The bad news is that once again we are learning the hard way.  I burned out our laser tube when water flow stopped unexpectedly (power strip?).

The good news is that we are now back in action.  Here is how we did it in case there are any other slow learners out there.

We tapped into the auto shutoff circuit that shuts off power to the laser when the door is open.  First we found this calibrated flow meter from SparkFun, then discovered this beauty from AdaFruit for only $9.99.  We went with the AdaFruit, of course.

Carduino is a prototype of a project meant to be a automobile battery monitor that can shut off a peripheral when the battery voltage starts to drop.  It was a simple matter to re-purpose the Carduino for this application.

The simple sensor has three leads: ground, 5V, and data.  The data line toggles between low and high when water (or air) is flowing and is steady (high or low) when flow stops.  The flow meter sample code provided by AdaFruit, gave us a jump start on the development.  The protection circuit code is simple, if the delay between low and high toggles on the flow meter is longer than expected, shut off the solid state relay.

I had to channel my inner Anool to figure out how to wire the unit and could not have done it without a healthy portion of Bald Wisdom.  It took a day.  Here is the result.  If you remove the side panel it is much easier.

The fluid circuit was a challenge of fittings and space.  I put the flow meter outside of the laser, but needed a right angle adapter to save space inside the laser.  I had to cut a spacer to mount the assembly (see in photo).