Any guesses what this will become?
(Leave a comment, with contact information, if you really want to know ).
Quick status update on the bullet flight sensor. This is heading into systems integration testing next, where I’ll be firing up each section of the circuit and making sure it all works. Missing is the break beam sensor that I put a air rifle round through by accident
Note the “unusual” arrangement with the pocket wizard. The “hot shoe adapter” is actually plugged into the sensor to simulate a camera’s hotshoe firing the pocket wizard.
Continued work on the bullet flight sensor electronics. Dad had suggested that I research a “monostable vibrator” circuit, and to look at the 555 timer IC, so after some tinkering and math, here’s my first draft circuit diagram:
Dad (a retired electrical engineer) had given it his blessing, so the next step up would be physically prototyping it. Looks like I’ll be placing an order with DigiKey or Jameco or some other online electronics component vendor.
While the circuit components are enroute – and they will be breadboarded up first – I’m going to try my hand at learning Eagle, a CAD design software. The circuit is simple enough that I can probably etch it myself using laser transfer paper, but I might also just job it out to someone like BatchPCB.com and deal with it that way. It’ll all depend on the mechanical fabrication lead time as well as other project loads.
Meanwhile, mechanical design needs to be refined a little bit. These are current CAD model screenshots:
Here the unit is partially disassembled to change batteries.
Here’s a top view looking straight down on the circuit card, with the detector barrel rendered transparent.
And finally an isometric view of the unit assembled.
Mechanical details for the battery contacts, as ewell as lead-in for the slots, needs to be integrated. Then it’s a matter of generating a file to drive Dave’s grandparent’s laser engraver to cut these acrylic parts!
W00t, can’t wait!
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 .