Some time ago, I decided it was time to start considering dual timers. Considerable amounts of time go into building the rocket and rocket electronics, and it would be nice to have a back up for deployment. While many rockets use dual deployment, I simply wanted a deployment back up. I considered using a single 556 timer, and running two deployment charges off it, but that wouldn't be as reliable as two separate timers. I also wanted a place for my FM transmitter to be mounted, so I decided a dedicated electronics bay was in order. I wasn't too concerned about the added weight, I am more interested in designing things that are dependable, I'll worry about maximum altitude later.
I'm writing this page a little after the fact. The electronics bay was built over a month ago, and has already been flown in Launch Test 24. But I didn't use the bay in Launch Test 30, and the result was a lost air frame. I think the failure in that flight was the deployment charge, I think it was Pyrodex, rather than black powder. It was simply my failure to label the charge that was at fault, but it reminded me of why I wanted dual timers.
The support structure was all made from 1/2" plywood, I know that's overkill, but it's what I had on hand so I used it.
The bottom of the bay is to the left. On the far left is two pairs of wires, they are for the pull wire to start the timers timing. There is an eye bolt on the bottom to give me something to grab on to pull the bay out of the body tube. The first battery is for the FM transmitter, which is just above the battery. The transmitter antenna is hard to see, but it zig zags to the top of the bay. The next battery is for the first timer which is just above the battery. At the top of the bulkhead is a socket to hold the mini bulbs I use as deploy initiators. You can see there is a bulb in the socket. I put an intact bulb in the socket for testing, when flying, there would be a deployment charge in its place.
This is the other side of the bay, it holds the second timer.
In this picture you can kind of see it all. You can see both bulbs in their sockets for each timer. I need to describe how this thing fits in the body tube. I should have taken a picture of the upper bulkhead, but I didn't and it's in the body tube now, so it's staying there. The bay slides in from the bottom of the upper body tube until it rests against the upper bulkhead. I cut a hole in the upper bulkhead so both of the deployment charges stick through. I use a piece of foam neoprene as a gasket between the bulkhead and the bay. Once the bay is in place, I drive two screws from the outside of the body tube into the plywood of the bay to hold it in place.
It seems to work well. The electronics can be turned on and the rocket assembled in just a minute or two. I used two different connectors for the pull wires, so I couldn't hook them up wrong. I tried tying both pull circuits to one single pull wire, but it didn't work well. The first timer to time out would reset the second timer, so it would go off late. I thought about adding a couple of diodes to the pull wires. But decided to keep it simple and use two separate pull wires.
I was going to use a pull wire on one timer, and a g-switch on the other. I haven't had a chance to make a g-switch yet, but when I do I'll go that way.
In the future I will build all of my 3.25" diameter rockets to hold the electronics bay. Then I can use the same bay in different rockets, or build other electronics bays with different payloads and they will interchange with any rocket.
One last thing I plan to do before using the bay again. I am going to put a small aluminum plate between the deployment charges. I'm worried when one goes off, it will set off the other. Not that it would really matter, but I don't need the nose cone coming off like a cannon ball!
A new electronics module has been built for the Cosmo rocket project. I decided to move the content here.
Electronics Module:
I decided I needed another electronics module anyway, so I might as well get one built for the Cosmo project that had all the features I wanted. There are only a couple of real differences in this module. First of all, I wanted power switches to each timer, so I could turn them on only after the rocket was prepped and on the launch pad. Second, I wanted dual deployment capabilities at both ends of the module, that would mean two deployment sockets at each end.
I cut the module frame out of 1/2" plywood, exactly the same dimensions as the first module so they would be interchangeable. The axial section was rabbited into the round end pieces, and glued and screwed in place. Four new deployment sockets were installed and epoxied into place.
I decided to mount my three new toggle switches on an aluminum plate, so I cut a plate to size and drilled it for three switches and three LED's. The LED's will light when the switch is flipped to the on position. The LED's are not happy with 9 volts, I think most of them are best at about 2.7 volts. So I installed 300 ohm resistors between them and the 9 volt source. I had to cut out a section of the plywood to give me enough room to mount the switches, so the wiring is accessed from the other side of the module. It worked out well, because the timers will be on the same side as the wiring. I mounted one battery on the switch side, and the other two on the timer side.
Here is the switch side. You can see the illuminated LED's indicating power on. The switches were mounted with "on" in the down position. I seriously doubt that g-forces would be able to flip the switch, but better safe than sorry.
Here is the back side of the switches. You can see the other two batteries and the battery retainer.
Here is a close up, you can see the resistors between the switch and the LED's.
I've spent 15 or 20 hours now on the new electronics module. I'm going to refer to the modules now as EL1 or EL2, as in electronics module 1 or 2. I'm not going to go into all the sorted details of construction, but rather give an overview of what I've done.
This was kind of an Oh, duhh thing. I noticed this after I had everything wired up and working. When either switch one or two was turned on by itself, it would power up both LED's. I wondered for a moment. Then the Oh duhh. I was running both switches to the g-switch, so when either switch was turned on it would power the other one. Not that it was a big deal, but then I could just as well only used one switch, which would have been fine too. So I soldered a couple of diodes in the power lines to keep the power flowing in one direction only. Easy fix.
Here is a side by side of em1 and em2. The only thing added to the switch side so far is a large piezo buzzer. It's on the same side of the module now as the transmitter will be, so it should give a little better volume.
Here is the timer side of em2. At the very top is the first timer. It is activated by a pull wire, and the main power is turned on by switch 3. The timer will initiate one deployment charge on the top, or one deployment charge on the bottom. Depending on if the module is to be flown with dual parachute or single parachute deployment.
At the very top of this picture you can see a new timer. This board has two separate timers on it, each with its own 9 volt battery. I mounted both on one board to save space. The timers are both started from the g-switch below the timer. Main power switches 1 and 2 are used for these timers. One timer is for the drogue deployment charge at the bottom of the module, the other timer is for the main deployment at the top of the module. Below the timer is the new g-switch.
Here is the back side of the switches, and the mess of wires going to and from it. I dropped some hot glue around the LED's, to make sure the g-force from launch didn't move any wires together. I used round staples to hold the wires in place.
After everything was in place I gave it a good test. Shook it up, banged it on the counter top, realized I didn't have a retainer for the single battery when it flew out, and tested all the circuits with intact light bulbs in the deployment sockets. Satisfied, I think the module is ready to fly. I will move the FRS radio to this module, or perhaps if I can find another radio similar to it I'll just buy a new one.
So here's what I've got.
Three timers, one to deploy a drogue chute at a time up to 63 seconds started by a g-switch. Another to deploy the main chute at up to 200 seconds started by a g-switch. The final timer is a 63 second maximum back up drogue timer started by a pull wire, it may also be diverted to the top as a back up for main chute deployment for low altitude flights not using a drogue.
The piezo buzzer serves several purposes. First, it provides an audible warning the timing circuit is running, second, it provides an in flight indication (via the FRS radio) the timing circuit has started. Third, it provides a tone on the signal to make locating the downed rocket easier. And lastly, it would provide some sound to help find a rocket in tall grass or crops.
The main power switches allow for complete assembly of the rocket prior to setting the rocket on the launch pad. This is for safety, as the g-switch could start from a bump of the rocket, or the pull wire could accidentally come off prematurely. LED lights indicate a power on situation, making it easy to know when the circuit is active.
There is still room for at least one more board for future projects, perhaps a recording altimeter.
