Okay, i am fed up with the cheap estes launcher that we have been using to launch the kids model rockets, so i decided to build a new one for them (and go way overboard for me of course)

The idea:
power 12v Gel Cell (so it can power higher end igniters and multiple launches)
Must Haves:
1. Key switch for power (so when the cat is away, the mice without the keys can't play)
2. Continuity check circuit (to check to be sure that the igniter is completing the circuit.
3. a safety "arming" switch to make accidental firing not probable (my thought was a momentary switch that must be held closed when pressing firing button)
4. Proper indicators for power on, armed, and continuity. (my thought was blinking led for power on, led and buzzer/transducer for armed, and led and buzzer for continuity.

this is for the most part so it is safe, and adheres to the 2 model rocket association rules. (just a little overboard with the key switch and 12v)

Extras for dad
1. Multiple launch pad support (6) (for the cub scout troop to participate)
2. Race capability (multiple simultaneous launch, the reason i went with 12v, so when it is divided out, there is still enough voltage/amps to fire all stations)
3. Toggle switches for each pad to arm/disarm. each with a safety toggle cap to ensure that it is very clear which pads are armed.
4. nice to have but not necessary, LEDs indicating each pad that is armed, and each pad have it's own continuity check.
5. Volt meter on 12v when powered on to display battery voltage, so it will be easy enough to see when it is time to recharge
6. charging jacks in enclosure to charge battery without removing.

I bought a nice hammone sloped enclosure with plenty of room for all components, switches, meter, and battery.

I have the toggle safety tops for 1/2" toggle switches, and 12v panel mount meter.

I am having trouble designing the circuit. I need the power running through the circuit on continuity check to remain under around 50ma. The estes igniters can withstand about 150ma or so without firing, but some of the higher end ones will launch with much less current.

I was hoping to isolate the continuity to a separate portion of the circuit, isolating it from the launch pulse. My thought was a spdt toggle to set to test/arm, with arm being momentary for the safety (must be held to actually launch)

I also thought i'd isolate the power indicators by using a spdt pushbutton for hte launch button. that way the power indicators are active in launch and test modes, but when the acutal launch button is pressed, it moves power from the power and armed indicators so there are no extra components in the way of the launch pulse.

I also thought that each pad would have a spdt toggle to arm/disarm each pad. The problem i run into here is that this allows no pads to be armed, which will in essence allow for a direct short circuit for the battery without anthing in the path. (not sure how bad that really is, but i figured 12v with no limits might heat up the wire and risk damage).

I also can't think of a good way to have each pad have its own LED indicator of armed and continuity check. I've hit the limits of my meager knowledge and visualization.

any help would be appreciated. I will try to work up a diagram that i can post for critique.

Thanks (from both dad and kids)

ron

 

Take a look at an 8-station dual gel-cell powered launch controller here:

http://www.mn-rocketry.net/masa/tech/8 Pad Launch Controller.pdf

You'll need to have Adobe Acrobat Reader installed, available here:
http://www.adobe.com

Here's another with similar capabilities; construction of which would be much less expensive:
http://www.kevincook.net/Hobbies/Rocketry/LaunchController/index.html

Additions to either of these launchers you might consider is:
1) Dual "LAUNCH" buttons on opposite sides of the controller, wired in series. This will greatly reduce the chance of an accidental launch, as both must be pressed simultaneously to apply current to the igniters.
2) A 10-second countdown delay once the LAUNCH button(s) are pressed, connected to a LOUD buzzer/piezo transducer that beeps once a second during the countdown. Lifting the LAUNCH button(s) at any point during the countdown aborts the launch, and resets the count. You could use a 556 dual timer IC with external components to build a 1-second time interval clock signal, the 2nd half being wired as a 1-shot multivibrator with a (roughly) 1/4 second duration to blast the buzzer/piezo. The 1-second clock pulse can be used to drive a CMOS 4017 IC (Decade Counter/Divider), the O9 (pin 11) being used to turn ON a power transistor. Don't use the carry out (pin 12); as that output toggles halfway through the count.
3) If you want to get fancy, you could use the decade outputs from the 4017 to drive super-bright LED's in sequence to show the progress of the countdown on the control panel, and optionally a remote indicator for the budding rocketeers to cheer on.


Fun project

 

Instead of using gel-cell batteries, you might consider using a large capacitor, such as this one:
http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=14643

Problems with gel-cell batteries is that they're heavy, take a long time to charge, and have a service life of 2-3 years. This can be really inconvenient when you have a bunch of teenagers that want to go launch rockets NOW!

Large 1 Farad capacitors are MUCH lighter in weight, have a much longer service life, and can be charged relatively quickly from your vehicle's battery; just use a 12-volt bulb in series with the connection. This does two things; 1) limits the initial charging current to reduce the risk of overheating your wiring or damaging your battery, and 2) shows progress of the charging, when the lamp extinguishes the capacitor is nearly charged. The particular capacitor in the link above has a digital display for the voltage; very convenient.

One caveat; you must be sure to never short out the terminals of such a large capacitor when it's charged, as there is a tremendous amount of energy stored. Either remove jewelry, or put insulating tape over it. Shorting the terminals will likely vaporize the shorting item, as well as destroying the capacitor.

You can limit the maximum current output of the capacitor by using a power resistor in series with it. An 8 Ohm 20 Watt power resistor will allow a maximum of 1.75 amps of current when the cap is charged to 14V, plenty to fire a string of rockets. It will also prevent your wiring from burning up if there is a short somewhere. Radio Shack carries these resistors as Catalog #: 271-120, but you can get them lots of other places too.