I've done some analog electronics, and I've done some model rocketry (basic Estes stuff) however I wanted to take the next step and build my own controller to support Boy Scout and School rocketry. I am thinking of going for 10 launch pads. Without practical knowledge, I am wondering about a few things - especially power supplies. Seems like it would be extremely convenient now days to use one or more tool batteries (such as Dewalt, Ryobi, etc) as these are compact, recharge quickly, easily swap-able, light weight, and can even be charged remotely using a car cigarette lighter accessory. I can hack an old tool from a garage sale to create a "snap in" socket for the battery type. A person could carry four or so batteries and and keep them on a rotating charge if they didn't last the full length of a day of rocketry. Question, could you get enough amps out of them to fire igniters?

Second issue, I have seen numerous designs out there and many seem to suggest using 12 v 30 amp auto relays in the firing circuit (others digital versions but still basically a relay) triggered by a momentary push button for a "Fire" command. I have also seen many that simply use a simple firing path involving an analog switch such as a momentary press plunger without any relay. What are the advantages of using a relay? Are they mostly for larger rocket types, higher amperage, or is there a reason to use a relay in basic rocketry?

My main idea is to use a simple 12v to USB converter to power all the logic indications and to conduct continuity checks on installed igniters, and have a separate 12v circuit for firing voltage/amperage. As others have done, to use the same wires to the igniter for continuity checks as for the firing voltage. Attached is a basic circuit diagram. Assuming 16 or 14 gauge wiring for all the 12V lines, Using LEDs for all indicators and appropriate load resisters for each (~300 ohm).

I want a safety key so anytime anyone is at the pads the key is out and the system cannot be energized. And I want the ability to have a remote "launch button" with the ability to disable or enable it so I can put the kids farther but control when their button works (and if they brake the button, switch back to local launch on my controller).

Sequence basically this:
1. connect battery, close battery switch - test range alert horn
2. Verify all switches "safe" - Firing voltage disarmed, Local switch "Local", all pad switches "OFF"
3. Insert safety key, sound warning horn to clear pads - Go "Live" position
4. In "Disarmed" mode - Test igniters by placing applicable pad switches in "Test" and press momentary "press to test light to observe Green Igniter Test. (remove Range key to troubleshoot at pads)
5. Move applicable pad switch to "Fire" as desired to observe Igniter Ready.
6. Move Firing voltage to "ARM" - verify Igniter ready AND Launch order on pad desired.
6. Start count down - When near zero switch Local Remote to Remote and allow launch.
7. After launch shut down to "Local", Firing Voltage "Disarm" and move pad to "OFF"
8. Return as necessary to step 5.

Any help on problems you note or recommendations I would appreciate it before I go spend money.

 

... just a suggestion ... how about locating a separate battery at the launch pad itself ... specifically to fire the ignitors.
... The basic reason for doing this is that for a typical safe distance, of say 100 feet, the intrinsic resistance in 14 gauge wire, at a current level of maybe 10 amperes will create a voltage drop of several volts ... that is, 100 feet of 14 gauge wire = ~0.25 ohms, plus there will also have to be a ground wire.

... This suggested overall concept can be achieved by using a relay for each rocket, and also a separate relay to activate the ignition battery.

... Furthermore, the selection control, for firing individual relays/rockets is implemented with a logic chip called a multiplexer. The control wiring, between the control panel and the actual rocket, is simply Ethernet cable, telephone wire, or something of that nature. Heavy gauge wire, like 14 or 16 gauge, is not needed for the substantial distance between the two locations.

... In order to select a particular rocket launch pad, a binary number is selected at the control panel, using a counter, and that binary number activates a firing relay at the actual launch location, being an output pin of the multiplexer. After a particular rocket pad has been selected, the separate battery ignition relay is activated, which supplies current to the designated rocket.

 

Question, could you get enough amps out of them to fire igniters?

That's your easiest question, and the answer is yes. Those batteries have plenty of current capacity to light up an igniter. There may be battery packs that make it harder to get at the battery poles directly.

What are the advantages of using a relay? Are they mostly for larger rocket types, higher amperage, or is there a reason to use a relay in basic rocketry?

