4. TeleMetrum¶
Fig. 4.1 TeleMetrum v2 Board
Fig. 4.2 TeleMetrum v1 Board
TeleMetrum is a 1 inch by 2¾ inch circuit board. It was designed to fit inside coupler for 29mm air-frame tubing, but using it in a tube that small in diameter may require some creativity in mounting and wiring to succeed! The presence of an accelerometer means TeleMetrum should be aligned along the flight axis of the airframe, and by default the ¼ wave UHF wire antenna should be on the nose-cone end of the board. The antenna wire is about 7 inches long, and wiring for a power switch and the e-matches for apogee and main ejection charges depart from the fin can end of the board, meaning an ideal “simple” avionics bay for TeleMetrum should have at least 10 inches of interior length.
There are two generations of the TeleMetrum design. The major changes in the v2 generation are:
- uBlox GPS chip certified for altitude records
- Higher power radio (40mW vs 10mW)
- APRS support
Otherwise, they’re the same size, with mounting holes and screw terminals in the same position.
4.1. TeleMetrum Screw Terminals¶
TeleMetrum has six screw terminals on the end of the board opposite the telemetry antenna. Two are for the power switch, and two each for the apogee and main igniter circuits. Using the picture above and starting from the top, the terminals are as follows:
| Terminal # | Terminal Name | Description |
|---|---|---|
| 1 | Switch Output | Switch connection to flight computer |
| 2 | Switch Input | Switch connection to positive battery terminal |
| 3 | Main + | Main pyro channel common connection to battery |
| 4 | Main - | Main pyro channel connection to pyro circuit |
| 5 | Apogee + | Apogee pyro channel common connection to battery |
| 6 | Apogee - | Apogee pyro channel connection to pyro circuit |
4.2. Using a Separate Pyro Battery with TeleMetrum¶
As described in Using a Separate Pyro Battery, using an external pyro battery involves connecting the negative battery terminal to the flight computer ground, connecting the positive battery terminal to one of the igniter leads and connecting the other igniter lead to the per-channel pyro circuit connection.
To connect the negative battery terminal to the TeleMetrum ground, insert a small piece of wire, 24 to 28 gauge stranded, into the GND hole just above the screw terminal strip and solder it in place.
Connecting the positive battery terminal to the pyro charges must be done separate from TeleMetrum, by soldering them together or using some other connector.
The other lead from each pyro charge is then inserted into the appropriate per-pyro channel screw terminal (terminal 4 for the Main charge, terminal 6 for the Apogee charge).
4.3. Using an Active Switch with TeleMetrum¶
As explained in Using an External Active Switch Circuit, an external active switch requires three connections, one to the positive battery terminal, one to the flight computer positive input and one to ground.
The positive battery terminal is available on screw terminal 2, the positive flight computer input is on terminal 1. To hook a lead to ground, solder a piece of wire, 24 to 28 gauge stranded, to the GND hole just above terminal 1.
