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Ignition circuit, advise

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ryanshow

Fri May 04 2012, 09:52AM

Registered Member #4720 Joined: Thu May 03 2012, 11:19PM
Location: Fresno, CA
Posts: 5

Hey everyone,

I'm trying my hand at designing a circuit that's going to be used for igniting fuses for model rockets. I've manged to cobble something together that looks like it might potentially work, but I'm a complete newb at circuit design. The idea is that there are basically 4 parts to the circuit:

- The charging portion uses a "9v" NiMh battery to charge up a 3.3F supercap at 2.7v.
- Once the charging portion is complete, the LED turns on, indicating completion.
- The arduino has a line hooked up to the A1 Analog input pin which indicates when charging is complete (ideally when it drops to 0)
- The arduino also controls a small relay via it's D1 digital out pin that closes the circuit between the supercap and the fuse.

Seeing as how I have no idea what I'm doing, I'd appreciate some feedback as to what I'm doing wrong :) Some of the part numbers are not correct as I'm just using the circuit lab built-in parts for now, but you should be able to get a general idea of what I'm trying to accomplish. In particular, I'm worried about the A1 sensor input. Is this the correct way to handle this? I've never used OP Amps before and I'm not quite sure if the voltage will drop over the capacitor will even out as it becomes charged.

Thanks in Advance!
Ryan-

Tetris

Fri May 04 2012, 03:48PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

I've seen people use NSTs. plug it in when you are ready to fire, andplace the fuse on one of the terminals where it is easy to arc to. :P

Steve Conner

Fri May 04 2012, 04:06PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It looks too complicated. You can fire the igniter using just a battery and a switch.

Also there's something not quite right about the charging system. Why is the negative terminal of the capacitor not connected to the negative terminal of the battery? There doesn't seem to be a path for charging current.

Ash Small

Fri May 04 2012, 05:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

It looks too complicated. You can fire the igniter using just a battery and a switch.

Maybe the OP has other reasons for wanting to use an arduino? We'd need to know more about the project. But I agree, it looks far more complicated than required.

ryanshow

Fri May 04 2012, 06:16PM

Registered Member #4720 Joined: Thu May 03 2012, 11:19PM
Location: Fresno, CA
Posts: 5

Sorry everyone, I'll elaborate a bit more on what exactly I'm trying to accomplish.

The end-goal of this system is going to be to have the rocket have a self-contained ignition system that can ignite not only on the ground but also multiple stages while the rocket is in mid-flight. I understand that launching these things is as simple as hooking the fuse up to a battery with enough current but I need to be able to control exactly when the ignition happens from a micro controller.

I know a few solutions already exist out there for this sort of thing but I've been really wanting to get into building circuits. Steve pointed out that I hooked my supercap up backwards... oops! Thank you for noticing.. I've corrected the issue and removed the diode on the discharge circuit.

Building circuits is not very intuitive for me coming from the physics class I took several years ago on electronics.. Everything is backwards!

mister_rf

Fri May 04 2012, 09:51PM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

What 9V NiMh battery type do you want to use for charging that supercap ? I hope thatâ€™s not the â€˜6F22â€™ style:

Anyway, from the electrical point of view, still there are many things to change in that schematic (you need to hook diodes up backwards to make them work properly, no direct path for charging the capacitor, the operational amplifier completely out of job, no justification for using Q1 ) and most importantly, in order not to waste so much energy charging the capacitor we need to change the standard LM317 circuit and try to replace it by a more efficient step-down DC-DC converter ( 9V to 2.7V).

In my opinion there are some major problems to be considered before we start designing a mid-flight control ignition, because such situation require supplementary extra weight loaded on that rocket, and thatâ€™s a huge task to carry out for a normal device.

In practice itâ€™s more efficient building the classical solution using a 12V accumulator and implementing some important features as for example the safety key, flashbulb-safe continuity check circuitâ€¦ Maybe for the mid-flight control ignition you may use directly a 9V alkaline battery to get the required ignition pulse.
See some electronic launch controller projects here:

Harry

Fri May 04 2012, 11:01PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

Does the mid - flight ignition need to be electronic? What about Battery -> Igniter (1 ohm 1/2W resistor?) -> First stage AND secondary fuse -> fuse delays ignition of the second stage.
How's that sound?

Mattski

Fri May 04 2012, 11:46PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

There's no need to build a fancy capacitor charging circuit just because you want microcontroller control. You can still use a battery and switch to trigger the igniter, just replace the switch with an N-channel MOSFET and drive the MOSFET gate with the microcontroller. You can add an arming switch and armed indicator too.

ryanshow

Sun May 06 2012, 01:40AM

Registered Member #4720 Joined: Thu May 03 2012, 11:19PM
Location: Fresno, CA
Posts: 5

Thank you everyone for your feedback!

mister_rf: I took your advise and modified my circuit to fix all of the issues you mentioned from the electrical point of view. I'm curious what's wrong with the "6F22" style 9v battery though? NiMh may be heavy and so I've been considering moving to a lithum source. In any case, I only have it in the schematic because it was high enough voltage for me to drop it down nicely to 2.7v and still be in the 5-12v range for the arduino supply. I'm planning on implementing some sort of safety mechanism eventually but for this initial design I'm only going to be testing with igniters without the actual rocket. In the end I'm hoping this entire apparatus will weigh only 30-40g or so.

Harry: Yes, I require electronic ignition because I need to control exactly when the secondary stages ignite. I'm planning on having an accelerometer (and maybe a barometric altimeter) on-board which I'll use to tell when the flight has reached it's peak altitude so the second stage can ignite. I'm also going to record ignition times and such so I can create some fancy flight graphs.

Mattski: I like the idea of lighting the fuse with just a battery but my concern is that a single, light-weight battery will not be able to provide me with enough current to ignite the fuse. I've found a few tests people have done online and it looks like these fuses can take anywhere from 200mA to 1.5A to ignite at around 2v. I'm of course going to try and purchase the fuses which are most likely to ignite with the least amount of current but ideally I'd like it to work reliably under all circumstances.

Here's my updated (and hopefully electrically sound) circuit. My concern is that I'm not using the LM311 voltage comparator correctly... The idea is that I'm dropping the inverting input down to just below 2.7 volts with a resistive voltage divider and then plugging the voltage after the cap into the non-inverting input. If I understand how the LM311 functions, I believe once the non-inverting voltage surpasses the inverting voltage, the emit out should jump from 0v to 2.7v with a 8mA max supply current? Then I'm trying to send that to the arduno and pushing it through an NPN transistor so the LED can light up. Does this make sense? I've spent the last few days studying these datasheets and various tutorials through and through trying to figure out how these ICs work.

radiotech

Sun May 06 2012, 03:42AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

One thing that I don't see in the schematic is a manual safety switch to disable ignition
to approach the rocket.

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