[TCML] LED Strobe Sync Idea

[<ConorPerry@xxxxxxxxxxx>]

I don't know if the attachment will come through, but maybe someone can link 
it somehow?

After reading the recent threads about syncing a rotor gap I thought that 
there must be a better way to do this than just trial and error.  The 
attached schematic I just made up and it seems to work, but your mileage my 
vary.

The idea is to use LEDs to flash at the peak of the AC waveform.  To do this 
I used a simple Opto-Isolator (4N37 but many types would probably work 
fine).  The 120VAC is dropped down through a 150k resistor, then through a 
blocking diode into the input of the 4N37.  A 5k potentiometer is across the 
4N37 input LED for something of a gain/sensitivity adjustment (an additional 
Variac on the input could do the same thing.)

On the output of the 4N37 are two NPN transistors, these are used to invert 
the signal and provide current gain for the LEDs (a single PNP could 
probably work as well.)  The LEDs I used was from a cheap LED flashlight. 
It already had the LEDs in series and was setup for 6V input, but since 
we're flashing the LEDs pretty quickly the 9V input shouldn't hurt anything. 
You'll want some bright LEDs because they're only ON for a moment at 60Hz.

I setup a standard 1725RPM 1/4HP motor with a pulley and made a single line 
with magic marker.  With the motor running and the 5k pot adjusted I could 
see two distinct lines on the pulley at 180 degrees apart.  Since this isn't 
a synchronous motor the marks slowly rotate at the slip factor.  So it took 
about 10 seconds for my marks to rotate through a revolution.  With your 
sync motors the marks should not move at all.  Then proper phasing of the 
disk to shaft can be done very quickly and safely since you don't need the 
gap actually running.

You could also add a bridge rectifier to the input and get 4 strobes per 
rotation.

Hope this is useful to someone, it was an interesting exercise for me.
CP8071 
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