Introduction: Sound Card Tachometer

About: I ain't no physicist, but I knows what matters.

I designed this as a substitute for a non-contact laser tachometer.

It uses an IR emitter/detector pair mounted in a hand held wand.

The resulting signal is sent to the computer's microphone input and the waveform displayed with a sound card oscilloscope.

Step 1: The Circuit

The circuit is a simple voltage source for the emitter and detector.

Input voltage can be anywhere between 7 and 35vdc. (See the 78L05 datasheet.)
R1 limits the current to the emitter.
R2 & R3 are pull-up resistors.
T2 is added as an output driver.

R4 is used to limit the current and voltage to the sound card. Anything over one or two volts can damage the card. A 2.2 Megohm resistor gives an attenuation of around 25:1.

Schematic created using Eagle 6.4.0

Step 2: The Case

If you guessed that the case is (was) a wall transformer, then you are correct! I saved the insides for another project.

On the left is a 5.5mm power jack. On the right is a headphone jack to feed the sound card. On top is a three pin connector to attach the probe.

Making a tiny circuit board with connectors is a good example of overkill. I must have been really bored that day.

The circuit is so simple that perfboard and point to point wiring is a better choice.

Step 3: The Probe

On the right you see my source for the IR diode and photo transistor, an interrupt detector from a dead printer. For the probe, I configured the IR components to act as a reflective detector.

The white plastic housing is part of a computer power supply connector. The emitter/detector pair is mounted inside the housing and pressed into an aluminum tube.

Note: The IR will leak out the sides of the emitter, causing false readings. A barrier must be placed between the pair.

For the probe cable I used shielded headphone wire. The connectors I had in my parts box.

Wires run through the tube to connect the input to the IR pair.

Step 4: Hook It Up

I'm using an external USB sound card.

On your computer plug into the microphone input.

The easy way to hook up the sound card is with a male-male patch cord. A male-male adapter with a standard cord will also work. Pink ones will cost you extra.

Step 5: Let's Run It

For checkout I'm using a DC motor scavenged from a cordless drill.

A reflective piece of aluminum (from a soda can) is glued to a plastic disk and pressed onto the shaft.

The probe is sensitive enough that a good signal can be acquired by using black tape to cover half of the motor shaft.

This is a good technique for measuring the speed of installed motors.

Step 6: It Works!

The first image shows a frequency of 419.47 Hz. Multiply by sixty to get 25,168.2 rpm. How easy is that?
The waveform is actually a square wave, but the sound card's characteristics cause it to be distorted.

Note: Sound cards are analog devices. They do not do well with digital signals.

The second image shows a frequency of 67.601 Hz, or 4,056.06 rpm. At this frequency the waveform distortion is extreme.
Since we know that this is a square wave, use the leading edges to determine positive- and negative-going pulses.

My thanks to Christian Zeitnitz for offering his sound card oscilloscope for free. This is the best (and easiest) scope I've ever used. The controls and functions are similar to a standard bench top scope.
Download and install your copy:
Sound Card Oscilloscope V1.41 ©2012 C. Zeitnitz

Build this tool and keep it handy near the workbench. Using an accurate tachometer is a lot better than guessing the RPM by the sound a motor makes.


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