I think it's a bit of overkill for the typical igniter. They just don't draw that much current, maybe not much more than the relay coil requires. On the upside though, automotive relays are inexpensive, easy to find, and can switch high currents. The current load on your switch contacts is limited to whatever the coil current is. So your switch life should be extended. A better option, in my opinion, would be a MOSFET switch. In essence, a solid-state relay. (There are also SSR, solid state relays which would be fine, but a MOSFET could accomplish the same job.) MOSFETs are cheap, small, easy to get, no moving parts, and can also switch high currents. Activating a MOSFET would put almost no load whatsoever on your switch contacts.

My main idea is to use a simple 12v to USB converter to power all the logic indications and to conduct continuity checks on installed igniters, and have a separate 12v circuit for firing voltage/amperage.

I don't see any reason to use a separate, 5V supply. You could do it all with 12V. Maybe I'm missing something. I'd use a comparator (such as LM339) to detect a small current (say 1mA) through the igniters, to test for continuity, and use the comparator output to directly light an LED. You can only get ~5mA through the comparator but that's plenty as an indicator. If you need more current to get more brightness, just add another transistor.

Like I said, I may be missing something, but I wouldn't want to rely on igniters not lighting up when supplied 5V. I'd much rather design something that supplies on a tiny test current.

 

I want to say it was about maybe 20 years ago I built a launcher for a co-worker's Boy Scout (Maybe Explorer) group. We kept it simple and a standard generic 6 volt lantern battery powered the entire affair. The Estes Rocket Starters did not draw any current to speak of. I think we used a 4017 counter and a 555 timer for 1 second pulses. Used a string of LEDs for countdown (actually count up but who knew) . On ignition it gave a beep from a small piezo buzzer. Today I would have likely used a uC like an Ardino and just at a glance a Google of "rocket launcher using arduino" fills a page with hits, including videos. While discreet components were the way to go 20 years ago, today I would program a uC and maybe even add a small display.

When I built the unit I built my friend was a mechanical engineer and woodworker. The thing went into a real sweet wood box with hinges and all. They named it the "Master Blaster". Anyway, I would look at uC solutions today and those igniters do not require much current at all as I recall.

Ron

 

The one and only rocket I ever launched was constructed from an expensive aluminum cigar tube, powered by an empty \(CO_2\) cartridge filled with sulfur and powdered aluminum. The ignition was a piece of nichrome. A jump off the car battery supplied the heat to fire it off. ... worked great.

 

One would think that safety is the #1 priority of rocket launch controller design.

This thing is very complex, complexity leads to confusion, confusion leads to accidents.
I also see absolutely zero reason for having a 5V regulator involved, just run it all straight from the battery.

 

Appreciate the feedback -all is well taken. Thanks for the suitability on the tool batteries. I think my final design is going to be based on these batteries as I like the appeal of easy field re-charge and the simple "plug in" design. I am pretty sure I can find cheap and ample supply of broken power tools to make the battery receiver (connector) and the purchase of a couple new batteries and the charger isn't going to brake the bank for what it is worth.

This design was a surrender to my limits of design capacity (which is why I'm here looking for feedback) I originally favored having the 12v and high amp stuff located at the pads and then using a controller to provide "logic" to activate appropriate relays to get the firing voltage to the right igniter. The obvious advantage is being able to use low voltage/low current cables to and from the launch pad and controller. I wanted to use common cabling such as Cat-5 as interface because it is low weight, low cost, easy to hook up and easy to replace if damaged and just two strands could supply enough lines for 10 launch pads worth of commands easy, but obviously you can't run 12v at up to 10 amps through that stuff. My dilemma is I really wanted to have continuous continuity test through the igniters prior to launch. However with my limited circuit design skills, I have not yet figured out how to do that with remote relays in the chain "out there" and still get back continuity status "back here" at the controller. I figure there is a way to have a relay for each pad that can be selected to provide a continuity path to the igniter and a separate "firing relay" at the pad as well that would route through any closed pad continuity relay to fire. Just haven't conjured up that design YET. Looks like I'm going back to that idea based on input here. Any comments on the revised partial attachment with all 12v at the launcher and logic at the controller? I'm open to if this is analog relays or the use of SSRs, MOSFETS, but for now just trying to get the logic flow right. This would require 22 low voltage signal wires between the pad and controller (Igniter Continuity + Relay select per pad = 20 + Fire Relay + Common Ground =22 total) A Cat 5 has 8 usable signal wires, so 3 cables carry all needed signals to /from launch pad and controller.

