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I recently hooked up a cheap laser module to a mircrocontroller (via a MOSFET), mounted on top of a couple servos.

schematic

simulate this circuit – Schematic created using CircuitLab

This worked fine until I invited my coworkers to access the controls for my laser turret. One wrote a script that moved the servos from 0° to 180° and back repeatedly as quickly as possible.

Now my laser is dead. It is very dim and gets hot when power is applied, and both my cat and I are very sad.

Unfortunately, I don't have much information about the laser module itself other than what you see on the linked page. The inside looks like this:

laser top laser bottom

So what killed the laser? My suspicion is reverse voltage from a servo motor. If that's likely the culprit, what can I do to guard against it?


Edit: As requested, a photo of the whole rig – although I'm not too sure it's very useful given the rat's nest of wires.

photo of full circuit

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    \$\begingroup\$ Show the circuit. With a competent circuit any voltage from the motor shouldn't be able to harm the laser. Also, check those ridiculous solder blob connections. You'd never be able see if you had a cold joint underneath somewhere the the 50 pounds of solder everywhere. \$\endgroup\$ – Olin Lathrop Jul 29 '13 at 23:24
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    \$\begingroup\$ Also, why is what appears to be a digital output connected to the drain of the FET with the gate tied to 5V? You may have fried the digital output by applying 5 V to the 3.3V logic. \$\endgroup\$ – Olin Lathrop Jul 29 '13 at 23:27
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    \$\begingroup\$ Does the GPIO pin of the uC board really go to the source and not the gate of the FET? Also, a picture of the whole setup might could help us diagnose a problem. \$\endgroup\$ – The Photon Jul 29 '13 at 23:41
  • \$\begingroup\$ Why is gate connected to ground? Shouldn't it be connected to the GPIO? \$\endgroup\$ – Arjob Mukherjee Jul 30 '13 at 5:59
  • \$\begingroup\$ According to your diagram, you have the same powersupply connected directly to servo and laser, which is another way of saying you have your servo connected directly to your laser. This is all kinds of bad idea because there is a lot of noise on a line used for motor power. EDIT: And while we're at it - it is often to your advantage when doing drawings like this to have one common ground-line at the bottom, just so you have a feel for what is connected to GND - is that Beagle IO port really supposed to be GND for the laser? \$\endgroup\$ – medivh Jul 30 '13 at 7:54
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Cost = Capability

Although there are many good suggestions above, it may simply be that this laser cannot handle the stresses you've applied to it.

Electrical Stress

Your direct connection between servo and laser results in the servo feedback into the power line propagating to your laser diode.

This is what the power rail looks like when a servo is commanded to move. Note the voltage brown-outs and positive and negative edge spikes. That can damage the laser diode if it isn't sufficiently protected.

enter image description here

Thermal Stress

This laser may simply not be designed to operate at the 100% duty-cycle you have applied to it. That is, it isn't designed to be able to dissipate the heat of continuous operation.

Most 100% duty-cycle lasers employ substantial thermal management devices (such as large extruded heatsinks as seen here).

enter image description here

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I believe what happened was that the servo moving sent back EMF onto the power bus, and as you have that coupled straight to your laser, you over-volted it, and killed it. (Or perhaps your MOSFET -- but the laser is likely more sensitive.)

When dealing with motors together with logic, you want separate power supplies. When that can't be done, at least put a fat capacitor across the servo power in, and a fat inductor in series with the servo power in, and perhaps something like a TVS diode or Zener diode across it to suppress spikes, for good measure.

Lasers are generally specified for a particular current, rather than a particular voltage. A given voltage may generate a file current for one laser, but over/under-drive its sibling that came off the same manufacturing line. Thus, driving the laser with a constant-current source (made from a simple adjustable voltage regulator) is typically a good idea.

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    \$\begingroup\$ We can see from his pictures that he has a "laser module", not just a laser. There's some kind of regulator-ish circuit there. So a constant-current drive in his design would not necessarily be a good idea. \$\endgroup\$ – The Photon Jul 30 '13 at 15:59

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