Using a Separate Pyro Battery with Altus Metrum Flight Computers
As we have discussed elsewhere, the most reliable solution for pyro control circuits in large and complex rocket projects is to use a separate battery to provide the current used for actually igniting the charges from the one used to power the electronics. This document explains how to do this with various Altus Metrum flight computer models.
Pyro Circuit Background
All Altus Metrum flight computers use low-side FET switches to control pyro events. What this means is that the pyro circuit provides the equivalent of a switch to ground on one pin of each pyro circuit, and a source of power on the other. The continuity sense circuitry is on the switch side of the pyro terminals, and was designed to cope with any pyro voltage through nominal 12 volts (15 volts DC maximum). Using a separate pyro battery is therefore as simple as just ignoring the terminal that provides a source of voltage and only connecting the side that provides the switch to ground.
The negative lead of your pyro battery needs to be connected to ground on the flight computer. The plating around the mounting screw holes is one good place to find ground. The negative terminal of the LiPo battery connector is also grounded. And on TeleMetrum boards there's an explicit ground connection hole you can solder a wire to.
The positive lead of your pyro battery needs to connect to one side of each e-match, with the other side of the match connecting to the appropriate pyro terminal on the flight computer. This obviously means you need to make connections off-board. Whether you choose to do this with a screw terminal strip, crimp connectors, or by soldering wires together is of course entirely up to you!
Early TeleMetrum boards had an unpopulated connector footprint on the board for a separate pyro battery, but due to a wiring error in the design of the board, that connector was never useful. Just ignore it.
If you look at the bottom of the board where all the surface-mounted components are, on the end where the pyro screw terminals are located, you can see a fat trace connecting three of the screw terminals together. This is the source of pyro power, which comes from the LiPo battery through the power switch and then to one side of each pyro channel. The pins that trace connects are therefore the ones you want to ignore. The others are the switches to ground.
Here's a photo of the pyro end of a TeleMetrum board augmented to show what to connect where:
Wiring a TeleMini board for use with a separate pyro battery works the same way.
If you look at the bottom of the board where all the surface-mounted components are, on the end where the pyro screw terminals are located, you can see a fat trace connecting two of the screws. This is the source of pyro power, which comes from the LiPo battery through the power switch and then to one side of each pyro channel. The pins that trace connects are therefore the ones you want to ignore. The others are the switches to ground.
Here's a photo of the pyro end of a TeleMini board augmented to show what to connect where:
Because TeleMega was designed to be especially well suited for use in large and complex projects, it includes extra screw terminals on board to make use of a separate pyro battery easy. In fact, use of a TeleMega with only one battery requires putting a jumper between two adjacent screw terminals to connect the main LiPo battery to the pyro power rail.
To use a separate pyro battery, just leave out the jumper and connect the pyro battery to the two adjacent screw terminals marked 'gnd' and 'pyro', using the 'gnd' terminal for the negative battery lead and the 'pyro' terminal for the positive side. Connect the e-match leads to the pyro screw terminals as usual... no off-board connections are required.
Here's a photo showing the a TeleMega board wired this way: