3. Using Altus Metrum Hardware¶
Here are general instructions for hooking up an Altus Metrum flight computer. Instructions specific to each model will be found in the section devoted to that model below.
3.1. Wiring and Electrical Interference¶
To prevent electrical interference from affecting the operation of the flight computer, it’s important to always twist pairs of wires connected to the board. Twist the switch leads, the pyro leads and the battery leads. This reduces interference through a mechanism called common mode rejection.
3.2. Hooking Up Lithium Polymer Batteries¶
All Altus Metrum flight computers have a two pin JST PH series connector to connect up a single-cell Lithium Polymer cell (3.7V nominal). You can purchase matching batteries from the Altus Metrum store, or other vendors, or you can make your own. Pin 1 of the connector is positive, pin 2 is negative. Spark Fun sells a cable with the connector attached, which they call a JST Jumper 2 Wire Assembly
Warning
Many RC vendors also sell lithium polymer batteries with this same connector. All that we have found use the opposite polarity, and if you use them that way, you will damage or destroy the flight computer.
3.3. Hooking Up Pyro Charges¶
Altus Metrum flight computers always have two screws for each pyro charge. This means you shouldn’t need to put two wires into a screw terminal or connect leads from pyro charges together externally.
On the flight computer, one lead from each charge is hooked to the positive battery terminal through the power switch. The other lead is connected through the pyro circuit, which is connected to the negative battery terminal when the pyro circuit is fired.
3.4. Hooking Up a Power Switch¶
Altus Metrum flight computers need an external power switch to turn them on. This disconnects both the computer and the pyro charges from the battery, preventing the charges from firing when in the Off position. The switch is in-line with the positive battery terminal.
3.5. Understanding Beeps¶
Altus Metrum flight computers include a beeper to provide information about the state of the system. TeleMini doesn’t have room for a beeper, so instead it uses an LED, which works the same, except for every beep is replaced with the flash of the LED.
Here’s a short summary of all of the modes and the beeping (or flashing, in the case of TeleMini v1) that accompanies each mode. In the description of the beeping pattern, “dit” means a short beep while “dah” means a long beep (three times as long). “Brap” means a long dissonant tone.
| Mode Name | Abbreviation | Beeps | Description |
|---|---|---|---|
| Startup | S | battery voltage in decivolts | Calibrating sensors, detecting orientation. |
| Idle | I | dit dit | Ready to accept commands over USB or radio link. |
| Pad | P | dit dah dah dit | Waiting for launch. Not listening for commands. |
| Boost | B | dah dit dit dit | Accelerating upwards. |
| Fast | F | dit dit dah dit | Decelerating, but moving faster than 200m/s. |
| Coast | C | dah dit dah dit | Decelerating, moving slower than 200m/s |
| Drogue | D | dah dit dit | Descending after apogee. Above main height. |
| Main | M | dah dah | Descending. Below main height. |
| Landed | L | dit dah dit dit | Stable altitude for at least ten seconds. |
| Sensor error | X | dah dit dit dah | Error detected during sensor calibration. |
Here’s a summary of all of the Pad and Idle mode indications. In Idle mode, you’ll hear one of these just once after the two short dits indicating idle mode. In Pad mode, after the dit dah dah dit indicating Pad mode, you’ll hear these once every five seconds.
| Name | Beeps | Description |
|---|---|---|
| Neither | brap | No continuity detected on either apogee or main igniters. |
| Apogee | dit | Continuity detected only on apogee igniter. |
| Main | dit dit | Continuity detected only on main igniter. |
| Both | dit dit dit | Continuity detected on both igniters. |
| Storage Full | warble | On-board data logging storage is full. This will not prevent the flight computer from safely controlling the flight or transmitting telemetry signals, but no record of the flight will be stored in on-board flash. |
For devices with a radio transmitter, in addition to the digital and APRS telemetry signals, you can also receive audio tones with a standard amateur 70cm FM receiver. While on the pad, you will hear igniter status once every five seconds.
| Name | Beeps | Description |
|---|---|---|
| Neither | ½ second tone | No continuity detected on either apogee or main igniters. |
| Apogee | dit | Continuity detected only on apogee igniter. |
| Main | dit dit | Continuity detected only on main igniter. |
| Both | dit dit dit | Continuity detected on both igniters. |
During ascent, the tones will be muted to allow the telemetry data to consume the full radio bandwidth.
During descent and after landing, a ½ second tone will be transmitted every five seconds. This can be used to find the rocket using RDF techniques when the signal is too weak to receive GPS information via telemetry or APRS.
3.6. Turning On the Power¶
Connect a battery and power switch and turn the switch to “on”. The flight computer will signal power on by reporting the battery voltage and then perform an internal self test and sensor calibration.
Once the self test and calibration are complete, there are two modes that an Altus Metrum flight computer can operate in:
- Flight/Pad
- The flight computer is waiting to detect launch and then fly the rocket. In this mode, the USB link is disabled, and the radio goes into transmit-only mode. The only way to get out of this mode is to power the flight computer down.
- Idle
- The flight computer is ready to communicate over USB and in packet mode over the radio. You can configure the flight computer, download data or display the current state.
For flight computers with accelerometers (TeleMetrum, EasyMega and TeleMega), the mode is selected by the orientation of the board during the self test interval. If the board is pointing upwards as if ready to fly, it will enter Flight/Pad mode. Otherwise, it will enter Idle mode.
For EasyMini, if the USB cable is connected to a computer, it will enter Idle mode. Otherwise, it will enter Flight/Pad mode.
For TeleMini v1.0, if a packet link is waiting to connect when the device is powered on, it will enter Idle mode, otherwise it will enter Flight/Pad mode.
You can see in Understanding Beeps how to tell which mode the flight computer is in.
3.7. Using an External Active Switch Circuit¶
You can use an active switch circuit, such as the Featherweight Magnetic Switch, with any Altus Metrum flight computer. These require three connections, one to the battery, one to the positive power input on the flight computer and one to ground. Find instructions on how to hook these up for each flight computer below. Then follow the instructions that come with your active switch to connect it up.
3.8. Using a Separate Pyro Battery¶
As mentioned in Hooking Up Pyro Charges, one lead for each of the pyro charges is connected through the power switch directly to the positive battery terminal. The other lead is connected to the pyro circuit, which connects it to the negative battery terminal when the pyro circuit is fired. The pyro circuit on all of the flight computers is designed to handle up to 16V.
To use a separate pyro battery, connect the negative pyro battery terminal to the flight computer ground terminal, the positive battery terminal to the igniter and the other igniter lead to the negative pyro terminal on the flight computer. When the pyro channel fires, it will complete the circuit between the negative pyro terminal and the ground terminal, firing the igniter. Specific instructions on how to hook this up for each flight computer will be found in the section below for that flight computer.
3.9. Using a Different Kind of Battery¶
EasyMini and TeleMini v2 are designed to use either a lithium polymer battery or any other battery producing between 4 and 12 volts, such as a rectangular 9V battery.
Warning
TeleMega, EasyMega and TeleMetrum are only designed to operate off a single-cell Lithium Polymer battery and cannot be used with any other kind. Connecting a different kind of battery to any of these will destroy the board.
