Stepper motor traverse mechanism on the cheap
Stepper motors are useful little gizmos, but controlling them is usually a pain in the butt, and all the supporting electronics are too expensive for a DIY savlage-junkie on a shoestring budget, so after pricing out the motor+driver+controller+psu at $12+$15+$25+$7, I decided to go the salvage route:
First step: I pulled the stepper, drive gear, and sled out of an old CD drive with the optics gutted.
I also pulled out the momentary switches
Second step: Salvage a relay. I could not find a latching relay, so I will have to rig up a solid state latch, but I found a decent one on a piece of telecom gear in my bin of salvaged PC boards
Steppers are supposed to be run from pulsed DC, but you can run them from AC with the pulse width, duty cycle, and frequency being set from the mains (60Hz around here). Which is fine if you don’t need a lot of traverse speed or to vary the speed electronically. I will likely rig up a timer and control board, but for now, I am running the whole thing from a 7.5v/1A AC wall wart transformer, salvaged from an old Netgear router.
The two stepper coils can be run in series, using the junction between the two as a center-tap and a capacitor providing the phase shift necessary to drive the motor. The relay is used to switch the direction of the traverse by changing which AC ‘leg’ the capacitor is connected to. The capacitor value doesn’t seem to matter much, but TOO large a value will cause the direction switch to be slower because of the time-constant of the discharge. Anything between 20uF to 1000uF worked:
Since the relay does not latch, I rigged up a transistor latch. Many versions can be found online. Here is what I used:
The resistor values are not crucial, but if I toast any transistors in the process of adjusting it, I’ll let you know. I used 22k for the inputs and feedback, and 4.7k for the the v+ resistor and on the base of the pop (but I used 10k and 100k for everything in the initial test, so whatevzzz). The diode I used is just a regular switching diode, but a power rectifier diode or schottky/fred/whatever will work as well. I used 2n4401/4403 transistors because I have a ton on hand… they are great little units in that they are cheap, they use the Euro pinout with the base on the center leg, they have decent voltage and current handling, low-ish but decent gain, and are (surprisingly) very low noise in audio applications (which is irrelevant here but whatever, but I actually have a DIY coffee-can guitar amp behind me using a 4401 in the driver and a 4401/4403 pair as a low-wattage power amp stage.)
Here is a pic of the completed board (in all of it’s hideousness):
The salvaged momentary switches will be mounted to an adjustable arm, with the sled itself hitting the button and reversing the movement.
Here is a video clip of the mechanism under test, switching by hand:
Consequently, if you don’t like the large stepper sled assembly from the CD drive, a smaller but nearly identical assembly can be purchased for around $10-$15 online. You could also pull a higher resolution stepper out of a different drive if that suits your purposes better.
Here is a video of testing the traverse limit buttons and solid state latching circuit (with an LED standing in for the relay coil… just because it makes for better video).