Building a better wireless ignition box

[AllDigital]

I've got a wireless ignition box (personal) that has been a real workhorse and has served me well, but it uses two "eBay special" wireless 315Mhz relays. I've got a microcontroller in the box to gate the logic, continuity check, safety switches, and key control on the launch side, but I've always been worried that someone else on the range bought the same cheap wireless RF relays. So, I've decided to rebuild a new one using higher power serial TTY radio/modems on each side (bi-directional). That way I can be absolutely certain the arm and fire messages are coming from me and I can see exactly what is going on with the remote controller. While I am at it, I've decided to upgrade some of the functionality to include an ohm check/report and additional safety controls. In the process a few questions have come up that I thought I'd pose to this group.

1. What is a "good" range I should use to test ignitor resistance? I did a sampling of everything in my garage and it seems that mine range from .5 ohm up to about 3 ohms, but I do not have a lot of variety.
2. My continuity test today is based on 3v and 10ma of power and so far I've never accidentally launched a rocket or burned an ignitor. I always test new ignitors on the box before inserting them, but what is generally a safe current limit for continuity check?
3. On the business end of the box I am using three 10A relays. One to "arm" and then I'm separately activating the other two on each side of the ignitor to complete the circuit across a 12V AGM battery. I've considered using Mosfets, instead of relays, but old school relays just seem safer to me (although contacts can fuse together). Pros/cons?
4. I've got a current limiting resistor in series on the firing circuit dialed in to about 5 Amps. I've also thought about adding an auto-reset breaker (10A), but I am just choosing these limits arbitrarily. My firing timer is usually set to 3 seconds and so far (80-100 launches) I have not fried anything with crossed leads. Thoughts on the limits?

I know this group has thought about these a lot more than I have, so I appreciate any advice or opinions.

-Mike

 

[AllDigital]

Apologies. This probably belongs under "Ground Support" and not Electronics. Moderator, feel free to move over. Thanks!

 

[kc9qzf]

I've got a wireless ignition box (personal) that has been a real workhorse and has served me well, but it uses two "eBay special" wireless 315Mhz relays. I've got a microcontroller in the box to gate the logic, continuity check, safety switches, and key control on the launch side, but I've always been worried that someone else on the range bought the same cheap wireless RF relays. So, I've decided to rebuild a new one using higher power serial TTY radio/modems on each side (bi-directional). That way I can be absolutely certain the arm and fire messages are coming from me and I can see exactly what is going on with the remote controller. While I am at it, I've decided to upgrade some of the functionality to include an ohm check/report and additional safety controls. In the process a few questions have come up that I thought I'd pose to this group.

1. What is a "good" range I should use to test ignitor resistance? I did a sampling of everything in my garage and it seems that mine range from .5 ohm up to about 3 ohms, but I do not have a lot of variety.
2. My continuity test today is based on 3v and 10ma of power and so far I've never accidentally launched a rocket or burned an ignitor. I always test new ignitors on the box before inserting them, but what is generally a safe current limit for continuity check?
3. On the business end of the box I am using three 10A relays. One to "arm" and then I'm separately activating the other two on each side of the ignitor to complete the circuit across a 12V AGM battery. I've considered using Mosfets, instead of relays, but old school relays just seem safer to me (although contacts can fuse together). Pros/cons?
4. I've got a current limiting resistor in series on the firing circuit dialed in to about 5 Amps. I've also thought about adding an auto-reset breaker (10A), but I am just choosing these limits arbitrarily. My firing timer is usually set to 3 seconds and so far (80-100 launches) I have not fried anything with crossed leads. Thoughts on the limits?

I know this group has thought about these a lot more than I have, so I appreciate any advice or opinions.

-Mike

1. Assume anything under 1000 ohms is a connection. This will prevent the odd igniter from causing issues. The idea is to see if the leads are connected not determining the quality of the igniter.
2. See if you can do at or below 1mA. It shouldn’t take a lot of current to be able to establish a reasonable continuity voltage for an analog or digital input.
3. MOSFETs will easily work in the range you are looking at. Just make sure your driver will be able to keep the gate in the off state to prevent inadvertent activation especially during power up when the battery is connected.
4. Ditch the resistors, auto resettable fuses are more reliable are lower in cost and are in use in automotive applications for years. Littlefuse makes devices in the ranges you are looking for. The only thing you need to look at is the amount of time the fault is present to trip the device. Just like a fuse it is current and time that determines the trip.

 

[AllDigital]

Thanks. This is helpful.

1. Assume anything under 1000 ohms is a connection. This will prevent the odd igniter from causing issues. The idea is to see if the leads are connected not determining the quality of the igniter.

My primary thinking on the "ohm test" was to prevent (or require an override) if the ignitor appeared to be shorted out. So when testing for continuity you would get open, short, or good. There is so little resistance in some ignitors, so it seems like my threshold for short would need to be like .1 ohms or maybe even lower. This would also get tested when the box is put into an "armed" state.

 

[UhClem]

Resistance range and resolution are related. (Decide how much resolution you need and your ADC will then determine the range.) You want enough range to cover 99.9% of igniters. Too low a range and you risk having it tell you that you are open circuit when you are not. You also want enough resolution to see differences at the low end. Wire resistance complicates things unless you get really motivated and go with a 4 wire connection.

I still remember using Fire In The Hole igniters with the Kloudbuster launch system. They had to use the special override button because the system thought it was a short circuit.


MOSFETs are actually more reliable than relays. If you are worried about limiting current you could try things like the VNL5050. I find that limiting for a high power relayer but just right for model rockets. (I tried a cluster of 3 Solar igniters.)

The worst thing you can do to a relay is switch the contacts under load. But if you have a relay on the high side that is switched when you arm the system and then use MOSFETs on the low side that are controlled by the launch button, you eliminate that.

The DARS controllers use that system and some are getting close to 20 years old now with no relay failures. (In sockets so they are easy to replace.) The high power relays (which only have the relay) cannot make that claim with one spectacular emolation due to welded contacts. Even using 40A automotive relays.

I am not a big fan of wireless systems because you are dependent on software which is difficult to prove correct. But you can mitigate that problem a little bit by including a pad side safe/arm switch that interrupts the igniter connection and have a piezo buzzer that warns when there is power out of the arming relay. At the very least the user is a little further away when things go bad.

 

[kc9qzf]

Thanks. This is helpful.



My primary thinking on the "ohm test" was to prevent (or require an override) if the ignitor appeared to be shorted out. So when testing for continuity you would get open, short, or good. There is so little resistance in some ignitors, so it seems like my threshold for short would need to be like .1 ohms or maybe even lower. This would also get tested when the box is put into an "armed" state.

Unfortunately, getting that kind of resolution at a current that will not scare you gets hidden in the variability of the connections between the detector circuit and the igniter. The leads and clips will increase the variability of the reading to where you may or may not be able to know a short from a higher than normal resistance at the connectors. A kelvin connection could work, but in practice getting it to work in the field will be a major issue.
I mentioned the scary part because you could do a high current burst pulse but with 25 feet of cable the inductance of the cable will be an issue.
I doubt you can get less than 0.1 ohms in the cables and interconnects so you have that also. Not wanting to be negative and you are welcome to keep at it, but KISS has been a good principle in this area.