Launch controller help

[Eric J. Seiberling]

I am in the process of building my own launch controller. I have came up with a schematic and am looking for more input on what I might need to make it safe and reliable.
I have included a safety key and indicator lights. My question is the continuity and not setting off the ignitors.

Diagram is attached!

Any help would be greatly appreciated

 

[jqavins]

I'll get to answering what you asked in a moment, but first I must ask you why you are building your own. What is it you are out to achieve that you can't with a COTS unit? I'm not trying to imply that you shouldn't do it, as there are many good answers to my question. Just make sure you've thought about it before proceeding.

You probably need a resistor in series with the red LED. Look at the data sheet for the LED to see the maximum current it may draw at 12 volts and compare that to the guaranteed no-fire current of the igniter. (I have the no-fire current somewhere, but not with me.) Odds are, the LED current is enough, or nearly enough to risk firing the igniter. I would keep the continuity test current no greater than half of the no-fire current; that may be more cautious than necessary, but certainly don't let it exceed 75%. If it does then you need a current limiting resistor.

On the other hand, you might find that, with a low enough current to be safe, your LED barely lights up. If that's the case then you need a lower power LED.

Finally, if I were you, I'd reverse the LED colors. As it is, you light up green as soon as the power is on and red to indicate continuity. I'd prefer red, a warning, any time the system is on and green, a GO confirmation, when continuity is established. But more important than that, if you've got two lights then you should have clear, obvious labels stating what each light means. And if you have labels then the colors are not so important.

 

[shreadvector]

Safety key/interlock must NOT be able to be removed when the system is ON.

Many, many people make a mistake and buy a key lock which allows the key to be removed in either position (ON or OFF), this is a typical key used with an alarm system, but it violates the N.F.P.A. code for a Model Rocket launch system.

Make sure your key switch can handle lots of amperage and that the key can only be removed when OFF.

 

[Eric J. Seiberling]

I'll get to answering what you asked in a moment, but first I must ask you why you are building your own. What is it you are out to achieve that you can't with a COTS unit? I'm not trying to imply that you shouldn't do it, as there are many good answers to my question. Just make sure you've thought about it before proceeding.

You probably need a resistor in series with the red LED. Look at the data sheet for the LED to see the maximum current it may draw at 12 volts and compare that to the guaranteed no-fire current of the igniter. (I have the no-fire current somewhere, but not with me.) Odds are, the LED current is enough, or nearly enough to risk firing the igniter. I would keep the continuity test current no greater than half of the no-fire current; that may be more cautious than necessary, but certainly don't let it exceed 75%. If it does then you need a current limiting resistor.

On the other hand, you might find that, with a low enough current to be safe, your LED barely lights up. If that's the case then you need a lower power LED.

Finally, if I were you, I'd reverse the LED colors. As it is, you light up green as soon as the power is on and red to indicate continuity. I'd prefer red, a warning, any time the system is on and green, a GO confirmation, when continuity is established. But more important than that, if you've got two lights then you should have clear, obvious labels stating what each light means. And if you have labels then the colors are not so important.

This is more of a co-build with my son. So he can learn wiring and electronics. He built a Estes Leviathan last year and the Estes launch controller took forever to ignite the motor.

The other issue is no matter where I need look for the forward voltage of the Red LED assembly I cannot seem to find it. These parts are off the shelf automotive and general tech parts, all rated for 12v DC. But detailed specs are impossible to find. I would like to have the box configured and premeasured with all pieces ready to install.

Another question is, would 18GA wire large enough, or will I loose voltage out to 30ft? Should I bump up to 12 or 14?

 

[Eric J. Seiberling]

Safety key/interlock must NOT be able to be removed when the system is ON.

Many, many people make a mistake and buy a key lock which allows the key to be removed in either position (ON or OFF), this is a typical key used with an alarm system, but it violates the N.F.P.A. code for a Model Rocket launch system.

Make sure your key switch can handle lots of amperage and that the key can only be removed when OFF.

The key safety switch is one that can only be removed when in the off position.

 

[jqavins]

An educational experience is one of the valid reasons for doing this. Keep your son involved with the design modifications and the reasons for them.

