Can you help me identify an electronic component (from a printer)?

July 16, 2009 6:07 PM

long ago, i disassembled a printer and inside found a component that was clearly a revolution counter of some sort. it consisted of a disc with radial slots and a "caliper" with leads attached.

for the life of me, i can't figure out what to call this thing or how to describe it to google. what was it?

(for the curious, i'd like to modify my anemometer with this, to make it more precise; the one-per-revolution counter sucks on light-wind days and i'm a big nerd).

posted by klanawa to Technology (14 answers total)

Best answer: It's probably a rotary encoder, used for motor control feedback in lots of applications.
posted by dhalgren at 6:16 PM on July 16, 2009

Best answer: I don't know what its called either, but it sounds like exactly the same thing inside just about every mouse wheel. There's a disk with holes and a little U shaped mount around it with an IR emitter on one side and a detector on the other. Should be simple to build or repurpose a mouse.
posted by Ookseer at 6:18 PM on July 16, 2009

dhalgren is right, it's a rotary encoder.
posted by fake at 6:33 PM on July 16, 2009

Response by poster: i knew it was something simple! a tachometer is a counter, basically. don't know why i didn't make the connection.

here's the exact device, from dhalgren's link: http://en.wikipedia.org/wiki/File:Basic-Rotary.jpg

thanks!
posted by klanawa at 6:34 PM on July 16, 2009

Even digital bathroom scales have these. Calibrate the number of windows that go by the LED sensor, and you have weight.
posted by weapons-grade pandemonium at 6:53 PM on July 16, 2009

Here's a nice write ups on how rotary encoders work: http://www.robotroom.com/Counter4.html
http://www.robotroom.com/Maze-Solving-Robot-All-Right5.html
posted by skwm at 7:02 PM on July 16, 2009

Sorry, that should be:

Here are two nice write ups on how rotary encoders work:
http://www.robotroom.com/Counter4.html
http://www.robotroom.com/Maze-Solving-Robot-All-Right5.html
posted by skwm at 7:02 PM on July 16, 2009

An encoder actually has two sensors arranged 90 degrees out of phase. The signal generated is called quadrature. With this you have direction and position information. It is technically incorrect to call this device a tachometer; a tachometer gives you velocity information, not position.
posted by fritley at 7:06 PM on July 16, 2009 [1 favorite]

If there are two sensors in quadrature, you can even tell which direction the wheel is turning.
posted by flabdablet at 7:09 PM on July 16, 2009

snap!
posted by flabdablet at 7:09 PM on July 16, 2009

Usually they produce 2-bit Gray code.
posted by Chocolate Pickle at 7:12 PM on July 16, 2009

I remember being very impressed at the simple way the original Macintosh mouse decoded quadrature signals from its rotary sensors. One of the quadrature outputs was treated as a clock input, and the other as data. Sampling the data signal on the rising edge of every clock signal caused the data to read as 1 if the mouse was going one way or 0 if it was going the other.

This method effectively cuts the encoder's spatial resolution by a factor of 4, but it's really really simple and means that the original Mac mouse needed very little in the way of internal electronics.
posted by flabdablet at 7:21 PM on July 16, 2009

Response by poster: An encoder actually has two sensors arranged 90 degrees out of phase. The signal generated is called quadrature. With this you have direction and position information. It is technically incorrect to call this device a tachometer; a tachometer gives you velocity information, not position.

true. the component i was trying to find originally wasn't one of these. it had a single slotted disk and a single sensor, so it could only have been a counter. dhalgren's answer led me down the (correct) rabbit hole, but for my application, the direction of rotation is irrelevant.
posted by klanawa at 8:52 PM on July 16, 2009

Not all encoders use quadrature, though it is very common.

Quadrature and 2-bit Gray code are actually identical.
posted by hattifattener at 11:03 PM on July 16, 2009