Putting and LED bladder indicator in a Nerf Magstrike

I present to you a write-up on how to make and install an L.E.D. magstrike bladder indicator. This can be installed in pretty much any air powered gun, although I think you will run into a bit of a space issue in an RF 20. This is a re-post of my original post from the wonderful Nerf-HQ forums...a place I whole heatedly recommend you visit if you are into foam guns.

Caution!:

This writeup assumes that you have basic soldering skills, and are already capable with using the tools presented here. Soldering irons are very hot, and very dangerous. They can destroy your guns, and your hands. If you dont know what you are doing, please dont attempt to do this. Many types of solders contain dangerous chemicals, sometimes even lead. Dont do this on your mom/spouse/gf's nice kitchen table.

Materials Needed:

-30w or higher soldering iron
-rosin core, lead free solder
-Wirecutters/strippers
-Hot glue gun and glue sticks
-some thin wire, I suggest you get colored wire. Scavenge old electronics for it or rip open old vga cables or something.
-Heatshrink tubing of appropriate size for your wire
-Electrical Tape
-2k ohm resistor
-1 red led (5mm, frosted. used for indicator not light source)
-1 green led (5mm, frosted. used for indicator not light source)
-2 5mm LED holders (Radio Shack Catalog #: 276-079)
-membrane "dome" switch. - I have a ton of parts, not sure where to source these, this is an example from digikey (digikey part: 401-1924-ND)
-Contact switch - these are in EVERY nerf gun with a light that is trigger activated
-Your choice of battery and holder

How To:

    Let me start off by apologizing that I didn't take pictures during the process, i took these after, so some things may not be clear, but you already know how to solder.
The goal here is to have 2 lights that indicate when the bladder is half full, and when the bladder is about 80 percent full. The red led will be for half full, and the green will be for 80 percent. On my magstrike here, once the bladder fills up enough that it pushes against the white retainer plates, it is half full. When the over-pressure valve hits the back of the tank area, it is about 80 percent full. So I will be placing switches in both of these locations:

Your first step is going to be to drill 2 1/4" holes someplace on the shell that you want to put the LED's. I chose to put them on the top of the tank because I was in a rush and didn't feel like doing much thinking. Make sure wherever you place them that you have sufficient room for the internal components.

Once you have made these holes, you need to wire up your switches. The first switch is the dome switch. These are small, nearly flat switches that work when they are pressed. Basically what is inside the keypad on your phone or your keyboard. I drilled a small hole for wires from the switch at the top of the retainer plate that keeps the bladder in place, and then glued the switch to the inside of the retainer plate. I then wrapped a few layers of electrical tape over the switch to make it a bit thicker so that it would definitely be pressed by the bladder, and so that it would not cut the bladder.

When you solder wires to the switch, leave plenty of wire attached, you can cut it and route it later through the blaster. Make sure you heat-shrink around any bare wire contacts so you dont have any shorts or sharp edges.

Next you are going to place the rear contact switch. For this I used a switch out of a firefly, the little metal contact one that triggers the flash. Cut a space in the stopper plate in the back of the magstrike that you can fit the switch into, and hot glue it in place so that when the bladder over-pressure valve extends, it will press the switch closed. I also glued a small piece of foam to the side of the over-pressure valve so that it would have no trouble pushing the switch. The size of this foam can be used to calibrate when the switch is pressed.

The next step is to wire up your LED's. I will give some additional tips on LED's at the end of this writeup, so if you have no idea what I am talking about read that part after this. Select the order in which you want to attach your LED's, and place wires for them through the 2 1/4 inch holes you drilled. Solder these wires to the LED's, and make sure you heat-shrink the cathode and annodes of the LEDs. Insert the LED into the holder and push it into the hole, it should snap neatly into place with a tight secure fit.

Lastly you need to place your battery. Because I was in a rush, and used what was availible, I stuck a small inline batter case and a 12v 23amp battery inside the blaster. You can use whatever you want, and place it inside or outside at your pleasure. I will give some additional information about batteries later in this writeup.

Wiring it all up

This is where it can get tricky, and if you dont know anything about electronics I am sorry. I will provide a wiring diagram here, and some basic information about LED's and batteries, but I am not writing up a detailed guide to electronics and wiring. I may be an electrical engineer, but there are plenty of better guides for that on the net already. Check out sparkfun for some great guides and tutorials on soldering, wiring, and electronics if you need more.

The positive end of the battery should be wired to the 2kohm resistor, and then from the resistor should split off and power each of the positive ends of the LED's (the longer legs).
The negative end of the batter should be split to go to each of the 2 switches, and then each switch should be wired to its respective LED's short leg.
Once you are done, you will have a working indicator!

Notes about electronics, LED's and wiring

Some important things here for those of you who dont know. Electricity has 2 basic components, voltage and current. Voltage is like water pressure, its the amount of force the electricity has to move through a circuit. Current is like the volume of the water, its the amount of electrons moving via the voltage that can actually do stuff. What makes things work, bulbs light, motors turn etc is NOT voltage, it is current. This is important to understand. Any device will draw only as much current as it needs, but will be forced to accept whatever voltage it is given. with devices like LED's, this will cause the LED to burn out very quickly. This is why the resistor is VERY IMPORTANT. Each LED only needs about 1.2 volts to light up, and less than 500ma of current. The resistor is very important in making the circuit work and keeping the LED's lit. Do not wire this circuit without a resistor. The value of the resistor can be changed and will alter the brightness of the led's. I selected 2kohms as it makes them bright enough to see, but not too bright to give me away.

Batteries: Ideally you should have about 3 volts to run this circuit correctly, more voltage wont hurt, less voltage wont work very well or at all. You MUST have the resistor wired to the LED's to protect them in the circuit. Something I want to drive home about batteries: they dont work the way you all think they do. Stop using freaking 9volt batteries for everything. All batteries have a voltage indicator, but they also have a current indicator measured in amp hours. That is how many hours they can provide 1 amp of current. 9 volt batteries BARELY produce any current and cant provide it for long. They are for trickle devices like smoke detectors and walkie talkies. NOT FOR RUNNING MOTORS AND LIGHTS. In my gun here, I used a 12 volt batter that supplies 1 amp for 23 hours. If I hooked a stampede up to this battery it would operate for all of 8 seconds. This is because the battery has a very slow discharge rate and would exhaust all the current immediately. However, with something like an LED that sips on the current, it will keep the led's illuminated for days, probably a few weeks.

VERY NON-TECHNICAL DISCLAIMER
(The following is extremely non-engineering speak)

Seriously. Not to go off on a tangent, but I want to yell at everyone some more (our hvz game just ended, I died in the last 10 minutes of the final mission, and im hungry. Sorry for being grumpy). Those of you that keep replacing batteries in vulcans, stampedes and barricades etc with 9volts need to just cut it out. It makes me so stabby and punchy. Look at that massive tray in your stampede. Takes 6 C batteries, about 9 volts. So how is it that if they are that big and 9 volts that you think one small 9volt will replace them all! But seriously to bring this back to relevant information, 9 volt batteries have a very very very low current output which is what gives batteries their "making stuff happen" power. Thats why AAA, AA, C, and D batteries all put out 1.5 volts, but are each larger. The larger batteries have a greater current output and last longer. Okay rant over.

The finished product:

Questions? Comments? Thoughts? Make sure you post your questions here and dont PM me, especially if they are technical, so that others may benefit from your ability to point out the flaws of my writeup!
Enjoy!