LPR/MPR Launch Controller Build

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Jan 7, 2019
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I have had it on the build pile to build a custom launch controller for when I need a solid, simple controller for low key launches. I built a wired, relay system two years ago for my HPR habit for when I need to provide my own equipment vs. using a club's setup. However, for simpler launches, like when my under age 6 grandsons are in town and we're mostly launching 1/4A through C impulse, the big rig is too much work to setup. I used an Estes launch controller the last time this happened, but it's wonky, it's only 6V, and the dang button is just too small for a 4 year hold to hold down until the igniter goes. I've seen more igniters go "pfft" from inadequate pressing of the button to last a lifetime.

So, I designed a controller based loosely on the relay design (without the relay box). I had some components left from that build (bought in bulk), so it made sense to leverage that effort. I had several specific objectives in mind that I wanted to meet.

  1. The controller had to be self contained, including the battery.
  2. The controller had to be had holdable, especially for smaller hands. I envision the kids being able to hold it themselves once they become old enough (grade school age).
  3. The controller had to meet NAR safety requirements.
  4. It has to be easy to use, in terms of battery access, removal for charging, troubleshooting, repairs, etc.
  5. It has to have lights to indicate power on, armed and continuity.
With that I set out to find the necessary parts to piece this together. The list of parts is in the attached PDF. The key part to decide on was the box - small enough to be hand held, big enough to hold all of the components and one that could be opened with latches. A lot of electronics utility boxes have screw on lids, which are a non-starter. The one I used fit the bill, although I knew the innards would be tight. It has a hinged top and front latches. The vendor has several larger models which would have made the build easier.

The other key part was the battery. Given the size, it had to be a rechargeable LiPo. Wanting 12 volts, I went with a 3S. 2200 maH should be plenty for a lot of launches. I also want to be able to launch LPR and even MPR clusters with it in the future as the kids get bigger, and need the juice for parallel whips.

I actually built it over the past two days and it works great. This build thread won't be a blow-by-blow detailed build, but the general arc will be discussed along with the challenges encountered and how they were thwarted.


  • LPR Launch Controller BOM.pdf
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A part of the initial design effort, in addition to pulling together a parts list, was to do a wiring diagram. The diagram is in the attached PDF. I used TinyCad to do the design. I found the program 2 years ago when I was looking for a simple (and free) program to do the wiring design. It's not industrial grade but it serves this purpose very well.

The key elements of the components include:

  1. A key Switch to power on the box in an unarmed state.
  2. A yellow LED to show that the box is powered. I wanted this LED because the key is removable in the on position.
  3. A toggle switch to arm the controller.
  4. A red LED to show that the controller is armed.
  5. A green LED to show continuity at the igniter.
  6. A big red push button switch for little palms to hold down for launch.
  7. Banana jacks for the cable to the launch pad.
The key switch and toggle may seem redundant, but I have them both for two reasons. First, it's an educational step for the youngsters. Box powered on is different than armed. Second, if I'm flying with all adults, I'll just leave the power switch on and use the toggle, but with the kids I can pull the key if I have to leave the launch pad. Safety first. Plus, it let me use three LEDs instead of two.


  • Simple Launch Controller Wiring Design.pdf
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Last part of the design, a rough diagram of component placement on the surface of the box an internally. I have one diagram attached. I got lazy and only did one, superimposing the external and internal components rather than doing two, but it's pretty self explanatory. The keey point is that this was a starting point, which changed a lot once I got into the build. Once you start adding the wires and connecting things together, parts get in the way of other parts and it becomes an iterative process to cram it all together, the downside of keeping the footpront small.
Pic below is of the core parts before assembly. Let the fun begin...


Yeah, the busbars are overkill but I had them left over from the relay controller build...
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Step one was to drill out the holes on the plywood board for the component and wire placements and box itself.

Marked up board pic and drilled board pics.

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Marked up box and drilled box pics.

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The first step in building out the internal board was to build the battery connector. I bought the mountable female plug so I could do a simple push/pull to connect and disconnect the battery. Being a small box, fiddling around with loose connectors and arthritic fingers was a non-starter.

I used 14 gauge wire for the power connections because I had it on hand. 16 gauge would have been easier to work with. All of my wire connectors are soldered and shrink wrapped. With the case lid bending the connections need to be solid so they can take the movement. Here is the complete XT-60 connector.


The first issue encountered was the length of the shrink wrapped connectors. This was being mounted to the board and the wires fed back up into the box from the bottom of the board. This required adding spacers under the board to raise it up enough to not crush and damage the wire connections. This required a combination of parts from the parts bin and a trip to the hardware store.
The next pic shows the partially built out board with the board components installed and some of the wiring completed.

The next head scratcher was how to connect the LEDs. The wires are extremely small (they must be 26 or 28 gauge), so they are hard to put connectors on and I wanted to not have wires running willy-nilly within the box.

The first issue was handled by using 22 gauge ferrules. The LED wires were stripped double length, twisted, folded over once to make it twice as thick, then crimped. The wires were still not absolutely secure, so I mixed up some 5 minute BSI epoxy and placed a couple of drops in the ferrule cap to bind the wire to the cap. They won't come loose.

