Need a simple circuit,idea to test speed of small motors,such as cassette,8 track,turntable motors.

[Doug]

I know of hall sensors,photo sensors.anyone have a circuit that work well?I have oscilloscope and pulse ,frequency counters.
Thanks
Doug

 

[gophert]

Since you don't know how many windings are in your brushed motor, a scope isn't great help so you'll have to add something to the shaft like a rotary encoder. Or even a piece of white tape with a wide black line on it (parallel with shaft. Then illuminate the tape with IR and use a photo resistor to see the voltage drop each time the black line passes.

 

[rjenkinsgb]

To be honest, you can buy ready-made optical tachos so cheaply it's not worth building one..
eg.
**broken link removed**

I've had a similar one for several years. Mine came with a roll of retroreflective tape as well, you just put a small tag of that on the thing you are measuring, if it's not got enough contrast with white paper or normal tape.

 

[gophert]

Point the IR LED at the shaft w/black line and point the base (lens) of a photo-Darlington transistor at the illuminated shaft. Measure frequency where the meter is attached.

 

[Doug]

I appreciate suggestions and ideas.
I hope the digital meter from china doesn't come with more covid
That circuit must be like the vcr sensors
Stay safe
Doug

 

[Nigel Goodwin]

That circuit must be like the vcr sensors

Showing your age!

But yes, some used reflective sensors, and some used slotted opto-couplers.

 

[throbscottle]

Make yourself a 60 segment optical disk and put photo-interrupter across it. Then you can use your frequency counter in Hz to read the speed directly in RPM from the photo-interrupter output (ie 1 rev per second = 60rpm)
(I did this to test a lot of small motors recently, worked very well)