A Rudimentary Photo Tachometer For Use With An Oscilloscope

Tachometers have long been an interest of mine. Back in 2007 I made a photo tachometer of sorts from a reflective object sensor (ROS) and a few odds and ends. Here's a view of it.

And here's the schematic.

To use the device, a bit of reflective tape or white correction fluid must be applied to the rotating object whose speed is to be measured. The tachometer is connected to an oscilloscope, and the sensor is held near the rotating object. The sensor will output a pulse train whose period is the duration of one revolution of the rotating object. Finding the inverse of the period yields frequency in Hertz (cycles per second). Multiplying frequency by sixty yields rpm. Accuracy is determined by the oscilloscope's timebase accuracy, and the degree of accuracy with which pulse train period can be read from the oscilloscope's display.

The tachometer pictured above has seen better days; the cables have stiffened with age, and a couple of terminals have broken off the ROS. The ROS is irreparable, and it's the one and only Omron EE-SB5MC that I have.

I've ordered some RPR-220 photosensors from Amazon. When they arrive, I'll be able to reconstruct my photo tachometer and get it back to serviceablity. I'll revise the schematic then and update this post.

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RPR-220 Photosensors Received -- MONDAY, FEBRUARY 25, 2019

They arrived in today's mail. I tried one out on a breadboard, and it looks like it will work fine.

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Sensor Reconstructed -- WEDNESDAY, FEBRUARY 27, 2019

Here's a view of an RPR-220 photosensor in place of the Omron EE-SB5MC.

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It works. It's not as good as the Omron item was. The Omron sensor had excellent noise rejection, and put out a nice crisp signal. The RPR-220's output tends to be noisy -- it responds to every little slightly reflective irregularity in the rotating object being sensed. Sensor distance from the rotating object is important. I've found that a distance of 20 mm works reasonably well.

Here's a photo off my oscilloscope of a signal from my wood lathe's spindle running at a nominal 2,650 rpm.

The timebase is 5 mSec/cm. Observed period is about 22.5 mSec. That works out to about 2,667 rpm. Speed as measured with a proper digital photo-tachometer is 2,625 rpm. That's not bad accuracy -- within 2%.

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Here's the pin-out of the RPR-220 device.

And here's the revised schematic with the RPR-220 in place of the Omron ROS.

The Omron EE-SB5MC ROS is now obsolete. They can still be had here and there, but the price is steep. Using the RPR-220 photosensor yields a useable instrument at low cost, so I'm more-or-less satisfied with the outcome here.

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