Use test leads to hook up the battery (or any 12 V source), the LED, and an ammeter in a loop. See what current you get. At the same time (if you've got two meters) or in a separate setup, measure the forward voltage drop. (In other words, if you can't look it up then measure it.)

60 feet (round trip) of 18 AWG wire gives you less than 1/2 an ohm. While it should be fine, I'd go bigger. 16 and 14 AWG speaker wire are readily available and not expensive. 16 AWG gives you roughly 2/3 the resistance of 18, and 14 AWG less than half. If you're looking ahead to lighting clusters it could matter.

Fred, good point about the key locking in place. I was only looking at the circuit. Locking the key would be a good idea even if it weren't required. I'm actually in the design phase of a launch controller of my own, and I'll be sure to remember that part.

 

[Eric J. Seiberling]

What range of voltage should I be looking fo when the "arm" switch is in then on position for continuity before then launch button is pressed at the ignitors. Btw the LED assembly is a 3 led 3/4" marker light so it would be easily seen during daytime.

 

[jqavins]

Strictly speaking, it's the current that matters, not the voltage. If you measure the voltage right at the igniter terminals then it's tantamount to the same thing (assuming you know the igniter's resistance, which I don't) but if you measure voltage at the launch controller end, or at the igniter clips without an igniter connected, the results will be misleading.

I've just googled the 002302 Solar Igniter maximum no-fire current and found it's 500 mA. (Verify this; I'm not so sure of my source.) That means that if you pass no more than 500 mA (TBC) though it Estes guarantees it will not fire. As I stated above, you should ensure some margin below that, so go for no more than, let's say, 350 mA (TBC). Don't rely on the igniter resistance to keep the current down, as it is a bunch smaller than the other circuit elements. The bottom line is to hook the clips up to an ammeter and do the continuity test; if you get under 350 mA (TBC) then you're OK. But if you get more that (you may or may not) then you'll need that limiting resistor.

When Quest Q2G2 igniters were still available they were much more reliable than the Estes igniters. There are similar ones still available from at least one smaller company. Those have much lower no-fire current, and to use them you'd surely need a resistor.

You can determine the resistance needed by trial and error if you like, but if you want to calculate it then you'll need the data on the LED that I suggested previously.

The other issue is no matter where I need look for the forward voltage of the Red LED assembly I cannot seem to find it. These parts are off the shelf automotive and general tech parts, all rated for 12v DC. But detailed specs are impossible to find.

Surely the package at the store has a brand name and part/model number on it? Try writing the company and asking for the data sheet. Say it's for a science fair project (close enough) and I bet they'll be happy to provide it.

 

[Voyager1]

Typically, the forward voltage of a red LED is about 1.8V. If you connect it to a 12V source and measure the current, you can at least determine the LED’s operating current. Assuming that there is a built in series resistor, you can determine this resistance as = (12 - 1.8)/(measured LED current).

I agree with Joe on the choice of 14 AWG speaker cable. Later on you might wish to go out further with your launch pad. If you use a higher AWG, eg 18, then your total cable resistance is becoming closer to that of the igniter. You will waste voltage that’s not dropping over the igniter.

 

[OverTheTop]

Here is some information on one I built quite a while back.
https://www.rocketryforum.com/threads/homebuilt-launcher-including-pics-schematic.108736/




Note that the pictures earlier in the thread are broken, so just scroll down for the pics and schematic.

 

[catman001]

Eric, searching a bit on the web, I found a number of retailers of the Meerkatt ,3/4 inch, 12 volt, 3 led marker lights. When spec's were listed they all stated a power usage of .5 watt. With the voltage and wattage known and a bit of ohms law, I come up with about 40 ma current draw (wattage = volts x amps = 12 x .040 = .48 watts.

 

[DaveW6DPS]

Most launch controllers limit continuity current to 10 mA or less. There are plenty of LEDs that light on 10 mA or less.

See what the maximum current is for your LED and size the series resistor for that current.

You should also consider short circuit protection. A series resistor in the battery lead can limit short circuit current sot hat you won't set anything on fire.

You should also include a fuse in case you still get too much current.

 

[jqavins]

I'd use a fuse and not a resistor directly in the battery line. A resistor could potentially get in the way of providing enough current for clusters, where a fuse adds no resistance to speak of until near its rated current, but it's still quite capable of preventing fires.

