Motor speed measurement

If you’re just here for the answer to the question “how fast does the Ptouch motor need to run”, the answer is 3000RPM. Ish.

I’ve confirmed this using two different measuring methods — measuring the timing between noise spikes on the motor’s power lines, and the old reflective-tape-and-laser trick. I didn’t use the motor inside the Ptouch; I actually found a motor in a defunct CD-ROM drive that was the exact same type as the one in the Ptouch. It’s a Mabuchi RF-300C-11440, FWIW. [Rapid Electronics]( stock them as “solar motors” (part number [37-0440](, and there’s a datasheet online too. Oh well, at least spares won’t be a problem 🙂

The spike timing trick is quite nifty, and involves using an oscilloscope to show the voltage across the motor. Then you measure the time between three spikes, halve the frequency (in Hz) and multiply by 60 to get the speed in RPM. It works quite nicely, for the most part. Unless the motor doubles up a spike or misses one altogether. As usual, I’m not the first guy to think of doing this — a certain Roman Black [beat me to it]( I never seem to have any original ideas…

To get a more reliable speed reading, I grabbed a laser pointer and a phototransistor out of my junk box. Then I promptly blew up the laser — seems they rely on the current limiting of the batteries — and the spare was broken (looks like the metal shell had worked loose, breaking the +VE connection). So I bought a 5V “idiot resistant” (that’ll be me then) 1mW laser from Maplins for a tenner and bounced a little spot of coherent red light off a piece of tinfoil I glued to the output pulley. That bit of tinfoil (shiny side up) bounced light into the phototransistor once per revolution. Once again, multiply the frequency (Hz) by 60 to get the speed in RPM.

Once again, I got a figure of 3000RPM. Well, 3030 actually, but what’s 30RPM between friends?

Next plan: make the laser tacho permanent. I’ve got some 5mW lasers on order (which should hopefully turn up in a few days) which are earmarked for other, more interesting projects (and 5mW is a bit much for this sort of thing anyway). This time I’ve bought five, so failed diodes shouldn’t be as much of a problem in future!

Well, that and they’ve got a built-in driver circuit. Only a single-transistor constant-current driver, mind, not like the photodiode-regulated three-transistor monster on the Maplin module.