My relay launch controller plans

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kruland

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Hi all,

I've been wanting to build a relay launch controller for a little while. I've researched the web, and John McCoy was kind enough to share his tried and true relay plans with me.

My design goals ended up as:

1) Safety switch at the relay. I want to be able to turn the thing off at the pad just in case.

2) Ability to test for faulty (welded) relay. People have mentioned that this has never happened to them, but again, safety is good.

3) Continuity test at the relay.

4) Ability to provide power to remote handset so I don't have to carry two batteries.

5) Ability to use any 12v remote system. This system could be used as a relay from any larger system like those used at clubs.

I ended up with the attached schematic.

The schematic does not show this detail, but to allow using any 12v remote system, I am wiring some studs (screws) onto the box tied to pins 1 & 2 of the connector block (labeled J). I can then attach the clips from the remote system to these studs to trigger the remote.

Design decision #5 required the remote electrical circuit to be isolated from the 12v supplied at the relay. To accomplish this, I use a DPDT relay so the remote system's continuity check still functions correctly.

The safety switch and relay test are combined into the same DPDT switch. At the relay, the switch can be set into either "fire" or "safe/test". When in "safe/test" the igniter is removed from the circuit and replaced with an LED. If the relay is tripped, the LED will glow.

I'm using a 50' Cat-5 for the wire between the remote and relay. I happened to have it on hand, and since it's not in the firing circuit, the impedence doesn't really matter. For the record, I'm wiring each pair together to the corresponding pins.

I also chose to put a buzzer in the remote handset.

Any comments?

Kevin

View attachment relay.pdf
 
for the "welded relay" is what i would worry about. this means that there is current to the leads and as soon as a kiddo goes to hook up that J528..
shunting your leads,, is a must.

i have often thought a good way is to not have the leads connected till you walk away from the rocket. then if there is false contact, it maybe burns you, but doesnt erode your face.

a parallel buzzer in the circuit to be audible when a set of leads is contacted, could be a catch to help detect a false connection.

I like the radio shack illuminated armed toggle swich with the red protector.. my son loves his launher i made for him.
ours disconnects the leads at the control panel so i am confident hooking up the leads, but just pluggs the other end into a car cigarrete lighter.
 
kruland said:
Hi all,

I've been wanting to build a relay launch controller for a little while. I've researched the web, and John McCoy was kind enough to share his tried and true relay plans with me.
Click to expand...

You can take a look at this design for ideas:
https://www.thrustgear.com/relayer.pdf

I like two batteries because it prevents "chattering" of the relay at ignition if the voltage drops below the relay coil hold-on voltage. It also allows using only 3 conductors, such as orange AC cords, cheaper and extendable.

The alarm path will sound when the pad is unarmed and the relay is stuck. Otherwise, arm the pad and it launches (which I've seen happen twice on launch systems without stuck relay warnings).

Pins 2&3 on the relay box can be connected to an existing launch system to control the relay.

-John
 
DPDT relays with high contact current rating are tough to find. About the highest you can get is 30 amp. It may be better to redesign for SPDT or SPST if you want ample contact rating.
 
John, Thanks for the link to the plans. I hadn't seen that one.

I understand the chattering comment and didn't account for it.

Extension cords are cheap, but not quite as cheap as cat-5 (even when bought in short lengths). Now that networking is very common, Home Depot carries cat-5 patch cables and connectors. Really though, I was trying to work with what I had on hand. I had a 50' patch cable, but didn't have a long extension cord I was willing to repurpose.

I had wanted the continuity test from the remote to be able to test all the way through to the ignitor. I have had clips magically release from the ignitor many times due to wind, bad springs, etc, and think its useful to know from the remote if that circuit should still be good. It doesn't appear that your circuit does this.

However, somebody pointed out to me in a private message that my circuit is flawed in another significant way. Since the ignitor and coil are essentially in parallel, and the ignitor is very low impedance, when the fire button is pressed, the activation current for the ignitor will actually flow through the remote and the relay will not trip until the ignitor burns out. This is obviously very bad.

I will have to work more on my circuit because after building it there are some problems. In particular, when the battery was attached, and the DPDT armed switch set to "armed", the continuity light on the relay box was lit continually.

jsdemar said:
You can take a look at this design for ideas:
https://www.thrustgear.com/relayer.pdf

I like two batteries because it prevents "chattering" of the relay at ignition if the voltage drops below the relay coil hold-on voltage. It also allows using only 3 conductors, such as orange AC cords, cheaper and extendable.

The alarm path will sound when the pad is unarmed and the relay is stuck. Otherwise, arm the pad and it launches (which I've seen happen twice on launch systems without stuck relay warnings).

Pins 2&3 on the relay box can be connected to an existing launch system to control the relay.

-John
Click to expand...
 
