Help with 12 volt rocket igniter

Hi all,

I was looking into building a simple (but hopefully cool) model rocket igniter. Yeah I'm bored, but stupid as a brick. About the only thing I know about electricity is 110 volt hurts when you touch it. :) Luckily this is only 12 volts. ;)

You can view the plans here:

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If you scroll down and hit "contents directory" it will show you the "how it works".

How does the lightbulb limit the current that is going through the circuit without igniting the engine igniter? I am assuming from what I have read is anything above 6 volts, ignites it like a fuse blowing.

I would love to build this unit, only I would like to change the incandescent bulb to an LED in addition, add a piezo for an audible sound as well. Where should the piezo be added in the diagram?

Can anyone shed some light for me on this? Thank you for any replies and have a great night.

Reply to
ULB
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For this to work, the light bulb has to have a much higher resistance than the igniter so that it limits the current. When you arm the box with the key switch the power is applied through the bulb to the igniter. The bulb, having a much higher resistance than the igniter, takes all the voltage, and lights up leaving very little current for the igniter. When you close the ignition switch you short out the bulb, the full 6V gets applied to the igniter, and the rocket (we hope) goes off.

Safety disclaimer: Rockets can kill an/or injure, I'm not getting paid, safety is your responsibility, if you're under 18 I didn't write this, you hurt yourself it's your dime, yadda yadda.

If you can get a piezo that will work on 6V you can hook it up in parallel with the lamp or LED. As long as it doesn't take too much current it'll sound without igniting the rocket. You can hook up the LED where the lamp was with an appropriate series resistor in series with the LED.

You should be able to put this together so that an LED and a piezo together will consume less current than just the lamp, and that's what you should aim for. I very much doubt that you can get detailed specifications on just what makes your igniter fire off, so you're reduced to either blindly copying what's already been done, or trying to pry safety information out of a manufacturer (they'll tell you: "just use our box, we don't release that information, yadda yadda". They'll do this because their product liability insurance requires them to).

Whatever else you do, do some research into exactly what makes these igniters fire, and do some measurements to make sure that your box won't make them fire unexpectedly. Whatever box I built, for each one I'd check it with an ammeter first, to verify that the current was too low to fire an igniter until all appropriate buttons/keys/wingwazzles were appropriately activated. In fact, if I were good I'd do this each time I was getting ready to set out to fire off some rockets.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

If the referenced plans are correct, then the lamp they use draws 150 mA when lit, and around 500 mA when cold. It would be wise to reduce that current.

When you use the LED as you plan to do, it will draw

*way* less, and that is safer. Modifying the diagram at the site to include the led and piezo:

/ +---[igniter]---o o---+---[330R]---[LED]---+ | SW1 | | | | | | +------[Piezo]-------+ [Batt] 6V | | | | / | | +--------o o--------+ | SW2 | | | +-------------------------------------------+

Use a piezo that draws 10 mA or less, such as CAT# SBZ-365 from AllElectronics, and the total draw for the LED and piezo will be under 20 mA. You can go to a 12 volt battery if you want, and change the resistor to 1K.

See and follow all the safety points Tim mentioned.

Ed

Reply to
ehsjr

Tim and ehsjr,

Thank you for responding. I really am having a hard time grasping the resistor. I understand it reduces voltage etc, but I do not understand if it is like a hose, whereas the ouput is less, and anything prior to that resistor is the original voltage.

I just drew up a plan based on the above rocket launcher. I was at work today and read the internet instead of working. Don't tell my boss. :) You can tell I didn't comprehend much.

In any case, I have changed the design slightly, well probably alot. The launcher is now to be keyed, when turned in the 'on' position, it shows the voltage in the battery(s), lights an LED and buzzes a piezo. There is no longer a connectivity test to the igntiter (probably safer I would hazard a guess). The launch button lights an LED and then shorts the igniter and hopefully the rocket launches. How do I keep the "launch" button LED Lit even if it doesn't fire? I think I am missing a wire.

Thanks for the patience. Here is the link to my umm err drawing.

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Reply to
ULB

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Reply to
ULB

You got it.

Potential (pressure) is in the main hose, the resistor is like a capillary tube and only allows a small bit of current (water) to flow and go back to the source (other terminal of the battery).

Same concepts as water flow. A high pressure can send a stream further, a high current means a lot of water is moving.

The battery is the source of pressure. When the led is in series with the igniter it is like a tiny capillary and relatively little power is allowed to go to the load (igniter). Close the "fire" switch and the full battery power goes (more current is allowed to flow) to the igniter and it gets very hot and ignites the fuel.