As for the comments about complicated 5V regulator included - my thoughts is that it isn't complicated at all. Since we are dealing with a 12v system to start with, it seems incredibly simple now days to grab an off the shelf cheap car USB outlet and wire it in providing all the voltage conversion, current limiting features needed to drive the "logic" circuit. Test it on a few igniter brands to make sure it won't fire them and tweak the circuit as needed with some resistors to keep it low, or even a potentiometer to provide adjustment capacity at the final output that does the igniter continuity test. I suppose if the controller was purely low voltage (12v at the pads) then a 9v or a few double AA's for the controller power would work just fine - but again since my 12V was already at the controller I just figured it was a easy solution rather than building enough voltage drop from scratch into the circuit to do the continuity test, just plug in someone else's product - the USB adapter.

Maybe it isn't an issue for others, but I remember my last experience with trying to get a lot of rockets done on a multi pad and we fought faulted igniters and bad continuity at the lead clips that kept delaying getting everyone off. Are they good enough now days that it isn't worth the time to worry about igniter status when you have 10 rockets sitting on the pad for a one by one light off?

 

... Something is missing here.

How do you intend to switch between specific launch pads?
... Is there a multiple pole rotary switch?

... Suggest using numerical values along with name labels on voltage sources.

... drawing needs reorganization somehow.

 

Sorry, it is only a re-draw of a section of the original posts drawing. I'll spend some time and make it a full drawing for clarity.

 

Electrical Current Requirements of Model Rocket Igniters

The above link was an interesting read. I had no idea that today's igniters had such a high current draw to ignite.I would still consider using a uC but would likely use DC relays or logic level MOSFETs for the igniters. While the current is high it is relatively a short lived demand and over real quick. I was wondering how to test an igniter as in how much current can be passed through an igniter before it ignites?

Ron

 

Here are revised circuit drawings for proposed 10 pad launcher, placing all 12V stuff out at the launch pad and then having a 5 V only based controller that triggers relays at the launcher. By doing so all cables going to/from the launcher can be handled with low cost easily replaceable Cat-5 cables.
12v power to be supplied using modern tool batteries (Dewalt / Ryobi / etc) and 5v controller to use simple AA battery (3 or 4) power with a voltage and current regulator circuit to ensure insufficient power to ignite starters during igniter continuity tests.

 

Yes ... The battery arrangement is more suitable, but as mentioned previously, there is another option. The thing about relays is that they require some definite amount of current ... maybe a few hundred milliamps. The other method is to use a Mosfet. This device can put 20 or 30 amps through the ignitior, and requires essentially no current to be turned on. The only requirement, say for an IRF540 mosfet, is to see 10 volts at its gate terminal.
... The IRF540 mosfet sells for less than one US dollar at a reputable dealer ... less than the cost of a typical relay. Extras might include a heatsink, and dust/weather protection of some sort.

.... whatever you think is a good choice.

 

Extras might include a heatsink...

I've used that MOSFET at 5-10A without a heatsink and it stays touchable. For the intermittent burst this circuit should do, I wouldn't worry about a heatsink.

 

I've used that MOSFET at 5-10A without a heatsink and it stays touchable. For the intermittent burst this circuit should do, I wouldn't worry about a heatsink.

I would also run with a MOSFET and in the interest of making sure it turns on like a switch or relay I would look towards a Logic Level MOSFET which can easily be driven by a uC (micro-controller). Also as mentioned they can be had for about a buck apiece ($1.00 USD). An example would be the Fairchild Part # FQP30N06L which will easily do what you need done.

