So about to buy parts for a Cub Scout model rocket launch control system! Yay!!!

It is made of multiple boxes, each with a big 12V battery and an Arduino, networked together. At the launch pads, each Pad Box needs to power both the static 5V control logic AND 12V through relays to between one and eight model rocket igniters. Although the RSO has the Master Arm, _each_ Scout/LCO has their own Launch Button.

The big difference between systems I have seen elsewhere versus our needs: it seems almost all of the other multiple-pad controllers I've seen--one-person launches for all. But we want to continue to allow each Cub Scout (after the RSO arms) to control their own individual launch buttons. Most Scouts comply with the shared countdown & "drag race." However there are often Scouts who want to "confirm their own control" or "call attention to themselves" by launching early or late! A little early or late, no problem....these "control freaks" may become real rocketeers someday! (But we do want the RSO's master-arm to keep it within bounds).

So all of the individual launch buttons are merely gathered as inputs to the RSO's Arduino control. Only when the Pad Boxes have their local power turned on do the Arduinos (or anything else) come on. Only when the RSO pushes the Master Arm do the Pad Boxes' Arduinos get commanded to power on the relay boards. Similarly, only when the RSO's countdown clock enables the two second window, then do the collection of launch button signals get forwarded to the Pad Boxes. So remove any of FIVE keys or momentary buttons, or the control logic, then NOTHING happens. Add in extra distance, buzzers, strobe lights, "Apollo-style" countdown/message display, Scouts launching from under a tent, "peanut gallery" watching/waiting under another tent... Well you see we take safety _very_seriously_ indeed!

"Two heads are better than one..." So here it is, please give me your recommendations... Three Arduinos of some model(s), RS485 interfaces, driver/relay boards, all of it is open to suggestions. "Not too proud to beg." Thanks in advance.

 

Yes, fairly simple stuff, but never hurts to ask....others might see something I missed (Safety). Or they may know of ways to save costs (A Scout is....Thrifty...)

Planning to use a "real" electro-mechanical key switch for the RSO's Master Arm, one that cannot be removed in the On position. For the Scouts/LCOs, planning to use a 1/4" TRS plug as a removable "key." The plug will properly cross two of the wires. But if someone tried to insert a pocketknife and "pick" the lock, the third wire will lockout that channel.

Do need a momentary button for the Scouts/LCOs, but not settled on one yet. Like the look of "missleman" red flip-covered toggles. But also like the look of a big push-button, especially if "ready" lit. The flip-covers don't seem big enough for the push-buttons. Or maybe fit the Continuity LED into the end of the 1/4" TRS plug (since no wires to come out anyway)?

Yes, almost all logic is in the RSO's Arduino. In Modbus, this is the Master. RSO Box will send Enable commands to the Pad Boxes, poll from the Pad Boxes for Continuity status on the igniters, and send to the Pad Boxes the Launch relay commands. So the Pad Boxes are simple Modbus Slaves.

I do need to define how the igniters' Continuity is measured... I show the igniter as on the Common pin of the relay. The NO pin dumps the full 12V 2A ignition current. But the NC pin needs to measure Continuity and feed it to the Arduino. Definitely need Open/High Resistance (no rocket igniter/fallen lead) and Closed/Negligible Resistance (rocket igniter ready). Would be nice to have Short/Little Resistance (igniter bent/leads touching). Probably means moving from digital inputs to analog inputs, and maybe some extra measuring circuit. Will have to account for resistance variance due to 5'-10' igniter cables from closest launch pad to the farthest launch pad.

And I have to find or build the Countdown/Message Display. Haven't found a 6"H, daylight-readable, six character, alpha-numeric display (SETUP, HOLD, T-SS.d, LAUNCH, T+SS.d). Or maybe jump to a 6"H LED matrix panel? That would be more to drive....but then again the Pad Box Arduinos aren't under much processing load. Seems I2C in order to keep the pin count down.

 

Should also note the Pad Boxes will have a multi-step power switch:

Off: Obvious, ZERO power allowed from the battery
Local Test: Powers Arduino & Continuity Test circuitry only. Keeps boot transients from accidental launch triggering.
Remote Control: Allows receiving of sequential commands from the RSO Box:
"Master Arm" adds power to Relay Driver board
"Countdown Complete" (2seconds window) adds power to NO bus

And finally, the RSO Box can command the Pad Box's Arduino to trigger whichever rocket's relay....again only during the "Countdown Complete" window.

 

It may be helpful to draw a flowchart, in order to include details and achieve a satisfactory final objective.
Flowchart Basics

 

you might want to consider the nRF25L01+ 2.4GHz wireless board family to simplify setup and reduce the number of wires running around. Each transceiver can have six pipes, effectively addresses, at the same time. Multiple boards can use different RF channels for more capacity.

It also means handheld devices can participate in the system in various ways, which might be interesting, and countdown displays can be placed anywhere without wires.

Just a thought.

 

This is going to be harder than you might think.

