12v launch controller system wiring - 4 pads

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Well-Known Member
Jan 19, 2009
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I am in the process of building a 4 line lanuch controller running on 12 volts. Some plans I have seen call for resistors but others not. So one concern I have is safety - how many amps can 12 volts create across the system that has a test and ready lamp? I appreciate any and all advice. thanks.
Not sure what you mean by test and ready lamps. When you say you're building it for 4 pads, are you going to use a battery and relays at the pad, or are you just running igniter connectors out to four pads?

If you're using a battery and relays at the pads, then the setup has to be different then it would if you're doing everything from the controller.

As for amps from 12VDC. Use the equation E = I x R. E = voltage, I is current, and R is resistance. You should probably keep the continuity current below about 25 milliamps (.025 amps). E/I = R so 12 VDC divided by the 0.025 amps give you a resistance of 480 ohms. If you get a bulb with that much resistance, you can hook that in the circuit to use as a continuity check.
I built a 6 pad launcher for my clubs low power pads using the attached schematic as a guide. I built it in banks of 3 so i could use CAT5 to do the connections since it's really easy for me to get for free. I found this on Rocketry Online, but had some problems getting the page to load. I have the article as a .pdf if you are interested.

The main "mod" I did was to add a second battery to maximize the number of pads per connection within the limit of 8 conductors between the controller and the relay box.
I have made up a couple of systems. The first system is a 8 pad controler that uses push buttons and leds with 1k ohm resistors. the led and resistors are for the contunity check circuit.
The 8 pad circuit is in the upper left corner of the diagrams and I do not show pads 1-4 but pads 5-8 are wired the same. the rest of the drawings are for a modification that I did to the controler to alow for a 2 pad relay remote set up for our HPR pads.
old new launch controller.JPG
also this is a pic of the face plate of the controller