This is a prototype of a mechanical/electronic trigger system for a BB Trap. The trap fires a Mosquito Molds 165 BB Shower grenade shell at anyone that wanders into its field of view.
It’s a little ugly, being a prototype, but’s it’s more solid than it looks. 
For those of you unfamiliar with the BB Trap, it’s a metal frame designed to hold one or two Airsoft Launcher Grenades. Launcher grenades work kind of like Airsoft shotshells – they hold a number of BBs, and launch them all in a big salvo when the grenade is triggered.
This particular BB Trap holds up to two grenades. For best results, the Mosquito Molds 165 BB shower grenade is used – it fires a CLOUD of BBs at the target! (Other grenades hold 16 or 24 BBs or so)
One side of the trap is untouched and triggered normally (by tripwire). The other can be triggered by tripwire OR the motion sensor (but not both at once).
WHY IS THIS USEFUL?
The trap is normally triggered by a tripwire. Pulling the tripwire pulls a pin that releases a spring-loaded hammer, which in turn strikes the rear of the grenade – triggering it.
That original functionality is retained in this prototype. But added is the capability of releasing the hammer as a result of the Infrared motion sensor’s signal. So there is no tripwire (much easier to set up), and allows for a much more flexible field of view and range of detection!
HOW IT WORKS
Here is the design I used for the BB Trap’s electronics. CORRECTION: The battery source is incorrectly marked as 6V. You should use at least 9V. The PIR sensor is an infrared sensor unit that outputs a logical HIGH when motion is detected. It has a 60 degree field of view (adjustable – see later in this page for how).
The entire system has a single switch that is integrated into the firing system so that the whole assembly ONLY uses power when it is armed, or triggering. After firing, the entire system is turned OFF. This maximizes battery life and reduces overall complexity. Those 8 AAs are going to last FOR EVER.
When the PIR sensor detects movement, the output signal triggers the motor driver to power the gearmotor. The gearmotor (much like a common RC servo) moves an arm. The arm holds the hammer back, but releases it when it is moved by the gearmotor.
SETUP
The trap is placed in an opportune place – either stuck in the ground, or held in some fashion. The hammer is then cocked and held in place with the safety pin. The IR sensor is adjusted (swiveled, and possibly has the FOV – field of view – narrowed if neccessary). The IR sensor can be “dry fired” if necessary to test (arm and trigger as normal, but do not engage the hammer of the trap).
ARMING
To arm the system, push the arm into position (push it up against the strut of the trap) and push the switch to the ON position. The system will power up. Then, pull out the safety pin and gently allow the hammer to rest against the arm of the motor. You have 15-20 seconds before the system is LIVE, so work quickly and get out of the field of fire.
FIRING
When the IR sensor detects motion, the gearmotor is engaged. The arm moves out of the way, releasing the hammer that was held in place. The hammer fires the grenade, and the arm continues to hit the switch we originally used to ARM the system, and pushed it to the OFF position to cut power to the system.
ADJUSTMENT
The normal field of view of 60 degrees may not be suitable for some uses, notably outdoors use. There are two ways to adjust the IR sensor’s field of view.
#1: Move the sensor. Just swivel the sensor left or right to make it “look” in a direction other than straight ahead. Note that corners, walls, etc can also be used to artifically limit the sensor’s field of view.
#2: Add a “blinder” to the sensor. This refers to putting some opaque object with a slot in it in front of the sensor to give it “tunnel vision”. Note that you can’t just use any old material. For example, you might be tempted to use black plastic or electrical tape, but those are BOTH not opaque to infrared! Your best option is tin foil or a card lined with tin foil. I have exactly such a thing pictured below. It works VERY well to limit the field of view so the sensor only “sees” motion directly ahead of it. (The blinder I used is visible at left in the last two graphics below)
SEE IT IN ACTION!
Here is a short video of the Trap in a demo . You can see it fires only when someone crosses its field of view.
PICTURES
