Tonight, I did more engineering validation testing of the IR breakbeam sensor mentioned in the previous article.
First, the setup. The sensor is securely mounted in my benchtop vise, with a phone book propped up behind the bullet path as a pellet trap. (Finally, a good use for those dead-tree edition phone books!). A regulated DC power supply is used to provide the power to the sensor module, and my oscilloscope is used to monitor the signal line. As before, we set the oscilloscope to trigger on a falling edge signal at a level close to DC Bus -.
(I need to get my garage sale O-Scope probes checked. They don’t seem to be reading the voltage right, but at least the signal generator test indicates a good test pattern. Probably something stupid I forgot to set in the software. I’m still learning how to use this thing).
Next, I set the oscilloscopes time scale to 100 nanoseconds per division. Yup, definitely picking something up! That’s a good sign. Rechecking at 1 microsecond per division shows a fairly clean signal.
To give Dad a good idea of what he’s engineering to, I need to take some measurements of the pulse width of the event. We’ve previously calculated about 18.3 microseconds for a round ball at 1000fps. (Note that we actually don’t know how fast the air rifle is shooting at, nor is the pellet perfectly round.)
Look at that! 20 microseconds. Love it when the calculations matches real life data.
The next shot clocked in at a mere 5 microsecond pulse. There could be 2 reasons: A) the angle of the flight path through the sensor might be changing, or I might be nicking the beam differently. Still, the oscilloscope clearly captures a 5ms pulse.
Another shot, this time generating a 10ms pulse.
Yet another 5ms pulse again – followed by a lot of electrical noise. That’s strange…
Looks like the round nicked the sensor housing. Yeah, that would explain the sensor noise.
Remarkably the sensor still works. Putting the gun aside, I grabbed the soldering iron sponge and started dripping water past the IR beam. It registers on the O-Scope! (translation – this can be used for those awesome water-drop shots!)
Finally, here’s a couple of pellets recovered from the phone book. Love how you can see the rifling marks on the pellets 
.
Tags: DIY, Engineering, Photo Gear, Photography
Love it. Why not get a cheap Chronograph to figure out FPS? You must want one, generally.
What model of pellet gun are you using?
I’ve had better luck making a trap that uses GB Duct Seal as the backing vs a phone book.
Hey Nick!
Thanks for dropping in! It’s a Beeman break barrel with interchangable .177 and .22 barrels. I might pick up a chronograph later on, but honestly i don’t do enough shooting and tweaking of air guns to really justify one at this point. (Besides, I can probably build one – just have to put a second break-beam sensor in, some 12″ down the line. The oscilloscope and electronics are more than fast enough to measure the bullet speed.)
Mostly I just needed to convince myself that this sensor will pick up a speeding pellet.
I’ll have to look up GB Duct Seal…