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# Using a Separate Pyro Battery with Altus Metrum Flight Computers #
As we have discussed
[elsewhere](http://gag.com/bdale/blog/posts/Batteries_and_Pyro_Circuits.html),
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](http://altusmetrum.org) 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!
## TeleMetrum, TeleMini, and EasyMini ##
Very early [TeleMetrum](http://altusmetrum.org/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.
Each of these products can be used with a separate pyro battery, but unlike
TeleMega and EasyMega, there are no extra screw terminals to make it easy.
On TeleMetrum, 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:
[TeleMini](http://altusmetrum.org/TeleMini)
and
[EasyMini](http://altusmetrum.org/EasyMini)
work the same way electrically, but the screw terminal physical
arrangements are different. The two pyro channels have a pin each that
are connected together and to the pyro voltage. Ignore those, the other
two screws are the switches to ground.
Here's a photo of the pyro end of a TeleMini board augmented to show what
to connect where:
And here's a photo of an EasyMini board augmented to show what
to connect where:
## TeleMega and EasyMega ##
Because [TeleMega](http://altusmetrum.org/TeleMega) and
[EasyMega](http://altusmetrum.org/EasyMega) were designed to
be especially well suited for use in large
and complex projects, they include extra screw terminals on board to make use
of a separate pyro battery easy. In fact, use of a TeleMega or EasyMega
with only one
battery requires putting a jumper between two adjacent screw terminals to
connect the main LiPo battery to the pyro power rail. All such boards
ship with this jumper installed from the factory.
To use a separate pyro battery, just remove 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.
Note that when using a separate pyro battery in this way, only the "main"
battery will be charged by the on-bard LiPo charger. You will need to make
arrangements to charge the pyro battery if it's a rechargeable variety.
Here's a photo showing a TeleMega board wired this way: