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An Automated Motion-Sensing Airsoft Gun Turret

This little beastie is one step closer to a robotic Airsoft player. It’s an automated gun turret.I made it mostly because I could, but also for use in CQBs (indoors would be best) where it could be placed on guard around corners, etc. It could also be used as a booby trap. Since it’s so short, it needs to be placed on a chair, table, shelf. etc.


The guts of the Turret is a motion sensor, a fire-control circuit, and a mini electric Airsoft gun. I used a mini AUG made by UHC. The AUG has the highest BB capacity of any mini (~170 BBs) which made it a good choice for an automated gun.

The gun’s power source (normally 4 AA batteries) has been replaced with a 6V lantern battery, which is able to cough up far more current on demand. This greatly increases the firing performance of the gun as well as lasts much longer than 4 AAs would.

The PIR (passive infrared) motion sensor is triggered by people or animals. It is most sensitive to movement across its “field of vision” rather than straight towards or straight away from the sensor. It works much like motion sensing floodlamps that are common in backyards.

So, when the motion sensor is triggered, the custom fire control circuitry I built triggers an approximate 2 second burst of BBs. The action of firing is jerky, and since the gun is spring-mounted inside the turret, this means the burst “sprays” somewhat – which is a nice effect.

The turret is very good at nailing someone coming around a corner, since the corner (or doorway) offers a very narrow and predictable target area for both the motion sensor and the gun itself.

There is a drawback in that the Turret’s motion sensor must “settle” for 30 seconds after each triggering before it can re-trigger. This is probably for the best, though. Otherwise the Turret may be a little *too* effective at what it does. This way, the humans have a chance.

The guts don’t look pretty. But they’re solid!

Gun Modification
The mini AUG was modified in a few ways. First of all, it was opened up and trimmed down (lengthwise) until all the stock was removed – right up to the motor and gearbox. This necessitated the removal of the battery compartment.

Wires were attached directly to the motor leads, bypassing the trigger and safety. Holes were drilled in the gun’s exterior to provide mounting holes for the springs.

A hole was cut for the barrel in the plastic Turret shell and some springs were used to suspend the gun in the middle of the turret shell. This takes care of the gun mounting.

Fire Control
With wires leading directly to the motor, applying power to the motor is all that is needed to fire. To control the firing, a microswitch was used. Originally, I used a relay but had troubles with the relay driver locking up due to the high amounts of EMF and instability of the power supply (all due to the very noisy – electrically speaking – motor in the gun). Rather than shield the relay driver I opted for a microswitch-based solution.

The digitally-controlled fire control now triggers a single revolution on a gearmotor (old disk-eject gearmotor from a Mac’s floppy drive) which applies force to a fulcrum, triggering the switch. Sounds a little like a Rube Goldberg device when described that way, but it’s actually extremely robust and stable.

The motor triggers the microswitch, which applies power (6V lantern battery) to the gun – firing it. The gear takes approximately 2 seconds to complete a revolution (which is adjustable via a small 500 ohm pot I added to the motor driver board) before turning the gun off again.

The digital control comes from the motion sensor itself, where I simply tapped into the signal that drives the indicator LED in the motion sensor. A better way to do it (or rather, a way that is more likely to work on different motion sensor models) would be to use an optoisolator. This would have the added benefit of isolating the fire control system (which drives the gun itself – which is noisy (EMF-wise) and high-drain) from the sensor. I in fact originally used a homemade optoisolator setup but had trouble shielding it from ambient light. Placing the turret in bright sunlight would “blind” it. It worked outstandingly otherwise.

There are a total of three switches and 1 button on the Turret, as well as an “ON” light.

One powers up the Motion Sensor. Another powers up the fire control circuitry. The third arms the gun (if the gun is not armed, the Turret can be triggered but the gun will not fire. Useful for testing). Finally, a button is on the rear. The button triggers a 2-second burst manually. It is possible to rig a tripwire or anything else to provide an alternate “trigger” to the gun besides the motion sensor.

Putting it all Together
The whole thing went into the plastic turret shell, then the bottom was filled with expanding insulation foam to hold it all together and form a nice pocket for the lantern battery to snugly fit in. The lid is fitted with a simple hinge.

All in all – not bad. It has a limited role to play, but it does what it does very well.