Dale Saukerson said:
DPDT relays with high contact current rating are tough to find. About the highest you can get is 30 amp. It may be better to redesign for SPDT or SPST if you want ample contact rating.
Click to expand...
You can pretty much ignore the current ratings for automotive relays used in rocket launching.

h.250,w.150,m.97a4ea8bad5424bd8834ef7713a3ea37,8812,rly_351.jpg
h.250,w.150,m.32639b549535883803fae61d0abfca86,3896,srly_2.jpg


12 volt auto relay matching relay socket

A 30/40 amp automotive relay is very conservatively rated for hundreds of thhousands of operations at the rated current, and is rated for 100 amps in rush currents without damaging the relay contacts which is just fine for rocket launcher applications. The relays will take at least 150 amps before you can weld the contacts. With a matching relays socket and some wire nuts, you have a nice $5 a high current 12 volt relay system.

https://www.allelectronics.com/make-a-store/category/500700/Relays/Automotive/Power/1.html

Typical data sheet

https://www.components.omron.com/co...4E59A56C985257201007DD68C/$file/G8JN_0607.pdf

Bob
 
Mine does check continuity all the way through. Look at the path from S2 through P1/J1 pin1, through the n.o. contacts and the igniter. R1+R2 limits the test current. R2 is there in case something shorts at the connector. Pad box continuity is S4 through R3.

I've seen many systems built with CAT5 cables/connectors. They tend to get damaged easily and corrode. With orange cords, it's just as low cost and easily replaceable. Also, the connectors and wire are lower resistance which allows longer drops without worrying about insufficient voltage to pull-in the relay.

My design also uses a SPDT relay which is easier to find.

Another feature is the autoreset thermal breaker. This protects against dead shorts (igniter leads touching) which will melt the wiring and possible weld the relay contacts. I've been using this for 15 years in multi-pad controllers and single controller. I also use them in the commercial electronics I've designed. The way it works is that anything over 20A for about 15 seconds will cause the thermal breaker to open. It cool down in about 20 seconds and is ready to try again. Normal ignition in a few seconds, even with clusters, doesn't affect the breaker.

For something more complex, here's my digital padbox design which is part of my multi-pad system design:
https://www.thrustgear.com/LCS02/lcs2_padbox_sch.pdf
Continuity check is better... produces a higher-pitch tone for lower resistance, low-pitch tone for poor igniter connection. Alarm tone when relay is stuck.
The overall system is described here:
https://www.thrustgear.com/LCS02/
 
John, I'm getting sold on your design. Though, I'm also using this as a learning experience, hence the questions.

jsdemar said:
Mine does check continuity all the way through. Look at the path from S2 through P1/J1 pin1, through the n.o. contacts and the igniter. R1+R2 limits the test current. R2 is there in case something shorts at the connector. Pad box continuity is S4 through R3.
Click to expand...

What I was referring to is when you use the relay box with an existing launch control system (ie using only J1 pin 2&3), you cannot see continuity all the way through to the ignitor. You only see continuity through the relay coil. I'm curious if this can even be done.

jsdemar said:
I've seen many systems built with CAT5 cables/connectors. They tend to get damaged easily and corrode. With orange cords, it's just as low cost and easily replaceable. Also, the connectors and wire are lower resistance which allows longer drops without worrying about insufficient voltage to pull-in the relay.
Click to expand...

Experience, that's the thing I'm missing. Corrosion and damage I didn't consider. I thought that since the trigger mechanism was more limited by the resistance of the coil, that the incremental impedance of cat-5 vs 12/14/16gauge extension cords wouldn't make that much difference.

jsdemar said:
My design also uses a SPDT relay which is easier to find.
Click to expand...

Right. I did find one big DPDT relay rated at 30amp/12VDC. But I'm not willing to pay the $26+shipping to get one. Also the failure modes of a DPDT relay seem to be more complex. I would guess it's possible for one half of the relay to weld but not the other. There were two reasons why I went down the path of a DPDT relay: I had one, and I though it could provide the continuity test I wanted.

jsdemar said:
Another feature is the autoreset thermal breaker. This protects against dead shorts (igniter leads touching) which will melt the wiring and possible weld the relay contacts. I've been using this for 15 years in multi-pad controllers and single controller. I also use them in the commercial electronics I've designed. The way it works is that anything over 20A for about 15 seconds will cause the thermal breaker to open. It cool down in about 20 seconds and is ready to try again. Normal ignition in a few seconds, even with clusters, doesn't affect the breaker.
Click to expand...

I really like this feature of your design. I thought this would be useful, but didn't have the knowledge about the kind of part needed.

jsdemar said:
For something more complex, here's my digital padbox design which is part of my multi-pad system design:
https://www.thrustgear.com/LCS02/lcs2_padbox_sch.pdf
Continuity check is better... produces a higher-pitch tone for lower resistance, low-pitch tone for poor igniter connection. Alarm tone when relay is stuck.
The overall system is described here:
https://www.thrustgear.com/LCS02/
Click to expand...

