This is part 1 of a multiple part build of a Nerf Gun Hack which will monitor an area using a infrared beam and fire on anyone who walks through the beam. The goal which is just a loose guideline at this point is to have a microcontroller monitor an IR beam. When the beam is broken by someone walking through it the Nerf Gun will fire a dart (or several darts) in the location of the beam. I think there will only be one beam and the gun will be in a fixed position pointing at the area the person would be when the beam is broken, this could change along the way though. To fire the gun the microcontroller will need to simulate the same sequence as the operator pulling the trigger.
See lots of pictures and some videos below.
Selecting the Weapon
The gun needs to be automatic so that it can be electrically controlled, this rules out many of the low cost units since most of them need the user to cock the gun (compressing the spring which propels the foam dart). There are a few manufactures who make foam dart guns, Nerf is the most popular and also the most expensive. I want this project to remain fairly inexpensive so the Automatic Tommy 20 by Buzz Bee Toys was selected.
Operation of the stock Automatic Tommy 20 Gun
The gun takes 3 AA batteries which are in series to provide a 4.5 volt gun power supply. The gun fires a dart by spinning 2 ribbed wheels which are located in the front section of the gun directly in front of the darts. A dart is pushed between the spinning wheels which propels the dart out of the barrel.
There are two trigger positions to allow the gun to function. Squeezing the trigger half way spins up the two motors in the front section, it takes about a second for the motors to attain full speed. To fire a dart the trigger is pulled all the way back, this ratchets the rotary dart holder by one location and then pushes the dart into the spinning wheels to fire it. If the trigger remains pulled all the way back the cycle continues until the user releases the trigger.
Opening the Automatic Tommy 20 Gun
Top open the gun there are a number of screws holding the two halves together. They are all the same size except for one which makes re-assembly simple. There is only one hidden screw, you can find this one under a sticker in the front section. What was very frustrating is that 95% of the gun came apart by removing screws however the but of the gun was glued on. This meant that there was no way to nicely separate the gun without cracking some of the plastic. I was careful and it didn’t damage the gun very much…
Inspection of the Internal Gun Operation
The gun operation is very simple. There are two motors in the front section, each motor is directly connected to a spinning wheel. The motors are wired so that they spin in opposite directions, power it transferred to the outer mechanism by two contacts. I would have expected a simple wire here which would have kept costs down. Under the battery box there are two micro-switches. When the trigger is squeezed half way the first micro-switch is activated and provides power to the two front section motors.
There is another mechanical portion located in the main portion of the gun just in front of the trigger. A single motor moves a lever which is pulled in and out ratcheting the rotary dart barrel around one dart position at a time. This is done using grooves inside the dart barrel. The same mechanical section activates a timed plunger which pushes the newly advanced dart forward into the spinning wheels. The trigger mechanism also interacts with this mechanical section by retracting the dart pusher when the trigger is released. The motor for this section is activated by the second micro-switch. It looks like this section is well designed for simplicity and it does work well when used with the stock trigger.
Hacking the Gun for Remote Fire Control
Based on the operational details of the gun hacking the gun for remote fire control is actually quite simple. I will simply be simulating the operation of the micro-switches. This was accomplished by running 3 wires out the but of the run, one common wire, one for the two front motors and one for the single motor in the main body of the gun. I did run into two issues though. The trigger needs to be pulled all the way back to allow the firing mechanism to operate, this is because when the trigger is in the normal position the dart pushing mechanism is deactivated. To overcome this problem a tie strap was used to hold the trigger in the fully pulled position. When this was done the second problem happened. Now that the trigger was pulled back all the way both of the existing micro-switches were activated essentially shorting together my independent remote control lines. The solution to this issue was to clip the positive line that fed the micro-switches from the stock battery box. This means that the gun will not function as a normal gun even if some batteries are installed but I can live with that.
Coming up Next
Next time we will be connecting the modified gun to a microcontroller and testing out some remote firing!
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