a reference clock at some point. Steve
mathews8 years ago
You will first need a way of measuring rotations of the motor, either a magnetic switch or a light gate. This will then need to be fed into a frequency counter, easily made up of decade counters, and output by 7-seg displays. You will need to hold on the display while it is counting. You will need to reset the counter every second with a resettable monostable.
seandogue8 years ago
If you have access to a frequency counter or oscilloscope and a few other parts, yes.

An optical interupter like this

or others not shown here can be used to monitor the rpm of a motor using a thin white stripe applied to the axle. The sensor is mounted near the axle, pointed at the axle so that it's transmitter shines on the stripe, and when the white line rotates by, it's receiver (a phototransistor or similar) turns on, allowing current to flow thru the load resistor, and so picks up the signal, (the load voltage across the load resistor) shown as a pulse on the oscope...a frequency counter does much the same, but it show more directly how many times per second the pulses occur without calculating from the scope's display and time settings.

(note that there is usually a maximum functional distance that the sensor can be placed from the object to achieve a good signal. Spec sheet usually covers that, but ime, hands on usually does a better job of fine tuning for one's application.)

A magnetic sensor can also be used for the same purpose. a fixed magnet is applied to the axle and as it passes by a flux sensor, a pulse is sent to the scope of frequency counter. This is how it was done in ":the old days", before optical sensors became cheap and easy to use.

it's then up to you to convert the output to rpm, where

RPM = F(the frequency of pulses per second)/60

The clever inventor can often get single quantity sensors by calling the manufacturer and asking for a sample.

I could have sworn there were TTL output devices, but i didn't see them in my quick look at digikey after reading your query.

the circuit for running one of the reflective sensors is pretty easy too. usually only needs a couple of resistors (one for limiting the supply for the transmitter diode, one for loading the output of the phototransistor, and a supply of the correct voltage for running the transmitter and driving the phototransistor so you can see the output. Usually the sensor has a spec sheet that shows a standard hookup...that's the simple circuit

also note that most modern DMMs, or Digital MultiMeters, have a frequency counter built in...Even most cheap ones.

best of luck..