There are still risks involved even if you minimize the current in the wires while checking the "armed" state with the tiny current. Transmitters and even lightening flashes can induce (couple magnetically) enough voltage into your squib/igniter to cause it to fire unexpectedly. The reason you see signs "Blasting - turn off all two way radios."

Your propellant is less active than some explosives, but do be careful. The battery should be the last thing you connect and then well away from the rocket. In my youth we used fuses and spark coils to ignite our rockets and cannons. If the rocket doesn't fire disconnect the battery and wait before investigating. An ammeter in series with the squib will tell you that current is going to heat the squib and you should see it go to zero when the squib burns out.

Your drawing leaves a lot to be desired. It looks as if you have a wire (pencil line) around both the armed indicator and squib - if that's the case neither will work since the wire has a lower resistance than either the armed indicator or igniter and electricity will flow via those routes in preference to the way you want it to go.

The ASCII schematic someone already sent it the way to draw and wire it and much more clear than block drawings.

Another good hobby of my youth was hydrogen filled balloons - all it takes is lye (caustic soda) and aluminum foil, some plumbing to contain the reaction, cool and direct the gas to a balloon. At night we sent up flares with long fuses and the balloons would be over a mile away before the flare ignited and eventually balloon. Big yellow flare in the sky and satisfying thump a few seconds later. We had a wide river and would watch the wind direction so there was little problem if the flare detached before it was expended. It is dangerous but more so to the people and objects downwind. disclaimer: Don't do it.

Reply to
default

I am not sure about the second link , because is asked to sign in and showed no image. The first will fire a rocket but the ignition led will never light because it is bypassed by (near) 0 resistance. You should hook up the launch button across the LED and it's resistor. Now when you try to launch and nothing happens, the led will tell you ( after releasing the button ) if the fuse is still good or not ( it is possible for the fuse to go off and the fuel not to ignite ) .

Cheers

(WOOOHOO another vicarious experience)

Reply to
Sambo

Oh one other tip about taking closeup pictures, tape piece of thin mate paper ever / in front of the flash. Loosely hanging out and over toilet paper works for me.

Reply to
Sambo

A slightly different, perhaps better, version: / +------o o--+-----+---+---[580R]---[LED-G]-+----------+ | SW1 | | | | | | | | | / | | | [580R] | +--------o o--------+ | [Batt] 6V | | SW2 | | or | [Piezo] [ignitor] | 12V | | | | [LED-R] | | | | | | +------------+-----+-----------------------------------+

SW1 should be a keyed switch which can only have the key removed when off. The key should always be in hand when approaching the rocket.

The LEDs will light, albeit not very brightly, with 6V. In my experience, 12V is more reliable for launching, and easier to find. If doing a dedicated 6V version, go back to the 330 or 280 ohms.

In this version, LED-R (red) and the annoying noise both indicate that the arming switch (SW1) is on, and that there is power. If the annoying noise is on, don't approach rocket - that's an improvement (IMHO) over Ed's version in terms of safety, if using a noisemaker. The current from those does not pass through the ignitor.

LED-G (green) indicates a good connection of the ignitor wires (though it can't tell a short from an ignitor, so be careful when connecting wires at the rocket.) SW2 is the launch pushbutton.

Regardless of what circuit you use, remember not to approach the rocket for at least a minute in the event of a launch failure. Sometimes they smolder and go off.

If you like, you can replicate the right side of the circuit (beyond the piezo) to make a multiple-rocket launcher (where you'd set up 2 or more launch rods and rockets, and launch either at the same time, or in quick succession). In practice I've found that this is not usually very useful, as it increases the likelihood of losing the rockets, trying to track more than one at a time.

--
Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

Thank you all very much.

I went down to the local radio shack and got a bread board. Wired it all up. Everything seems fine. I do have two questions.

When I push the launch button all LED die off If an igniter is in place). I even tried using 12 volt incandescent lights minus the resistors and same problem. I am assuming this is because of the "short"? Is the above by design? Perhaps I mis wired it but I do not believe so. I played for 12 hours today. LOL.

Second question: While trying to mimic the Igniter (I went through 8 of them), I just used a jumper cable, same issue, all lights go off. However, if I leave the jumper on, boy does the battery (12 AA 1.5 volt) get hot. To the point I can tell something is definately amiss. I went back to Radio shack and got some diodes. Didn't fix the issue. I "Worry" about this as the final solution would be to use my car battery, and I certainly don't want to fry anything in the vehicle

Thank you all for any responses and helping a newb.

Reply to
ULB

Yes, the circuit I provided is designed to turn off the LED when the launch button (sw2) is pressed.