Ron

 

Ok, so I understand the benefits of going solid state / mosfet etc. However I don't have a firm grasp on how to integrate them into a circuit. I can pretty much figure out how I would use one as my "firing relay" - using a MOSFET that when fed a 5v to the GATE will turn on the 12v igniter voltage/current sitting ready on the Source out the Drain lead and down a path toward the igniter. However how would you replace the relays I'm using to select the specific launcher that also need to pass the 5v igniter continuity prior to passing a 12v firing voltage? is there a MOSFET that you can "switch" on with the Gate that is ok sending the +5v sitting on the Source lead that you can then hit with the +12V on the Source as well when the launch "firing" button is pressed? Would it look something like the attached? Do I have the SOURCE lead and the DRAIN lead configured correctly?

 

Keep in mind that semiconductors tend to fail SHORTED, a very dangerous situation relative to firing rockets.
Make sure the system fails SAFE.

Relays are tough and dumb, just what you want in a safety related system, not delicate, finicky transistors.

Your regulated 5V is also working against you, 12V is a much better voltage for driving the gate of a MOSFET.
The Rds-ON rating of the MOSFET is a function of the gate-source voltage, the MOSFET will conduct harder (less resistance) with a higher gate voltage. Ditch the regulator.

The series connected MOSFETs are problematic, better to use ONE with the logic handled in the gate circuit.
it's simpler, cheaper, and has lower resistance.

Having the drain of the MOSFET connected only to a long external wire is an invitation to ESD damage, (SHORTED FET!) connect a diode between ground and the drain in reverse bias, this protects the FET by clamping the drain to ground for negative ESD events, positive voltages are already clamped by the FET's intrinsic diode.

This ESD rant is intended to illuminate the pitfalls of this approach, when's the last time you heard of a mechanical relay failing from static electricity?

I am still vaguely horrified by the complexity of this monster.

 

• ... For the initial version of your launcher, just simplify and try to get the basic concept to work.

• ... Get some one sided copper circuit board and a few 12v MOSFETs. Fasten each transistor to a square of PCB. Figure out what sort of selector switching you want ... individual switches, a rotary switch, whatever.
• The large copper board area will help to prevent static electric charge build-up.
• See what kind of difficulties show up ... what works.

... Your basic firing circuit has to have Vgs as the firing signal pulse, with the source terminal at ground. The 12v bus would be at the ignitor terminal.

 

Keep in mind that semiconductors tend to fail SHORTED, a very dangerous situation relative to firing rockets.

A fair concern, but relays can fail to a short also. Which event is more rare? Both are so unlikely that I'm not convinced it's a big design concern.

Relays are tough and dumb, just what you want in a safety related system, not delicate, finicky transistors.

Maybe I can buy that. Maybe this is an argument to ditch both and just use a good momentary switch. That's what most model rocketeers start with anyway. I never got past a simple box with two switches, one to arm the box and the momentary switch to launch.

Your regulated 5V is also working against you, 12V is a much better voltage for driving the gate of a MOSFET.
The Rds-ON rating of the MOSFET is a function of the gate-source voltage, the MOSFET will conduct harder (less resistance) with a higher gate voltage. Ditch the regulator.

Agreed, it has no valid function. The right MOSFET will do a fine job with 5V on the gate, but why bother?

The series connected MOSFETs are problematic, better to use ONE with the logic handled in the gate circuit.
it's simpler, cheaper, and has lower resistance.

Total agreement.

 

Ok, so I understand the benefits of going solid state / mosfet etc. However I don't have a firm grasp on how to integrate them into a circuit. I can pretty much figure out how I would use one as my "firing relay" - using a MOSFET that when fed a 5v to the GATE will turn on the 12v igniter voltage/current sitting ready on the Source out the Drain lead and down a path toward the igniter. However how would you replace the relays I'm using to select the specific launcher that also need to pass the 5v igniter continuity prior to passing a 12v firing voltage? is there a MOSFET that you can "switch" on with the Gate that is ok sending the +5v sitting on the Source lead that you can then hit with the +12V on the Source as well when the launch "firing" button is pressed? Would it look something like the attached? Do I have the SOURCE lead and the DRAIN lead configured correctly?