Eric, I mean no offense if you already know this. In case you're a bit confused about the talk of a fuse, you should know that a fuse is not primarily there to protect equipment. A fuse's main job is to protect wires (and nowadays those lithium cells that don't have other protection built in) so they don't overheat and cause a fire. If it also protects a device that's gravy. Should you choose to use one (many launch controllers don't) then you'll want to compute the maximum expected current for the largest cluster you anticipate using, add about 20%, go up to the nearest fuse size larger, and use a slow blow fuse.

 

[Eric J. Seiberling]

Eric, searching a bit on the web, I found a number of retailers of the Meerkatt ,3/4 inch, 12 volt, 3 led marker lights. When spec's were listed they all stated a power usage of .5 watt. With the voltage and wattage known and a bit of ohms law, I come up with about 40 ma current draw (wattage = volts x amps = 12 x .040 = .48 watts.

Cattman -

So with now knowing the amperage of the LEDs from what I am reading is that this build should be good to go.

 

[jqavins]

I agree, though I'd still be a lot more comfortable if you make the measurement, just on the off chance that yours are different from what catman001 found. It's probably not different, and I urge you to ake sure all the same. (The worst case would be coming in over the guaranteed no-fire current, but under the guaranteed all-fire current. Then it could work fine the first time you try it and fire early the next.)

 

[dwightr]

The schematic that OverTheTop posted is probably a better way to go for continuity check. The igniter would only see the base current of the transistor thru resistor R6. You could easily limit test current to 10ma or even 1ma while still providing rated current to the LED.

 

[Eric J. Seiberling]

This is one of those times I wish RadioShack was still open. But until I build this I won't know. This is Version 1.2.
Thought??

 

[Eric J. Seiberling]

Or this for an individual pad select

 

[Voyager1]

This is one of those times I wish RadioShack was still open. But until I build this I won't know. This is Version 1.2.
Thought??

Your output has a short circuit across the igniter from the output side of the ignition switch to -V.

 

[Eric J. Seiberling]

Your output has a short circuit across the igniter from the output side of the ignition switch to -V.

There are 3 leads to the ignition switch. (lighted switch).

 

[Voyager1]

This is what I'm referring to:

 

[Voyager1]

In fact you have another short straight after switch (3).

 

[Voyager1]

 

[DaveW6DPS]

Or this for an individual pad select

Since they parallel at the switch both ignitors would be energized with either switch on and the fire button pushed. You would need either two switches or a double pole switch.

Are you sure you don't want to use a commercial controller, or start with a simpler (and less risky) project?

 

[Voyager1]

Since they parallel at the switch both ignitors would be energized with either switch on and the fire button pushed. You would need either two switches or a double pole switch.

Yes! Switch (4) will need to be a double pole switch. The dashed line represents the mechanical link between the 2 switch poles. .
You also have an issue with the common connection to the buzzer (6) after the select switches (3). This will drive both igniter circuits regardless of which switch is on. You will need to move the buzzer.

 

[Voyager1]

Try this!

You could also place the enable switches before the DP ignition switch, directly after the buzzer connection

 

[Eric J. Seiberling]

Awesome thanks for the input

 

[dwightr]

My big concern is the current thru the continuity LED's. These LED's will be putting current to the igniters any time the arm switch is on.
You need to make sure the current is well below the nofire current of the igniters. If the continuity circuit had a momentary pushbutton I would be okay with 50%, maybe even 75% of nofire current but in an arrangement like this I would feel better with keeping the current to 25% of nofire and 10% would be even better. 2 transistors and 4 resistors would add maybe $0.14 to the project and make it a lot safer. Check out the rocket ignition system on RobotRoom.com. David Cook does some really neat projects, well thought out and documented.

 

[Voyager1]

As mentioned in an earlier post by catman001, the expected LED current is about 40 mA. The OP really does need to confirm this value before using his circuit.

Personally, I wouldn’t build a controller with parallel channels in this way. You would be safer with a rotary switch selector for channel selection. You don’t need parallel ignition channels unless you are drag racing, and there are better ways of doing this.

 

[Arthur Brown]

Just be very careful what igniters you intend to use. Estes starters will pass 100mA without firing but a firework igniter will have a test current in the order of 25mA