Use a 12 volt tail light bulb in place of the igniter for your tests. See below for what the indicators should do.

With the 12 volt bulb installed in place of the igniter, you should be able to see (literally) if there is a wiring error. The 12 volt bulb must not glow until both switch

1 and switch 2 are on.

You mentioned 12 AA batteries - that gives you a total of 18 volts. Use 8 AA batteries to get 12 volts.

With a jumper installed in place of the igniter (not a good thing to do), the only way that batteries can get hot is if SW2 is in the on position. That assumes the system is "armed" by switch 1 in the on position. But aside from that, the jumper will short your batteries when switch 2 is on (again, if the circuit is "armed" by switch 1 being on). Don't use the jumper! You don't want to short your batteries, and you don't want them to get hot.

When you install a 12 volt bulb in place of the igniter, and use 12 volts to power the circuit, the indications should be as follows: (view in fixed font)

Condition INDICATORS SW1 SW2 LED PIEZO BULB ========= =============== off off off off off ON off ON ON off ON ON off off ON

Ed

Reply to
ehsjr

=A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0|

--
> >>> =A0[Batt] 6V =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0|
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Reply to
ULB

As Ed said, 12 1.5V batteries are 18V, not 12. Cut back to 8 (or 10 if nicad/nimh rechargeables at about 1.2V each).

It is normal for the the LEDs to drop out. In my version, LED-G will definitely drop out (SW2 shorts it) and LED-R and the piezo may or may not drop out (but will definitely dim) depending on the battery source impedance (internal resistance of the battery + resistance of the wires leading from the battery) and resistance of the wires running to the launch pad, as well as the ignitor resistance. When testing with no long wire (which adds some resistance) to the launch pad and AAs (particularly non-rechargeable ones, which have a relatively high source impedance) as the battery, I'd expect those to both drop out as well.

A light bulb is a better test item than a dead short/jumper.

With a car battery as the source and long wires out the launch pad, I'd expect LED-R and the piezo to stay on in my version - but it's not a big deal if they don't. The main thing to check for correct/safe operation is that nothing happens with the test bulb if SW1 is not on (pressing SW2 should have no effect), and nothing happens with the test bulb if SW2 is not pressed (SW1 on or off), and that the test bulb lights if SW1 is on and SW2 is pressed.

Use a lighter plug to connect to the car - that will put a heavy fuse in line with the battery supply, which I neglected to draw.

In any case, SW2 should not be held on for more than a few (1-5) seconds. If the ignitor has not gone by 5 seconds, something is wrong.

--
Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

I'm not sure where/how "best practice" fits in or what you have in mind by the phrase. The fact that the LED turned off gives you more information than if it remains on - it tells you that sw2 worked properly, and that a launch was attempted. If the LED stayed on when the launch button (sw2) was pressed in the circuit I posted, it would tell you that sw2 was not working, or that you did not press it hard enough. If it goes out when you press the button and comes back on when you release it, it tells you that the igniter in the rocket did not work.

So the question is, what do you have in mind when you want the LED to stay on - why do you want that?

Ed

Reply to
ehsjr

=A0 =A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0 =A0 =A0|

--
> >>>>> [Batt] 6V =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0|
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Reply to
ULB

Sounds like you are measuring the adapter voltage with a resistor (or resistors) in series with your meter leads. You will get very little voltage drop that way, because the meter draws an extremely small amount of current - roughly 1.6 microamps assuming a 10 megohm meter and a 16 volt adapter. The voltage drop is equal to the current times the resistance, so your 3 560 ohm resistors (1680 ohms total) would drop .002688 (1680 * .0000016) volts.

But, if you installed the 1680 ohms in series with an LED and connected the adapter, the LED would draw roughly 8.5 mA through the 1680 ohms, and the voltage drop across the resistance would be 14.28 (1680 * .0085)

Ok all that said, you won't be able to use the adapter with a 560 ohm resistor in series to provide _12_ volt power for the igniter. However, if the igniter will work at _16_ volts, you can use the circuit posted. Just replace the single resistor in the posted circuit with the three 560 ohm resistors you have, wired in series for 1680 ohms total. The resistor in the circuit posted lowers the voltage to the LED, but does not lower the voltage to the igniter.

I don't know what your igniter is rated for, but I'll bet that using 16 volts will be no problem for the igniter, at all. I don't know if your adapter will be able to provide the current that the igniter needs. If you look at the adapter, you should see its current rating on the label. The other piece of information needed is the current rating for the igniter. It would be helpful if you could post those numbers.

Ed

Reply to
ehsjr

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