Hey i am making this attachment for my arm bot, it shoot model rocket engine’s. if yah know what im talking about please let me know so i dont break somthing but, if i were to get two igniters and attach them to an exsention wire (one on the black with red and one on the white and red) and if i were to press full power button for one direction do you think that the micro controller would frie from the sudden discharge, aka short in the circuit?


attaching two leads to the red and black terminals will give power to whatever they are attached to. in order to be able to tell the fuse when to go, you need some sort of relay.

The motor controller at the bottom of this page sould work, but you might want to confirm it with someone else.

I would use a relay and a motor driver over just the motor driver for one reason, if there was even a low voltage of 1 volt it can set off the rocket. With a solid state relay that operates between 6-12v you have to get over 6v to switch it so those low voltages that are present won’t effect it until you go full speed and the voltage rises above 6v.

Do not hook it up to the motor port like that there is always power flowing through it and the igniters will go off as soon as you connect it even if the motor is not running.

I wouldn’t shoot just a rocket engine. They won’t go straight, at all. I’d have small model rockets, with small engines so they don’t shoot into oblivion. Not saying you can’t do that, though :wink:

I wouldn’t do this at all. VEX and things that can explode or cause harm seem like a really bad idea…

I completely agree with corpralchee here - you are asking for trouble by doing this. A software glitch, radio interference, or other unforeseen problems could cause your rocket to fire at any time. Scan the forums for many reports if intermittent controller spasms…

If you insist on doing this, add an easy-to-reach safety switch in series with the igniter so that you can visibly ensure there is no way the igniter can fire.

Better still, use three relays driven by different I/O ports, and run the launch power through all three of them in series. You can wire it such that two relays must be on, and the third be off, before the igniter sees power. This will greatly reduce (but not eliminate!) the chance of a single failure launching your rocket unexpectedly.

Best of luck,

  • Dean

yah, same, my friend made one a while back but it shot actual rockets.

at least add fins to the engine itself.

also, heres a great thing to do for controlling a circuit:

have the negative wire connected to the micro controller and whatever you have the circuit connected to (igniter).

have the positive wire for the controller to an alligator clip and a metal platform. have the platform on a servo.

from the positive end of whatever your connecting to (igniter) have an alligator clip to a metal shaft.

To complete the circuit simply turn the servo to hit the metal shaft.
we used this for a fan and led system.

I suggest you do this just like the motor igniter they give you, it has two switches, you must first push the key in and then push the button to fire.

So what you should do is have say the up button on channel five as the safety button with a 2 second delay and the up button on channel 6 to fire.

That way if you hit channel 6 on accident it won’t fire. In order to fire you would have to hold the up button on channel five for 2 seconds to disengage the safety then you could hit the up button on channel 6 to fire instantly.

You could also put some sort of rotating light on the robot and a pezio alarm so that when safety is off, the robot will sound an alarm with a flashing light so you will know for a fact that you are ready to fire.

This will greatly increase the safety when you have measures in place to stop a sudden launch.

That would work - basically a DIY relay. If you are looking for a Vex-compatible relay, I just came across this and this, which are a bit less than a Spike. They should hook into a Vex controller OK if you add some pins to the servo connector.

  • Dean

No, no, no, no, no. … NO

I understand the good design principle you are suggesting; but without extra circuitry between the Vex microcontroller and a rocket motor, what you suggest is a bad idea.

Assuming the microcontroller is like most simple electronic devices that are not isolated from the external world by relays; when you turn on the microcontroller there is a brief period when its outputs’ values are undefined, and then they are set to the microcontroller’s default, and then they are set to whatever your software instructions tell them to be.

The twitches motors and servos exhibit when the microcontroller is turned on are evidence that my hunch is correct or close enough.

These undefined (at a black box level) transitions in the signals at the microcontroller outputs (digital/analog I/O along with the PWMs) are just one of the reasons why you should never use the Vex equipment to attempt to control anything approximating a weapon or other dangerous device.

It’s a good thought; but the equipment in the Vex kit simply can not implement fully it.

PS: This is also at least one reason why Quazar said that even if you line up three relays in series, you do not eliminate the likelihood of a very dangerous mistaken/premature launch.

anyone who’s been on this forum for long enough will know exacly what im talking about here:

we have seen many threads about dangerous things and they all turn out deciding that this is a huge safety concern and you should not attempt it

this is the kinda thing Mythbusters is for (hey they have lots of Vex)…send 'em and e-mail!

The vex controller is not the absolute best, its a controller designed for a robotics kit not a weapons control platform. The only thing that can increase the safety is relays and multiple switches. For every relay you add it gets safer because the robot would have to magically turn on all of the relays to complete the circuit so the chance of it happening will decrease.

Thats about all I can say, just do this in an open field not in your backyard and make sure that its not pointed at any human, animal, or anything you don’t want a rocket flying at.

To test and see if it works the first time do not use a rocket, for at home tests use a light or a rocket igniter (Just the igniter not the motor). This will show that power is flowing which is all you need to fire the rocket.

Heh, I wasn’t thinking when I posted…

I’ve only used that for leds and fans, nothing harmful.

Of course my friend wouldn’t let me program his igniter by a digital out so thats what he used… luckily he didn’t blow himself up.

You could also not put on the engine/ alligator clip the igniter until it is on.


I’ve been assuming the title of this thread means the O.P. understands the risks inherent in what he is attempting.

He must assume that any time the igniter is hooked up and the Vex microcontroller switch is on, that the rocket can fire spontaneously. He must treat it like it can launch at any time while it is “hot”. This is true with or without additional “safety” relays.

  • Dean

So if i buy the relay and plug her into the igniters it should work just dandy? shloud the relay go into what port again motor or the digital? Im trying to use the joy stick to launch the rocket, also im no longer using model rocket egines they go out controll im using little bottle rockets by removing the fuse and puting in place an igniter.

If you can’t do this electronically you should do it mechanically. Using the orginal concept that its a push key in, and push button to fire, actually have that on board the robot, and using servos or motors to actually push the key in and push the button. This removes the problem of relying on the vex controller to work perfectly. The twitch on start up wont be a problem anymore.

for the same reason my circuit completing servo won’t work, quirks in the system.

instead of using the launcher that comes with a model rocket, just use a 9-volt battery and some wire. I ended up using that method as a backup to our broken launcher when a few of my friends and I made a model rocket, and it worked fine.

what would be the point of that? we’re trying to make this launch when you want it to, not the moment you connect the wires. not to rain on your parade, sorry if i did :frowning: