Designing a custom launch controller?

Oh, I should point out, for the LEDs, you need to have the red lead on the positive side, and the black on the negative side - if you put them in backwards, they won't be damaged, but they won't work either.

One thing which is missing is a "battery hooked up backwards" indicator. Since the LEDs won't work if the battery is backwards (though the firing circuit will work just fine), you probably want to make certain the battery is hooked up correctly. If your jump starter has an accessory socket (cigarette lighter...), you can get a connector for that from RadioShack and use it. Or, we can work a diode and buzzer into the circuit which will buzz if the battery is hooked up backwards. Or, you can color code the terminal clips. On my controller, I have red & black clips and just do it correctly.

Kevin

Just some thoughts~

I'd make sure the LEDs have current limiting resistors built in. Most that are sold already in panel mounts do have the resistors, but it's worth verifying before closing the switch that pushes the full amperage of your car battery through them!

This very worthwhile article should be mandatory reading for anyone building a launch controller. It's what pushed me over into installing transistors for the continuity circuit. And the article gives a good understanding of what ignitors you can (and can't) use with whatever you choose to build.

In the meantime, I will return to watching out for the zombies...!


All the best, James

BTW, attached is a simple transistor-driven continuity circuit. This will limit the continuity check to 2.38 mA, far below any ignitor threashold. I believe the 2N3906 costs $0.95 IIRC.


View attachment SimplePNPContinuityCircuit.pdf



All the best, James

kruland said:

Instead I bought a mono headphone jack and mono headphone plug to use. In the headphone plug, you solder a wire bridge across the two terminals so it operates as a short. When the plug is in the jack, the circuit is complete.

Kevin

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So looking at this thing....I think it's pretty obvious what you are saying. Just bridge those two screw points with wire?



Now this thing is a little more confusing. There are 3 leads going off of it not really labeled at all.

Green Jello said:

View attachment 108433
So looking at this thing....I think it's pretty obvious what you are saying. Just bridge those two screw points with wire?

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Yes, bridge between the screws with a piece of wire. You'll want to fashon some kind of "handle" to pull on

Green Jello said:

View attachment 108434
Now this thing is a little more confusing. There are 3 leads going off of it not really labeled at all.

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Yes, that is confusing. You're probably going to have to experiment some. I suspect the third tab is connected to the nut which attaches it to the panel and in an audio system would be connected to ground. You will only need to use two of them. To figure it out, I would first bridge the plug, then insert it into the jack. If you had an ohm meter, then you could figure out which two are shorted (by the plug) pretty easily. You can probably do this with one of your LEDs and a 9v battery. A 9v should light the LED dimly (you should check first). Then use the led and battery to figure out which pair of tabs are shorted by the jack. Once you find the correct pair, those you will use in the controller. Sorry, I can't be more certain.

Yeah do exactly what Kevin's said. we're section mates. he knows exactly what he's doing. I'm currently working on a launcher of my own. 3 pad thing with lights on the rack and the controller. Using an 8x8 inch pencil box from Michael's to build the controller in. its based on a schematic that I've posted on the NAR facebook page. Kevin really knows his stuff and has helped me out a ton with mine.

kruland said:

...


Yes, that is confusing. ...

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It appears to be a switched audio jack. Inserting the mating plug will spring a pair of the contacts apart, and will open a circuit. I often use such a jack for the charging plug of an internal battery, as when there is no plug present, battery is connected to the control circuit, and with the plug present, power is disconnected from the circuit and the battery is directly connected to the external charger.

I thought it might be a stereo jack but wasn't so certain. In any case, if you already bought it you can still use it. The mono plug fits mechanically and will connect with two of the three tabs. The tip will connect with the third. Just pick a pair of tabs which works and use them.

Ain't rocketry fun... you can break the rules and still get away with it.

Kevin

Actually Kevin, I just had a closer look at the picture, and in fact it is NOT a stereo jack. It is - as John noted in post #69 - a switched jack.

Sorry about any confusion I may have caused in my (now deleted) post earlier.

s6

put the red lead of the LED to the key switch, and the black lead of the LED after the launch button (same wire as goes to the igniter).

The LED should basically be a short around all the switches to the igniter.

Green Jello said:

Ok...I came home and rigged up a crude prototype on some basswood. It actually works perfectly except for one minor thing.

<img src="https://www.rocketryforum.com/attachment.php?attachmentid=108458"/>

I even tested it on an Estes igniter with success. I just can't figure out where to connect the continuity LED leads. I was able to connect them according to Kevin's diagram (I thought) and it lights up. The problem is that it doesn't function as a continuity light. It remains lit if the igniter is connected or not.

