Building a full-auto Nerf Demolisher Part 6: It’s not the voltage that kills you, it’s the amps.

Safety first kids.

A favorite TV dad of mine would remind us all, “…goggles might make you look cool, but they are also a part of proper safety attire.” – Hank Hill. Before you go there I am well aware of the likelihood (or lack thereof) of injury by blaster being related to electricity – it is extremely improbable, given the fact that at the end of the day all electrical connections and components are encased in plastic which is non-conductive. I won’t debate the merits of overkill, I’ll just state that my preference is always to do a little more than needed when designing or building anything, guess it’s just a trait of my generation.

You should watch some videos.

If you are a layman when it comes to electricity and electrical components it’s probably a good idea to get your self familiar with some basic concepts. With the amount of resources available there is really no excuse not to put in some time watching videos on the subject. I’ll skip the ones that should be considered entry level knowledge and focus on a few that will get you off to a very good running start.

You should get a decent understanding of things like voltage, amperage, diodes, resistors, mosfets, etc:
It’s not the voltage that kills you, it’s the amps
Transistors – The Invention That Changed The World
N-Channel MOSFETS
MOSFETS for Nerf

While you are at it might as well watch some videos on assembly techniques and soldering:
Collin’s Lab: Soldering
Soldering XT60 Connectors
Soldering 101| Nerf Modding Tips

Plan it out.

I can’t emphasize enough the importance of good planning. In the case of wiring up blasters, thinking through your plan to connect all the various components before you break out the iron, strippers, and solder will help you focus on assembly and not remembering or figuring out what you’re doing in real-time.

Consider the following wiring diagram that incorporates an N-Channel MOSFET to activate a set of flywheel motors.

Consider this example where a 2S Li-Po is driving 2 DC motors, a buck converter steps down voltage to create an isolated signaling circuit.

I like using a Buck Converter, also known as a step-down converter, to create a signaling circuit that is isolated away from the main draw of the flywheel motors. While this does add more complexity to the circuit it allows for the use of an easily sourced and more common sub-minature microswitch typically rated at 5A and it means you can use smaller gauge wire in the grip area making the routing of wires easier and much more orderly.

So much wirez.

It’s gonna be a lot of wires, at least if you insist on taking an approach similar to mine. I’ll be the first to admit, just like I did above, that I like to lean on the overkill end of the spectrum – you don’t really need to use a MOSFET or a Buck Converter or even use Capacitors on the motors to filter out electrical noise, you could just slap a 15A microswitch directly inline with the motor circuit and wire the whole thing with 18 or 16AWG wire then call it a day. I obviously didn’t do that. Have a look at the business end of the loom inside my test mule below, I used a fair amount of mini Dean’s connectors because I plan to swap components in and out of this blaster frequently.

The loom in my prototype mule uses way more mini Dean’s connectors than you’ll ever need to.

DC/DC Buck Converter, don’t mind the fraying wires this whole thing is encased in heat-shrink.

N-Channel MOSFET, to ensure that the Gate “sees” as close to 0v you should run a 10k Ohm Resistor across the Gate and Source ends of the transistor.

The bare minimum safety measure I would recommend as a must is using a 1N5400 Diode to prevent reverse current flow in the flywheel circuit.

That was a little too easy.

The 2 piece shell design of the Demolisher is suspiciously friendly for routing wiring, it’s almost as if the designers had intended it that way. Like some sort of modders easter egg hunt wiring channels and holes are conveniently placed, the battery tray is very thoughtfully positioned in the left side of the shell, and let’s not forget the perfectly positioned ledge for the gearmotor to sit on. The left side outer shell just to the right of the battery tray is another conveniently hollowed area making it the perfect place to mount a voltmeter.

If you don’t take a second look, you might think the voltmeter was always there.

If you have have or have access to a 3D printer feel free to grab the file for my Pressure-fit Voltmeter Bezel from Thingiverse here.

Next time.

Building a full-auto Nerf Demolisher Part 7: Final thoughts and a project update.

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