Getting a reliable continuity test will require measuring the resistance within a fairly narrow band, the leads going to the igniter will have some resistance that needs to be accounted for.

Worst case- 16 Amps flowing- watch for voltage drops over long cable runs.

Make sure your command protocol has error checking or multiple authentication, don't want com glitches firing rockets

 

As an alternative to the I2C communication, a 74HC595 shift register allows just 3 arduino pins to send a high or on signal to a group of 8 terminal nodes (specified by writing an 8 bit byte) such as mosfet gates or drivers. There is a specific arduino command to initiate this process.The 74HC595s are series connectible, so two of them, using only 3 board pins, could turn on up to 16 end devices of your choosing. ... reference material upon request.

 

It may be helpful to draw a flowchart, in order to include details and achieve a satisfactory final objective.
Flowchart Basics

I would like to apologize for jumping in this thread

@drc_567 Thank you very much for providing excellent information. I was looking for that type of tool.

 

This is going to be harder than you might think.

Getting a reliable continuity test will require measuring the resistance within a fairly narrow band, the leads going to the igniter will have some resistance that needs to be accounted for.

Worst case- 16 Amps flowing- watch for voltage drops over long cable runs.

Make sure your command protocol has error checking or multiple authentication, don't want com glitches firing rockets

I'm a network & communication engineer by trade. But yes, I expect a few surprises. Hence asking for many opinions, on even the easy stuff, before buying parts!

Our local NAR/Tripoli club uses a _big_ truck battery, and lots of heavy electrical extension cords. But by placing ignition batteries right at the launch pads, we don't need to send that amperage 150'....just lightweight command signals. And by introducing the Arduinos we don't need to wire for individual analog commands to the launch pads either....just two wires, and maybe a ground and/or shielding. Again, by having the igniters close to the Pad Boxes, short runs of heavy wire which should not drown out the igniters' resistance as we try to measure. Will probably have to opamp the help the Arduino's analog input pins differentiate three states though. And might add a potentiometer to each igniter circuit, to make up for the 5'-10' varying resistance from the short leads between Pad Box and closest rocket versus longer leads to the farthest rocket.

ABSOLUTELY adding internal app-protocol headers to validate both the IDs of the sender/reciver and the checksum of the commands & any packed bits! Hoping to leave phy and comm-protocol validation to whatever RS485 boards we choose. Could go Wi-Fi, and see lots of personal, singe-rocket, controllers doing this. But I have four FCC licenses (ham, clocks, site radios), and have no interest in the added complexity of either transmission or security for a system with this many rockets and Scouts to account for.

 

I see lots of 8x Arduino driver/relay boards. Any opinions on brands, quality, etc?

I've used 4x logic chips many times, for the original purpose of simple logic on ADA accommodations.. Might consider using an AND gate to buffer some of the boot-up transients. But open to suggestions on how to wire such.

But has anyone seen 8x or even 4x opamp circuits? Would seem kind of nice to have the symmetry of 8ea. igniters->single 8x resistance measurement board->Arduino->single 8x drivers/relays board->8 ea. igniters. Then again, maybe it all collapses to a single, shared PCB anyway.

 

As an alternative to the I2C communication, a 74HC595 shift register allows just 3 arduino pins to send a high or on signal to a group of 8 terminal nodes (specified by writing an 8 bit byte) such as mosfet gates or drivers. There is a specific arduino command to initiate this process.The 74HC595s are series connectible, so two of them, using only 3 board pins, could turn on up to 16 end devices of your choosing. ... reference material upon request.

Sure, always appreciate S2P (or any other) examples. Thanks.

Someone else suggested a mechanical "flip dots" type of countdown/message display in front of the launch pads. But that probably isn't I2C. Seems a bit like a LED matrix in how it might be driven. Not sure which would be more power-efficient. Seems "flip dots" would be energy intensive to set, but then "free" to maintain at a "brightness" to be readable 15'-100' from the pads & clock. Whereas the LED matrix stuff is "free" of the mechanical set, but ongoing energy burn for as long as the message is shown.

I mention the energy because the "rocket range" is a transient setup on a massive camp field. There is _zero_ grid power available at the range. All system batteries have to be charged elsewhere overnight (hopefully just once per week), then dragged back to the field. Hoping to keep this down to a moderate-sized 12V lead-acid or gel cell battery seen in e-lights, power tools, or maybe lawnmowers...one battery in each Pad Box (igniter loads), maybe a smaller one in the RSO Box (Arduino and all those status LEDs).

 

You could switch in a constant current source. LM334 or LT3092 and measure the voltage at the ignitor to determine the quality of the ignition circuit. You can use Kelvin clips https://www.aliexpress.com/item/32812901480.html to do so. You can parallel the LT3092's to increase current.
These have 2 contacts. One would be voltage measuring, the other current carrying. For ignition, you would bypass the current source.