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I've never seen one in person - but I do notice a little metal tab on the tip end. You say it's open when the plug is out and closed when the jack is in? If so, that's great because then you don't need to short the plug at all, it simply becomes a mechanical activator.

stealth6 said:

Actually Kevin, I just had a closer look at the picture, and in fact it is NOT a stereo jack. It is - as John noted in post #69 - a switched jack.

Sorry about any confusion I may have caused in my (now deleted) post earlier.

s6

Click to expand...

Kevin;

It appears to me to be the other way. That Jack is normally closed, and when the plug is inserted its tip presses the hooked tab at the back down, opening up the contact between that hooked tab and the one just above it.

John Brohm said:

Kevin;

It appears to me to be the other way. That Jack is normally closed, and when the plug is inserted its tip presses the hooked tab at the back down, opening up the contact between that hooked tab and the one just above it.

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Ah, yes. That is what it looks like and makes sense.

John,

Since we have your attention, do you think there is a problem with an led in parallel with the igniter used to indicate 'launching'?

Kevin

Well the main problem would be that the LED would never illuminate, as the igniter would bypass the current i.e.: short out the LED, at least until the igniter failed.

John Brohm said:

Well the main problem would be that the LED would never illuminate, as the igniter would bypass the current i.e.: short out the LED, at least until the igniter failed.

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I keep thinking of the igniter as a component with meaningful resistance. The only way to make it work would be with transisters then, right?

Green Jello said:

While I have you here....seeing how ridiculously simple these things really are, does anyone have good plans for a home made controller. It might be a fun project.

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I built a Ghetto-Brand(tm) 4-pad launch controller based on Mike Harris' design a few years ago for a Scout Camporee (taught Space Exploration Merit Badge). I don't seem to have photos of it online (PM me if you want them), but the "schematic" is relatively simple to follow, and I listed the parts out for anyone else to build one. Basically, all of this will easily fit inside a $2 hard plastic Carlon project box from Lowe's or Home Depot, and you can either use speaker connectors or do what I did and mount up an 8-wire towing hitch connector that just plugs in. (Photos would help -- sorry don't have those readily available online in not-8MB-each-file form... PM me if seriously interested.)

https://www.sschnege.org/scott/Rocketry/MHarris_Launch_Controller_Schematic.jpg

kruland said:

put the red lead of the LED to the key switch, and the black lead of the LED after the launch button (same wire as goes to the igniter).

The LED should basically be a short around all the switches to the igniter.

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That did it. Thanks! Now just time to start building the real setup in the project box.

Good to hear! Send pics.

Today I drilled the holes and installed the parts to fit in the case. Next I need to sand, prime and paint the metal face.

Looking good!


All the best, James

Now wasn't that easy! It certainly looks good.

kruland said:

I keep thinking of the igniter as a component with meaningful resistance. The only way to make it work would be with transisters then, right?

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Yes, the ignitor has resistance... that's why it heats up when you deliver full battery power to it... but you have to have resistance ELSEWHERE to prevent firing off the ignitor prematurely--

That's why Quest ignitors will fire off as soon as you insert the key with an Estes controller-- the flashlight bulb has too little internal resistance and thus allows too much current to pass, exceeding the "firing current requirement" of the Quest ignitor. The Estes ignitors can pass more current without firing (and thus require more current to fire when the button is depressed. If you have an LED it will choke down the current to below levels that would fire off the Quest ignitor, and MUCH below that required for an Estes ignitor.

Thing is, the semiconductor bridge in an LED is inherently low resistance, in and of itself... so if you're using more than just a few volts, you need a resistor in series with the LED to prevent the semiconductor bridge in the LED from prematurely burning out... If you've ever hooked up an LED to a 12 volt power source without a resistor, you'd see that it 1) burns much brighter than normal, BUT 2) the color is off, a rather sick color compared to the standard LED color... this is because the voltage is pushing too much current through the LED. An appropriate sized resistor in series with the LED will "choke back" the current to levels safe for the semiconductor bridge to pass...

"LED indicators" from Radio Shack already have the appropriate resistor soldered in to the LED... and they work great...

later! OL JR

I've never hooked up an LED to a 12-volt power source before, but the last few times I hooked one up to a plain 1.5v alkaline cell, the plastic lens blew off and bounced around to who-knows-where. None of the magic smoke remained inside the LED, unfortunately...

CarVac said:

I've never hooked up an LED to a 12-volt power source before, but the last few times I hooked one up to a plain 1.5v alkaline cell, the plastic lens blew off and bounced around to who-knows-where. None of the magic smoke remained inside the LED, unfortunately...

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Interesting... I've never had one blow up, but if you hook it to a power source without a resistor you can DEFINITELY tell that it won't be long for the world in that situation...

Later! OL JR

After a long break for holiday travel, I finally finished the paint on the plate.

Pieces are in, but not wired up yet.

Probably the best looking controller I have seen poster here. Nice job.