The second issue was to use terminal blocks to manage the wiring. Wires from the LEDs to the terminal blocks (one per LED) and then wires out to the correct attachment point. The only (and best) place to put them was on the lid above the switches, so I cut a plywood board, drilled holes and put in 4-40 weld nuts. The board was epoxied to the box lid with West Systems epoxy with some colloidal silica added to keep it from running. Pics of pre and post installed board below.

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The pic below shows the terminal assembly components. The screws I had were a bit long, so I had to add a spacer. When mounted, the spacers went under the screw head (shown here atop the weld nut to verify the screw wouldn't bottom out on the case lid). I had to trim the sides of the spacers to fit between the terminal block headers.

The pic below shows the completed (mostly) internal wiring. The next problem is that the terminal blocks extend too far down and hit the battery - the lid won't close.


No worrries. I detached the board from the box (kept most of the wires attached, disconnecting everything would have been a nightmare), trimmed the board to allow the battery to drop down to the bottom of the box and built a battery holder using more plywood. Oh, and I had to move the battery ground from in front of the busbar to under it to make room for the battery. More snipping, drilling soldering and shrink wrapping.


The battery fits like a glove now and the lid closes.

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The last step was to run a test. On the first try the amber Power On LED did not light up. Breaking out the multi-meter showed the LED was fine, but the connection at the busbar was broken. The post on the keyed switch was right over that connection and broke it when the lid closed. Snip, re-make and move it over one position on the busbar to the left. Time to re-test.

Keyed Switch on...


Armed switch toggled on, no continuity...


Armed switch on with continuity (second green LED connected to the output leads) ...


Launch button pressed (box LED goes out, continuity LED gets brighter) ...

I used the final 25' of 14 gauge wire I had remaining and built a wire lead with banana plugs on the controller end and alligator clips on the other. The final step will be to print and attach labels to the switches and LEDs.

All-in-all, a challenging and fun build. This will be easy to carry to the field and use. Now I'm itching to use it. Oh, it's winter in the Midwest and it's -2 degrees outside. The grandsons are 1,000 miles away in Denver (another really deep sigh). I guess I have to go back to building rockets....
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If I wanted to add a small electronic buzzer that would sound when the arming toggle was in the on position (with continuity), where in the circuit would I place that? Would this buzzer work? [Edit] (https://www.amazon.com/dp/B09HGQRNLC/). (https://www.amazon.com/uxcell-HYD-2312-Passive-Electronic-Sounder/dp/B00O9VX4C2) If not, do you know of one that would?

Parallel to the Armed LED, but without continuity. With continuity included, you would need to modify the circuit so that the continuity circuit wasn’t loaded by the buzzer.

You could possibly have it parallel to the continuity LED, but you would need to make sure it didn’t allow too much extra current through the igniter. With a series current limit resistor on the buzzer it might not work. It would depend on the resistance of the buzzer.
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So running parallel to the red 'armed' LED, I would get tone when the arming switch is toggled, and the tone would continue until the firing button is pressed, correct?
Yes, that is correct. You already have the continuity LED to indicate continuity. It’s better to have the buzzer to warn that the system is armed.
I left a buzzer off this design because I just didn't have room in the box I used. The other thing I left off is a circuit breaker, which I have on my relay design. Arguably, I need it more for this box, which uses a 3S LiPo, than the relay system, which uses 12 SLA batteries, to protect again a dead short, but again the amount of room made it a real challenge. The breaker I used in the other controller might fit (in one very specific location), I just haven't looked hard at it to see if it would. I can provide that component info if anyone is interested.
So running parallel to the red 'armed' LED, I would get tone when the arming switch is toggled, and the tone would continue until the firing button is pressed, correct?
Actually, if you run it parallel to the red LED, it will continue to buzz even when the launch button is pressed and after the igniter blows. The red armed LED is always active as long as the toggle switch is in the on position. You actually want it that way, since you don't want power available to the leads when you are at the launch pad attaching the clips to the igniter. In my relay system, the buzzer is on the relay box controlled by the arming switch on the launch controller box, so if you are at the pad 100' away, you know if it's safe.
I see what you mean about having enough room in the box.
There may be smaller breakers available, I just never looked for one. For what it's worth, I installed this breaker in my relay system on the outside of the box with only the posts on the inside. That was because the box I used for that controller had a screw on top and I needed to be able to do a reset without opening the box. I might fit (posts only on the inside) but I haven't done the measurements. If you went with a smaller 3S battery (say 1500 maH) then it might fit on the left side of the box towards the front, since the battery should be shorter.
@Mike Haberer - The 30 amp breaker you refer to seems like a lot for the 12 volt on your relay, or is the '12 SLA batteries' you mention actually 12 batteries of the SLA type?

I'm looking at a single, external 12 volt 7Ah SLA battery connected to the controller with 10 feet of 18AWG zip wire. The run out to the pad is a 100-foot 16/2 medium-duty extension cord. Could I get away with something along the lines of a 10 or 15 amp breaker? I'm thinking of just putting it inline on the positive lead coming from the battery, before the controller, using these:

@Voyager1 - Your thoughts?

Yes, you would only need to use a 10 A fuse, typically. My homemade controller uses a 10 A resettable circuit breaker and it’s never blown, but it would if there had been a dead short for a few seconds. The ones you’ve identified look ok. I also use the 7 Ah SLA batteries on my standard controller.