If you wanted to make it totally digital, use an LT6700 series comparitor

 

You could switch in a constant current source. LM334 or LT3092 and measure the voltage at the ignitor to determine the quality of the ignition circuit. You can use Kelvin clips https://www.aliexpress.com/item/32812901480.html to do so. You can parallel the LT3092's to increase current.
These have 2 contacts. One would be voltage measuring, the other current carrying. For ignition, you would bypass the current source.

If you wanted to make it totally digital, use an LT6700 series comparitor

Funny that you mention clips. I am also looking to make a _single_ bipolar clip for each rocket. Something of a cross between the stock 2 alligator clips, a terminal strip w/ barriers, and an office binder clip. Would like to quickly clip _both_ leads onto the igniter, with one motion. Should also reduce the number of misfires due to fallen clips.

 

Those are Kelvin clips. You connect two wires to each clip. You would have a pair per ignitor.

Low value resistance is a 4 or more wire measurement. Each Kelvin clip gets a wire that carries current and a wire that measures voltage.

If you know the current through the device and the voltage across the device, you know the resistance, The clip to launch resistance would be out of the picture. So, you might want to measure the voltage at the launch controller end, so you include the wiring to the ignitor, not just the ignitor.

 

Those are Kelvin clips. You connect two wires to each clip. You would have a pair per ignitor.

Low value resistance is a 4 or more wire measurement. Each Kelvin clip gets a wire that carries current and a wire that measures voltage.

If you know the current through the device and the voltage across the device, you know the resistance, The clip to launch resistance would be out of the picture. So, you might want to measure the voltage at the launch controller end, so you incluse the wiring to the ignitor, not just the ignitor.

I think he's talking about something different. Each jaw would connect to one wire and be insulated from the other, unlike those for Kelvin measurements that are both connected to the same lead.

 

I think he's talking about something different.

I know the TS is talkig about something else. I just wanted to make clear what Kelvin clips are.

At the same time, I don't think they are appropriate in this application.

1. A constant current source makes sense below the amount of current that causes heat or ignition.
2. Measuring V at the source makes the most sense because it includes the wire resistance.

You can do a compare using the LT6700 series of comparitors with a 400 mV internal reference and they are an "Over the Top" product meaning inputs can exceed the supply.

Say you put 100 mA through the wires and <1 ohm was the target. That's <0.1V for the comparitor. You woulld have to amplify. If you supply 12V through 1 ohm, it drops 1.2V, so say >10 V or is an open ignitor. The <10V creiteria could be Vbat-(some number).

You could put test points there to get the values of V if you needed them to actually measure R.

 

Ironically, driving home today, a new concept ran through my mind: Instead of "reinventing the wheel" building the Scouts' individual Keys, Continuity LEDs, and Launch buttons....why not just build an interface to clip the _existing, OEM controllers' alligator leads onto? Reduces what I have to build, yet still gains most of the other benefits from my RSO and Pad boxes. Still only a single cord from RSO/LCOs to the Pads. Still a single RSO battery instead of scads of AA batteries in individual controllers (well still have the big batteries in the two Pad Boxes). Still have the added safety of the RSO's "Master Arm" key. Still have the Arduinos enforcing the "Countdown Complete" 2 second window to allow Launch commands. Still have the strobe, siren, & other warnings. However....then all that mass of loose OEM controllers spread across the tables, instead of a single "stick" panel. Not sure...

 

Ironically, driving home today, a new concept ran through my mind: Instead of "reinventing the wheel" building the Scouts' individual Keys, Continuity LEDs, and Launch buttons....why not just build an interface to clip the _existing, OEM controllers' alligator leads onto? Reduces what I have to build, yet still gains most of the other benefits from my RSO and Pad boxes. Still only a single cord from RSO/LCOs to the Pads. Still a single RSO battery instead of scads of AA batteries in individual controllers (well still have the big batteries in the two Pad Boxes). Still have the added safety of the RSO's "Master Arm" key. Still have the Arduinos enforcing the "Countdown Complete" 2 second window to allow Launch commands. Still have the strobe, siren, & other warnings. However....then all that mass of loose OEM controllers spread across the tables, instead of a single "stick" panel. Not sure...

you cold do a hybrid. Something that could work with any controller by pretending to be the igniter, and a controller of your own that does more. The interface for the igniter simulating connection could be a box that connects to where your version connects but simply provides a place to clip on and go.

 

If there is any interest, the book "Arduino Workshop" by John Boxall covers basic usage of the 74HC595 SPI chip as a seven segment led controller that could be used for other purposes. An interesting aspect is the setup of a 1xn array to select various individual led digits.

 

If there is any interest, the book "Arduino Workshop" by John Boxall covers basic usage of the 74HC595 SPI chip as a seven segment led controller that could be used for other purposes. An interesting aspect is the setup of a 1xn array to select various individual led digits.

Will have to check that out....Arduino book number four! Working my way through Monk's second, "Next Steps" now. Then Banzi & Shiloh's "Make."

If I build the countdown/message clock, it'll be either 14 or 16 segments per character. But may just go LED matrix panel.