I'm not suggesting the following is useful, it's just a thought. A MOSFET can be installed in in either configuration of S and D. It contains a body diode that will conduct in one direction (S to D), regardless of the gate voltage. We usually don't want that so we orient the MOSFET to be only off or on depending on the gate voltage. If we design with the 'off' state conduction through the body diode in mind, we can actually take advantage of it. I'm thinking about your small continuity current. Something in the 1-10mA range could easily be sent through the body diode without worry of heating that diode. (The specs will describe the current capacity of that diode.) I'd have to think about it a while to figure out how to exploit this.

As for your 5V continuity test, you need to get over the though that the voltage matters. The key to the continuity test is not the voltage, it's the current. Applying 5V to the igniter is far too risky, in my opinion. You want to apply just a few mA, far less than could ever fire the igniter. You can use your 12V supply for this, or you could use 120V AC for this as long as the current is limited. Limiting the current could be as simple as using a resistor in series with the igniter and power source. It might be safe to pass 10mA through both the igniter and your indicator LED. If so, that's how I'd do it. So with a 12V supply your LED would drop ~3V and the resistor needs to drop ~9V. Using Ohms law ∆V = I•R , 9V = 0.01A • R Ω and R = 900. So you could use a 1000Ω resistor. Of course you need to know that 10mA is in fact safe. I'm pretty sure but it would be wise to nail it down for safety.

 

So, I believe your making good argument for my design using relays and against MOSFETs.

1. I don't want the need of larger higher voltage/current cables running back and forth between the controller and the launcher as this makes for expensive and bulky interface wiring, so placing the 12V at the controller or using 12V to drive the MOSFETS is out unless I want to use low coil voltage relays to drive MOSFETS which seems very redundant.

2. Failing SHORT isn't good. However that is also why I have a strobe light out at the launch pad to tell me anytime the firing voltage is present in circuit. While relays COULD also fail short, they are much more likely to fail OPEN. Automotive relays can be had cheap enough (even if not as cheap as MOSFET) and easy to replace when failed if mounted correctly.

3. As I want "series" logic safeties - if MOSFETS don't play well in series that isn't the best option. I want to be able to bring things up step by step toward launch, observing indications for normal circuit response before going "HOT" so to speak. I want multiple events all required to meet final "go" status.

4. Don't want "sensitive" ESD issues with a system that is going out into a field and being assembled/dissembled repeatedly. I have worked in sensitive electronics work for decades and this isn't going to be done in a protected environment or handled with kid gloves. It is going to be stored in a garage, transported in the back of a truck, pieces and parts carried by youth, and assembled and disassembled numerous times.

5. Long external wires are a fact of life with this. The controller must be at a safe distance from the pads. Since this is for large groups of kids, even farther. I know I can use 5v logic through a CAT-5 out to at least 300 ft without inducing signals across to other twisted wire pairs.

6. Understand the disdain of complexity. If I was building something just to launch "A" rocket this could be some wire and a battery. However my goal is something more like 100 rockets plus in a single afternoon and being very much in control of safety while those 100 plus kids are on site. I need to be able to safely send 10 kids out to mount their rockets, make sure they are clear, be able to identify problems at a distance, fire off all ten one at a time, deal with misfires, verify safety before letting anyone back near the pads, and cycle this routine repetitively all while letting each rocket owner have fun and the experience of pushing the button. So - safety key, testing circuit, alarm and visual warnings, controlling and limiting the moment firing voltage is present and where it is present, controlled manner of which of multiple pads is able to fire and lots of LEDs to ensure I can know what is going on each step of the way. Then build it with as many easily available and easily replaceable, and easily maintainable parts.

Thanks for your feedback and comments as this has greatly helped me refine and visualize the final project. I can now more clearly see where MOSFETs and RELAYs are both great approaches to solving "remote control" of switching depending on the specifics of the project. I'm confident now that I need to stick with relays with a 5v coil and capable of switching 12v power with a good current rating to make this project perform like I want it to. If you just want to launch a couple rockets once in a while - probably not a design for you, nor if your launching big league stuff -multi engine, sophisticated stuff where you probably want something commercial grade. I'm looking at LOTS of little basic rockets. High throughput but tight controls.