Detecting pyro charge on another board

[Chad]

I would like to use the number 3 and 4 pyro channels on my Raven4 to provide a logic signal to a separate board. This other board has general purpose IO 5v pins, if i connect these pyro channels to an IO pin is the voltage the only thing i have to worry about? For example, is it as simple as a 5v regulator between the pyro channel and the other pin? It's the current i can't get my head around. Current is "pulled" by the load/resistance and the pyro channels can deliver up to 25A i believe. However, if the IO pin isn't pulling anything will the current be high? On the other hand, I = V/R so the smaller the resistance the greater the current and i doubt there's much resistance on that IO pin. Do i have that right?

 

[cerving]

Use an optoisolator between the Raven (transmitter) and your logic board (receiver) with a suitable dropping resistor, and it should work fine. That also provides isolation between the two boards, which may be a good thing. The choice of a dropping resistor and optoisolator will be tricky... you don't want the continuity current on the Raven to trigger the optoisolator.

 

[Kelly]

I agree with what Cris said - using an optoisolator is a good recommendation - but to answer your question directly, the main thing you need to worry about is "what is the voltage output of the Raven" (to be safe, you should assume this is whatever battery you're using), and "what is the voltage rating of the IO pin" (which you've said is 5V). Current doesn't come in to play here. You are correct in stating that I=V/R, but typically input pins have very HIGH resistance/impedance, so the drawn current will be very LOW regardless of what the output capacity (impedance) of the driving circuit is.

 

[jqavins]

I endorse both of the above answers. And since cerving does this professionally, I would be a fool to dispute what he says.

But maybe I can add a bit to it. Do you know how the Raven's test current is regulated? Constant current source? What current? Resistor limited voltage source? What voltage and resistance? If you know the Raven end then the optoisolator circuit isn't hard. If you don't know the Raven end and need a circuit that will works safely no matter what, that's hard. So start with finding out what the Raven has, and then take it from there.

 

[Chad]

thank you all for your help, I'll first try to profile the Raven4 pyro channels and then look at my options. Just the general concept of optical isolation is really cool. (i'm not a hardware guy)

 

[Grog6]

IIRC, the pyro outputs are open collector drivers, an input would be a ttl- style gate input.
Interfacing those is well known. Use pull up or down resistors to set how you want it.

 

[OverTheTop]

I use Ravens (lots of them) and the outputs are indeed open-drain (similar to open-collector, but MOSFET) outputs. If you have the wherewithal to put an optoisolator in there that would be probably the optimal solution. If you share the ground/0V lines of both systems you can just have a pullup (to the relevant logic level voltage) on your logic system input and have the output of the Raven pull it down to ground when it goes active.

If you have any telemetry transmitters in the area you might want to keep the antennas away from that link to prevent noise pickup too. A small capacitor (1--10n say) to ground at either end would likely help with mitigating RF pickup in that situation. Ground testing is a must.


[edit] Chicken-scratch added.

Make sure the pullup has sufficient power rating (depends on the voltage it is on) so it doesn't cook when Pyro goes low.

 

[cerving]

The Raven uses a common-positive, that is, all of the deployments are connected to the + terminal of the battery and the - is switched on the other lead to fire the ematch. Connect your optoisolator so that the anode is on the deployment channel terminal (NOT the one connected to the battery), and connect the cathode to the - lead of the battery, through a suitable dropping resistor (we use 10K in the WiFi Switch). If you use an optoisolator with a bidirectional transmitter (LED side) or simply use two back-to-back (like we do in the WiFi Switch), you don't need to worry about polarity, as long as you have them connected to the right places.