# Continuity indication LED integration (I'm new to this)

#### Dr.FillyBlunt

Joined Jan 12, 2020
5
Please note: As the title reads I'm new to this and while I understand some basic concepts my knowledge with building circuits and theory is minimal.

I am currently in the process of trying to prototype a simple circuit that allows me to remotely send 12V to a speaker terminal. By off sourcing much of this to one of those prefabbed cheap RF relay modules, I was left with wiring, dropping in a speaker terminal, switch then adding a few LEDs. I was successful in achieving my main goal, however I wanted to implement an additional LED that would illuminate if there was continuity between the two speaker terminals.

I used the NC terminal on the module to add a red power LED and the NO terminal to send power to a green "signal" LED and the speaker terminal when the remote is activated. I have absolutely no idea how to achieve the LED that indicates speaker terminal continuity. I'd appreciate any help I can get.

After going through several different iterations of the circuit on paper I realized I don't know how to draw schematics but this last one seemed the best of all so I hope it's readable if you can't tell what's going on from the pictures alone.

#### dl324

Joined Mar 30, 2015
13,809
Welcome to AAC!

The L designator is typically used for inductors. For LEDs, we use D or LED. Having the LEDs in series with your 12V source, they're going to limit the maximum current you can provide to the speaker terminals.

I am currently in the process of trying to prototype a simple circuit that allows me to remotely send 12V to a speaker terminal.
A couple of questions. Why would you want to apply 12VDC to a speaker? And why do you want to know if a speaker is connected to the speaker terminals?

#### wayneh

Joined Sep 9, 2010
17,160
Another way of saying it is that L2 and R2 could be placed in parallel to your output load instead of in series with it. The downside of that is that the indicator will light whether the load is connected or not. There are ways to deal with that also, if you want, but it's trickier.

#### Dr.FillyBlunt

Joined Jan 12, 2020
5
Welcome to AAC!

The L designator is typically used for inductors. For LEDs, we use D or LED. Having the LEDs in series with your 12V source, they're going to limit the maximum current you can provide to the speaker terminals.

A couple of questions. Why would you want to apply 12VDC to a speaker? And why do you want to know if a speaker is connected to the speaker terminals?

Thanks for clearing that up, for my application it would probably make more sense to have the LEDs in parallel then because I'm going to need that current, or at least enough current to bring a strip of nichrome up to a certain temperature.

I'm not actually wanting to apply 12VDC to a speaker, I'm just using a speaker terminal because it would be the easiest way to output for my intended application. I guess it would be helpful if I provided a bit of context. I want to make something similar to this except a 1 channel version.

Notice when it's on, the #1 LED is lit because that terminal is populated with a good electric match. I've attached an illustration of the style of ematch I intend to utilize. The two wire ends are bridged with a thin gauge piece of nichrome wire then dipped into a pyrogen material that will emit open flame and hot sparks when current is driven through. As a result of course, the nichrome wire will break and there will no longer be continuity or in the case of a match that was made incorrectly or damaged somehow it won't have continuity to begin with. This indication is a convenience and a safety feature to let us know if a match is live and ready to fire.

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#### Dr.FillyBlunt

Joined Jan 12, 2020
5
Another way of saying it is that L2 and R2 could be placed in parallel to your output load instead of in series with it. The downside of that is that the indicator will light whether the load is connected or not. There are ways to deal with that also, if you want, but it's trickier.
Current requirement could be flexible depending on the design of ematch I suppose but more importantly the indicator should only light during continuity.

#### Wolframore

Joined Jan 21, 2019
2,483
Do you know the current needed for “no fire” condition? As long as the led current is well below that you should be fine... personally I would send a lower current (uA range) and detect that using an amplifier, microcontroller or something similar. A parallel test circuit with a test button is a simple but effective solution.

#### Dr.FillyBlunt

Joined Jan 12, 2020
5
Do you know the current needed for “no fire” condition? As long as the led current is well below that you should be fine... personally I would send a lower current (uA range) and detect that using an amplifier, microcontroller or something similar. A parallel test circuit with a test button is a simple but effective solution.
I'm not quite sure I follow what you're asking but do you mean the current draw of the resistor and LED after the normally closed output? My meter tells me 26.5-26.8 ma.

The amp/microcontroller solution sounds like it'd be above my capabilities but I do have a few momentary push button switches sitting around. How would that seperate parallel testing circuit look? Could I run that off the same power source or do I use a seperate battery? (forgive me if that's a dumb question)

#### Wolframore

Joined Jan 21, 2019
2,483
all fire current is current that the electric match will ignite. Every one is a little different. If you’re making this for amateur rocketry the Aerotech ones require much lower current than The ones from Estes.

the Quest ingniters can be set off by as little as 170 mA @ 25 mJ. The Estes ones have lower impedance and need 2A and about 4500 mJ for operation.

First 26 mA is too high for a continuity test light, it can be as little as 5 or 10, start by doubling the resistor currently on the led which should half the current to 13 mA.

try this... test button gives you momentary check so you’re not wasting power and heating up the igniter... then when you’re ready to fire have it held down before you hit fire. NMRA requires an interlock again not sure if you’re using this for model rockets but it adds a level of safety.

#### Dr.FillyBlunt

Joined Jan 12, 2020
5
all fire current is current that the electric match will ignite. Every one is a little different. If you’re making this for amateur rocketry the Aerotech ones require much lower current than The ones from Estes.

the Quest ingniters can be set off by as little as 170 mA @ 25 mJ. The Estes ones have lower impedance and need 2A and about 4500 mJ for operation.

First 26 mA is too high for a continuity test light, it can be as little as 5 or 10, start by doubling the resistor currently on the led which should half the current to 13 mA.

try this... test button gives you momentary check so you’re not wasting power and heating up the igniter... then when you’re ready to fire have it held down before you hit fire. NMRA requires an interlock again not sure if you’re using this for model rockets but it adds a level of safety.
View attachment 196937
I've been making my own igniters based on the design posted earlier and I pretty much got that down, just need to figure out a good pyrogen. I figure nitrocellulose lacquer mixed with BP (meal powder) should be plenty for my purposes, but that's another discussion entirely.

Instead of a push button I utilized the on/off/on toggle switch already in my circuit. One side sends power just to continuity light and speaker terminal (test) and the other side sends power to RF relay module then out from NO into the output (arm).

I did what you suggested for the continuity test light and just dropped in a 1.2k resistor in series with it and that dropped current (measuring at output) from 26ma to a nice 6.5ma. I can hold my finger across the bridgewire on the igniter during test mode and it doesn't appear to even get remotely hot so it seems safe to me, although I noticed 10.5v when probing the outside of the speaker terminal. This shouldn't be an issue with the current being this low right?

Can't figure how to get the signal LED to work with this design but I figured I probably don't really need it because when I switch to arm mode and use the remote button, the power LED will go off as long as I have the button held down, which obviously means it's getting signal anyway.

The interlock certainly seems like a cool idea but this is radio operated and the remote part is just one of those tiny little keyfob things and I'm figuring that'd be easier for me to implement an interlock on a wired handheld unit than this.

Thanks, you've certainly been a help thus far and that I appreciate.

#### Wolframore

Joined Jan 21, 2019
2,483
I’ve built such a device but using my own radio and not a key fob. An interlock is just a momentary switch to lock out the fire button for safety.

voltage drop is fine, it means you have higher resistance through your igniter than the rest of the circuit. It’s the all fire current you need to worry about.

potassium nitrate is also good for pyrogens but why use it when BP is easy to get.