That will have to wait until next winter for me!
 
kruland said:
John, I'm getting sold on your design. Though, I'm also using this as a learning experience, hence the questions.
Click to expand...

You're more than welcome to ask any questions. I've been designing hardware & software systems for 30 years and I'm currently teaching college courses in electronics.

kruland said:
What I was referring to is when you use the relay box with an existing launch control system (ie using only J1 pin 2&3), you cannot see continuity all the way through to the ignitor. You only see continuity through the relay coil. I'm curious if this can even be done.
Click to expand...

Without redesigning the main controller, you will only see the resistance of the relay coil if you add a pad relay box to an existing system. What I do on my multi-pad design is to use the same wire for the relay coil and the continuity signal. A full 12V DC will kick in the relay. A small AC signal, about 1/2V, is sent on the same line. A forward biased diode requires about 0.7V before it conducts, so I use a diode to keep the AC signal out of the relay coil. The small signal carries an audio tone back to the main controller, where it's amplified and drives a piezo element or speaker. The same tones that are heard at the pad box are available at the main controller (continuity pitch, armed pulsing, and warning tone for stuck relay).

kruland said:
Experience, that's the thing I'm missing. Corrosion and damage I didn't consider. I thought that since the trigger mechanism was more limited by the resistance of the coil, that the incremental impedance of cat-5 vs 12/14/16gauge extension cords wouldn't make that much difference.
Click to expand...

If you look at the reistance per foot of a 22 or 24 gauge and double the distance (out and return path), it adds up pretty quickly. Add contact resistance from the connectors and it can be 30 ohms or more for a 500ft drop. Driving a 60-ohm relay coil, 1/3rd the voltage is lost in the wire. 8V might not be enough to actuate a 12V relay.
 
jsdemar said:
You're more than welcome to ask any questions. I've been designing hardware & software systems for 30 years and I'm currently teaching college courses in electronics.
Click to expand...

Let the lessons begin! I've been doing software for 20 years but it's so large scale that it's mind boggling ("We have 16G ram, lets use it!") I want to try some small stuff, pic arduino, but have been really scared of the commitment. About all I know about 'lectrix is not to stick nails in outlets!

I wanted to stretch my knowledge and try something new with this project.

First my ambitions are pretty low. I'm really doing this to just get enough power to step up from single BP to small clusters. I don't need to be 500' away and don't plan on launching 30+ crazy big clusters like some of those other guys.

This one may be a dumb question, but I don't know the answer. One reason why I tried to use a DPDT relay was I thought it was necessary to keep the two powered circuits separate. I don't think I'm being clear right now, but suppose you have two 12v bttys - one in the controller and one in the relay box. The controller circuit would essentially power the coil, and the relay circuit the ignitor. Can you for example tie both the negative terminals of the two batteries together and not damage the batteries?

Without redesigning the main controller, you will only see the resistance of the relay coil if you add a pad relay box to an existing system. What I do on my multi-pad design is to use the same wire for the relay coil and the continuity signal. A full 12V DC will kick in the relay. A small AC signal, about 1/2V, is sent on the same line. A forward biased diode requires about 0.7V before it conducts, so I use a diode to keep the AC signal out of the relay coil. The small signal carries an audio tone back to the main controller, where it's amplified and drives a piezo element or speaker. The same tones that are heard at the pad box are available at the main controller (continuity pitch, armed pulsing, and warning tone for stuck relay).
Click to expand...

I noticed the opamps and other things in your design and was wondering what they were for. Now I understand. I don't think I'll be trying this right away.
 
John,

Would you mind walking me through this? I've tried to remove all the complexity that I added in the other one. Instead, I reduced it to the bare minimum.

The idea is some 12v launch controller could use pin 1&2 of J (think clips from an existing controller). I am hoping that when that controller is attached, its continuity system will still function correctly.

The thing I am most worried about is the fact the negative terminal of the btty for the relay is tied to pin 2 of J. Is this ok?

Kevin

View attachment simplerelay.pdf
 
kruland said:
...This one may be a dumb question, but I don't know the answer. One reason why I tried to use a DPDT relay was I thought it was necessary to keep the two powered circuits separate. I don't think I'm being clear right now, but suppose you have two 12v bttys - one in the controller and one in the relay box. The controller circuit would essentially power the coil, and the relay circuit the ignitor. Can you for example tie both the negative terminals of the two batteries together and not damage the batteries?

...
Click to expand...

Yes, you can. It's a common ground or return, and works fine. You'll notice that's what John is using in his very elegant single pad control system in the earlier post, and it's what enables his remote checking of continuity through the relay contacts.

