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Continuity Light?

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Continuity Light?

Unread postAuthor: ammosmoke » Mon Jan 25, 2010 7:42 pm

Ok, so I want to launch multiple model rockets individually from the same board. I have devised a scheme that would work using CAT6 wire, so I should be able to connect 5 ignitors to a common ground (the 6th wire). Each of the 5 firing circuits has its button or momentary switch. Each has a continuity light to let me know when the circuit is firing. However, I don't know how to make the ENTIRE setup have a continuity light. Using my current diagram, there is a master switch that gives power to all the circuits. I want a light to illuminate when I turn this switch on. How do I do that without stealing the power from the sub-circuits? BTW, I will only be firing one circuit at a time.

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Unread postAuthor: jimmy101 » Mon Jan 25, 2010 8:13 pm

"how to make the ENTIRE setup have a continuity light"

You mean you want a "Power On" light?

Light bulbs are resistors and are generally self limiting so you should be able to just wire a light bulb from the red wire after the main power switch to the ground (black) wire.

It the light bulb pulls a significant amount of the power supply's power then you either need a larger power supply or a lower current light bulb.

If you really want to drop the power drawn by the power light you can use an LED with a series resistor. LEDs typically draw about 20mA which is pretty insignificant. (A fressh AA battery will supply nearly 10,000mA for a short time.)

Take the voltage of your power supply and subtract 1.5 V. Divide that number by 0.02 and the result is the size of resistor needed to keep your power supply form burning out the LED. An resistor within about 30% of what was calculated should be OK.

It would be helpful to konw what the power supply is (votage and current) and what types of lights you are using for the continuity lamps.
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Unread postAuthor: ammosmoke » Mon Jan 25, 2010 8:22 pm

Yes, I meant a "Power On" light.

It's really that easy? :lol:

I'm planning to use automotive brake lights. Or any 12v lamp that I find which happens to be cheaper. I want to use a 12V DC transformer if I can find one around here. I know I have a 15v but that is a little high I think...

What do you think?
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Unread postAuthor: ramses » Mon Jan 25, 2010 8:32 pm

yep, it is that easy. Depending on the resistance of the ignitor and the maximum current the power supply is capable of, the light might go out while the button is pressed, until the ignitor burns out. Voltage doesn't really matter, but it would be best not to overvolt the lamp. You could probably run the whole thing off 12v AC if you had to. If worst came to worst, you could put diodes in series with the 15VDC transformer to drop the voltage by ~1.5V each. Just make sure they are rated for a decent amount of current.
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Unread postAuthor: ammosmoke » Mon Jan 25, 2010 8:36 pm

Cool, thanks guys.

The transformer is capable of.... lemme check...

15.95VD and 8.4W

No mA given...
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Last edited by ammosmoke on Mon Jan 25, 2010 8:41 pm, edited 1 time in total.
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Unread postAuthor: bullrees » Mon Jan 25, 2010 8:37 pm

By "light" do you mean LED? What power source are you using?
If you are using a LED the voltage used if you wired it from source to ground after the main switch should only be about 1.2-1.5 volts depending on the diode.

That's what I would do. Lowest possible drain..and if it breaks(shorts or opens) the result would not launch.

EDIT: too late lol
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Unread postAuthor: ammosmoke » Mon Jan 25, 2010 8:49 pm

15.95VDC* not VD... I'm sorta wondering how I'm supposed to figure out what current the thing puts out...
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Unread postAuthor: bullrees » Mon Jan 25, 2010 8:55 pm

you could also use a double pole switch on the individual circuits that turns off the main light when pressed if current is the main concern.
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Unread postAuthor: ammosmoke » Mon Jan 25, 2010 9:43 pm

bullrees wrote:you could also use a double pole switch on the individual circuits that turns off the main light when pressed if current is the main concern.


Meh, as long as it works I don't really care. These ignitors can be fired up with less than 9 volts.
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Unread postAuthor: rp181 » Mon Jan 25, 2010 9:47 pm

watts= voltage x current
so 500ma
should work fine.
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Unread postAuthor: Technician1002 » Mon Jan 25, 2010 10:33 pm

I would recommend using an LED with a series resistor wired across the fire buttons for each rocket. The current limit resistor would be high enough resistance to limit the current through the ignitor to a low enough value so it can't ignite. Depending on your ignition voltage, the resistor value needs to be calculated to limit the LED current to a value so it will light, but not run in overcurrent. For 5 volt systems, 470 ohm is common for indicator lights (red LED's) and 1K to 1200 ohms is common for 12 volt systems.

In commercial (proffessional ) fireworks and blasting industries, a higher voltage stored charge system is often used which uses a charged capacitor much like a camera flash for guranteed ignition to prevent miss fires from low battery supply current or long wire lead resistance. These use a low voltage continuity check and a high voltage fire current. The advantage is if the battery has enough current to charge the cap, it will fire. If the battery is low, it won't charge and provide an arm signal. This prevents miss fires.
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Unread postAuthor: jimmy101 » Tue Jan 26, 2010 4:09 pm

Like Tech said, LEDs are a better way to go, though I would drop the resistance by a fair amount. The LEDs won't last as long but they'll be much brighter. If this is for night use the brightness of the LEDs won't matter much. In full sun, a standard LED at the standard current (20mA) will be hard to see. Boosting the current (to 25 - 30 mA) drops the lifetime but I figure the LEDs total lifetime usage in this app is probably only an hour or two, which is nothing for an LED.

Filling out Tech's suggestion (I think), attached is a sample circuit using LEDs. It assumes that the resistance of the igniter is much less than the resistor for the LED. If the igniter resistance is less than about 50 ohms you should be good. The actually value of the limiting resistors should be fiddled with a bit to get the LEDs as bright as possible without cooking them instantly. RadioShack should have all the needed parts. The LED, resistor and push button for each channel will probably run about $1.50 total.

The design uses all push button switches for safety reasons. The main "Enable / Test" button should be well separated from the others and a protective cover would be a good idea.

Pushing the "Enable / Test" button will light up all the LEDs for launch channels that have continuity. If there is a wiring problem for a particular channel, it's LED won't light up. If the channel has already been fired it's LED also will not light up.

To launch you push the "Enable / Test" button and the button for the particular channel you want to fire. Requiring two button to be pushed at the same time is a good safety setup.

For power, the best thing would be a smallish 12V sealed lead-acid "gel" battery. They can usually be gotten pretty cheap and aren't too big. You recharge them with a standard car battery charger. The car battery charger is also a good power supply as long as it has a "manual" type setting that bypasses any logic that controls it's output. You can also recharge the battery by hooking it up to a car using a set of jumper cables.

I've salvaged nice sealed lead-acid batteries from uninterruptible power supplies for computers. They are also used as power backups for home alarm systems. All Electronics has ones for as little as $10.
http://www.allelectronics.com/make-a-st ... ERY/1.html

All Electronics would also have the switches, resistors and LEDs for a heck of a lot cheaper than RadioShack.
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Unread postAuthor: jhalek90 » Tue Jan 26, 2010 7:17 pm

Perfect diagram

That is exactly the way i would do it, and i tend to dabble a bit in electronic circuits 8)
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Unread postAuthor: Technician1002 » Tue Jan 26, 2010 10:48 pm

A key switch is recommended to prevent onlookers from launching anything during your setup. Keep the key in your pocket while setting up igniters.
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Unread postAuthor: jimmy101 » Wed Jan 27, 2010 11:31 am

A key switch is a cool idea (and a safety factor so it's a double bonus). I would still go with a pair of pushbuttons in addition to the key switch.
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