LED indicators on stepper driver outputs

[xkadet]

Hi, all, I have an electronic question about something I saw on a demo stepper driver kit years ago. This standalone driver had a set of 4 LEDs connected across the output to the stepper motor, I think. These LEDs would oscillate in a kind of a pulsing pattern as the stepper drove the motor. My question has to do with duplicating this display on my Probotix based 3 axis CNC that I've designed and built from scratch (nothing in the electronics wierd or non-standard). I'm using the Probotix stepper boards in a typical 1/2 or 1/4 stp config. I'd like to have these displays on my 3 axes and I need a working schematic and resistor spec to keep me out of trouble. My LEDs are 3.5V bright white 5000Mcd. I don't want to drive them hard, I just need enough current to light them obviosly under room light. Why you ask? Well, I use my DIY CNC for automation demonstrations in Jr. High classrooms from time to time and it would be great to have these displays on all of my 3 axis to further grab and hold the attention of the kids. Can anyone help me with a schematic? I hate to start blind here and possibly fry something. A starting point is all I need. Thanks in advance for any and all help.

keith

[mike_Kilroy]

i'll bite i cant scan something here at home so will describe and see if it is sufficient?

from radio shack or digikey or newark etc.... one complete kit for each output; either 2 or 4 would/shud be sufficient.... full wave bridge rectifier rated 100ma, 100v to resistor to led each. thats it.

fw bridge ac input terminals across each coil. dc +&- out of it to LED in series with the resistor. resistor size depends on ur stepper dc power supply voltage (Vsuppy-2v))/.03a for ohms rating. shud be nice for the kids to see.

this would get u started. u may need to add another resistor and zener diode to the LED side of fw bridge to make led consistent at all speeds...... but that can wait until after u get initial circuit up and running. if no lite first time thru, reverse led polarity.

[Al_The_Man]

I think the problem you may have is that anything above just a very low rpm, the LED's are going to blur together, due to the speed and the persistence of a LED.
I put together a simple phase rotation detector and pole counter circuit for BLDC/AC servos and I can turn this by hand to get the read out, If I just spin the motor even slightly, it is impossible to distinguish between pulses, and this is only for 1 to 4 pulses for one revolution of the motor for a 2pole/8pole motor span.
Al.