Picture of Sound Card Tachometer

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.

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Step 1: The Circuit

Picture of 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

Picture of 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

Picture of 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.

thank you for introducing me to this software! i just tested reading a pwm from an arduino, works great!