As another experiment in accurately measuring my turntable speed I have been investigating these cheap and cheerful Laser Photo Tachometers.
These are available for less than a tenner on eBay, so I thought I would see how if fares for measuring the rotation speed of my Dual CS505 turntable.
The tachometer is intended for measuring the speed of rotating machinery. It works by counting the pulses of light reflected from a reflective label attached to a rotating shaft. I don’t want to go sticking things to my turntable, so I used a spare 45 RPM spindle adaptor and stuck a bit of reflective tape to that.
It worked, but not particularly well. Referring to the instructions confirmed that the tachometer is not great with slow speeds. It suggests adding more reflectors at regular intervals around the item to be measured. That way you get more pulses per revolution and the device is better able to work out an accurate speed. You do have to divide the result by the number of reflectors to get a true reading of revolutions per minute. With 6 reflectors I was able to get consistent readings of 200 rpm. which, divided by 6 is 33.333 rpm.
We can do better than this though. The Dual 505 has strobe rings around the platter. That’s equivalent to having loads of very accurately positioned reflective tapes. Using the 33 RPM at 50 Hz ring the spacing is such that one passes 100 times per second. That’s 6000 per minute. If the tachometer can be made to read off the strobe ring then, it should report 6000 RPM if the turntable is running at 33-1/3.
Problem is that the sensing beam is quite wide, about 10 mm. The strobe “bumps” are only a couple of mm across. Adding a simple lens to the front of the tachometer to focus the sensing beam worked a treat.
To prove the idea I just fired it through my Lupe, which has a focal length of about 10cm. Adjusting the distance I was able to get a sharp point of light on the strobe ring and the tachometer registered 5998 rpm. That within 0.3 % of 6000 rpm. Pretty much bang on.
Blu tacking my Lupe to the laser tachometer is not a great look, so I raided my junk box and found an old microscope. A suitable lens was extracted from the eyepiece assembly and fixed to the front of the tachometer to create a compact practical measurement device.
The microscope lense is a good physical fit. The focal length is quite short. Only a few cm, so you have to get up close and personal. Results are excellent though.
here you see it in use. I’m running within 0.3 % of the expected 6000 rpm. That’ll do me.