John
 
kruland said:
John,

Would you mind walking me through this? I've tried to remove all the complexity that I added in the other one. Instead, I reduced it to the bare minimum.

The idea is some 12v launch controller could use pin 1&2 of J (think clips from an existing controller). I am hoping that when that controller is attached, its continuity system will still function correctly.

The thing I am most worried about is the fact the negative terminal of the btty for the relay is tied to pin 2 of J. Is this ok?

Kevin
Click to expand...

As a simple circuit, it looks fine to me. And I see that you've solved your pad continuity check circuit sneak path problem as well!

John
 
Hi all,

I took another stab at it. I added a +12v line back to the controller for optional use.

Now a traditional controller can be wired to 1&2 of J1 and should see continuity and trigger the relay.

On the other hand, I can wire between 1&3 of J1 with my battery-less controller and still have fully functioning system.

Does this look workable? I still wonder if I need some kind of current limiter to prevent the controller from trying to power the ignitor without tripping the relay coil first.

Kevin

View attachment simplerelay.pdf
 
With the relay coil in parallel with the igniter, it's very likely the relay will not engage, or it will at least chatter. When pressing the launch button, the system is trying to supply current to the igniter as if it wasn't a relay coil. Depending on the resitance of your wire and connectors compared to the resistance of the igniter, there may not be enough voltage to kick the relay. Also, if you have a wimpy battery at the main controller, it's internal resistance will cause the output voltage to drop while trying to power the igniter before ever getting to trip the relay. The problem is worse with a cluster... with 5 1-ohm igniters in parallel, that's 0.2 ohms. If your wiring and connectors are also 0.2 ohms, you'll only get 6V at the relay coil. Typically automotive relay coils are around 60->100 ohms and need at least 9V to actuate.

If you reduce your contiuity test voltage to <0.5V and put a forward diode in series with the relay coil, it would work. Put a 100 ohm resistor in series with the NC side of the relay contacts. With 12V from the controller, the diode will be forward biased and 11.3V will kick in the relay voil. With <0.5V, you'll have a 5mA test current to the igniter. The diode to the relay coil will not be forward biased enough to conduct, and the relay coil will be essentially out of the circuit (except for a small diode leakage current). However, you'll need to amplify the test current (a transistor) to get enough current to light an LED bright enough to see it in the sun.

Other things I see in the schematic: be careful having 12V pins on a connector next to a pin that goes right to the igniter without any current limit. A bent pin or frayed wire will cause the rocket to launch.

I assume the "speaker" you show from 12V to GND is actually a piezo beeper with self-contained oscillator, like the ones I show on my schematic? It'll beep continuously whenever the interlock is switched on. A regular speaker will click once and burn out with DC across it.

Let me know if you need me to modify the schematic to show what I'm describing above.

-John
 
I think you're raising the key point, John. This design is constrained as is by the tie between the continuity check and control of the relay. There needs to be a way to separate this, as you suggest with the diode and resistor.

Another way is shown in the attached schematic. But the consequence here is that the controllers would have to be modified to bring the continuity check out on a separate pin.

John


jsdemar said:
With the relay coil in parallel with the igniter, it's very likely the relay will not engage, or it will at least chatter. When pressing the launch button, the system is trying to supply current to the igniter as if it wasn't a relay coil. Depending on the resitance of your wire and connectors compared to the resistance of the igniter, there may not be enough voltage to kick the relay. Also, if you have a wimpy battery at the main controller, it's internal resistance will cause the output voltage to drop while trying to power the igniter before ever getting to trip the relay. The problem is worse with a cluster... with 5 1-ohm igniters in parallel, that's 0.2 ohms. If your wiring and connectors are also 0.2 ohms, you'll only get 6V at the relay coil. Typically automotive relay coils are around 60->100 ohms and need at least 9V to actuate.

If you reduce your contiuity test voltage to <0.5V and put a forward diode in series with the relay coil, it would work. Put a 100 ohm resistor in series with the NC side of the relay contacts. With 12V from the controller, the diode will be forward biased and 11.3V will kick in the relay voil. With <0.5V, you'll have a 5mA test current to the igniter. The diode to the relay coil will not be forward biased enough to conduct, and the relay coil will be essentially out of the circuit (except for a small diode leakage current). However, you'll need to amplify the test current (a transistor) to get enough current to light an LED bright enough to see it in the sun.

Other things I see in the schematic: be careful having 12V pins on a connector next to a pin that goes right to the igniter without any current limit. A bent pin or frayed wire will cause the rocket to launch.

I assume the "speaker" you show from 12V to GND is actually a piezo beeper with self-contained oscillator, like the ones I show on my schematic? It'll beep continuously whenever the interlock is switched on. A regular speaker will click once and burn out with DC across it.

Let me know if you need me to modify the schematic to show what I'm describing above.

-John
Click to expand...
 

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