Simple, Multiple-Pad Launch Controller
Plans, Etc By John Hruby
I have been a rocketeer for many years with the typical hiatus during early adulthood. Several years ago I came back to the hobby and launch mostly mid and high power stuff, but many times I am asked to conduct launches for local Cub Scout groups as well as my own children.
I wanted a multiple pad controller like our Tripoli club had, but did not need the capacity they had. A smaller version would work well with the Cub Scout launches and would be simple enough for our family launches.
The Internet being what it is, I set out on a quest for plans detailing a simple Multiple-Pad Launch Controller that even I could wire without screwing up. I consider myself quite technology savvy, but when I have to figure out a wiring diagram and then actually start soldering things together, something's bound to end up in smoke.
I found a system that was close to what I wanted, but it had one major drawback. The operator had to initially select a pad and then throw multiple switches to launch more than one rocket. I wanted something that was one switch per pad and hit fire. I later found this to be the hurdle - stay tuned.
Initially, I prototyped the controller mentioned in the previous paragraph. It had all sorts of continuity circuits, buzzers, etc, and every time I wired it, something went up in smoke - oops. See, I told ya. I set this prototype to the side for many months and did other things.
My rocket group, Tripoli Oklahoma, had to refurbish one of our relay boxes and it gave me a good look at the innards of that box. One day, on a whim, with all of my previously purchased components, I started putting things together with test leads. A test lead is just a wire with alligator clips at either end. I kept it simple.
After attaching a 12 volt hobby battery, I clicked on the "master arming" switch. The red LED came on and viola, no smoke. I next threw the "pad select" switch. Again, no smoke, and a pretty LED let me know the pad was selected. Finally, I hit the "fire" switch. The LED I had wired blinked every time I pressed the fire button - and no smoke. I finally made the circuit work!!!
Bolstered by my controller's success, I started drawing the diagram for it and started to figure out where and how multiple pads would fit in. Below is the diagram for a single pad circuit. Simple and easy.
I then figured out how multiple pads would fit in. Here is the diagram for multiple pads.
|Figure 1 - Relay Box|
|Figure 2 - Controller Box
The key component to make the multiple pad controller function as I wanted was a specific switch to select the individual pad circuits. It has to be a Double-Pole, Single-Throw(DPST). With a single throw of the switch, part of the circuit lights an LED, indicating that that pad is selected without sending a signal down the control cable. The other pole, is energized when the "Fire" button is pressed and sends a signal(current) down the serial cable to the appropriate relay - thus energizing the right igniter. If you use a SPST switch, the LED will only light when the "Fire" button is pressed - not too useful to see which pads are selected.
I used a DB9 serial cable for signaling the remote relay box and its circuits. A DB9 cable is lightweight, easy to solder, comes in many lengths, and can support up to a 8 pads without continuity(8 pads + 1 ground). A DB25 cable would be another good choice, and could support additional features like continuity along with 8+ pads.
Below are construction photos.
Click to Enlarge Photos
|Box with mounting plate and 1st terminal strip.||Negative in, wired to 10A fuse then to terminal block(4 positions on left).|
|Lower area complete with positive in(feeds to lid term block, DB9 wired to 4 right positions of term block.||Lid with holes drilled and filed. (Master arm on right side)|
|Lid with switches mounted. Temporary screws in center.||Underside of lid with wiring up to pad select switches, Note circuit board and upper term block wiring, jumpers and standoffs.|
|Underside of lid complete.||Lid connections to lower portion of box.|
|Relay box innards. Top term strip feeds positive power through relay. Middle term strip feed negative to plugs on lid. Lower strip is a common negative signal from the relays and back to control box via DB9.||Relay box showing plugs in lid.|
|Relay box finished.||Controller box complete.|
|Controller box showing red armed LED.||Controller box showing armed LED and one pad selected.|
|Controller box showing armed LED and all four pads selected.||Wide view of controller box showing armed and one pad selected, battery and serial cable.|
|Wide view of relay box showing serial cable, battery, and ignitor lead.||Overal system shot, showing both control and relay boxes, serial cable, batteries and ignitor lead.|
All of the components in the project were purchased from either Radio Shack or Digi-Key. Digi-Key is a LARGE electrical part supplier and will have just about anything you may need. All Electronics is another good parts house.
The most difficult part of the build was finding a box big enough to house the wiring without breaking the bank. I bought the largest plastic box I could find in the Digi-Key catalog. Other, larger boxes were available, but they were all metal(nice) and expensive - $35 and up.
Although, I could have wired the boxes more efficiently and saved space, I tried to keep things neat and reduce stress on components when opening and closing the cases. This is why I used terminal strips wherever I could.
|Qty||Description||Supplier||Part #||Cost||Total Cost|
|1||Project Box. 8x7x3, Relay Box||Radio Shack||270-1809||6.99||6.99|
|1||50 ft DB9 Serial Cable||CablesToGo||09453||28.99||28.99|
|4||Relay, 12vdc, 30 amp||Radio Shack||275-226||6.29||25.16|
|1||Fuse Holder 10amp||Radio Shack||270-367||2.29||2.29|
|1||Fuse Holders 30amp, Auto Style||Radio Shack||270-1234||2.59||2.59|
|4||Toggle Switches - Pad Select||Digi-Key||360-1191-ND||8.37||33.48|
|1||Rocker Switch - Master Arm||Radio Shack||275-690||2.59||2.59|
|1||Momentary On Sw, Fire Switch||Radio Shack||275-1566||2.69||2.69|
|1||SuperBrite LEDS, Red||Digi-Key||67-1611-ND||0.46||0.46|
|4||SuperBrite LEDS, Green||Digi-Key||67-1755-ND||2.73||10.92|
|5||LED Holders||Radio Shack||276-079||0.26||1.29|
|5||1K Ohm Resistors||Radio Shack||271-1118||0.20||0.99|
|1||DB9, Female, Solder Cup||Radio Shack||276-1538||1.59||1.59|
|1||DB9, Male, Solder Cup||Radio Shack||276-1537||1.59||1.59|
|2||Banana Plugs||Radio Shack||274-721||2.59||5.18|
|2||Banana Jacks||Radio Shack||274-725||2.59||5.18|
|2||110v Std Wall Outlets||Electrical Supply Store||0.97||1.94|
|1||Double Gang Face Plate||Electrical Supply Store||5.00||5.00|
|4||Alligator Clips||Radio Shack||270-347||0.82||3.28|
|4||Extension Cords||Big Blue||2.00||8.00|
|5||Terminal Strips and Jumpers||Radio Shack||274-659||2.49||12.45|
|4||Terminal Block Jumpers||Radio Shack||274-650||1.99||7.96|
|1||PC Board for Resistors, small 2x3||Radio Shack||276-150||1.79||1.79|
|1||Misc Wire, connectors, etc||30.00||30.00|