Today I ordered the parachute (a 48″ model – on the “slow and safe” side for the expected weight), a GPS antenna, power regulator for the GPS, and of course some duct tape! Every project needs some of that! It it’s good enough to save the Apollo 13 astronauts, it’s good enough for me!
So this evening I worked on the serial comms for the flight computer. The serial devices will be GPS (connected to the main (hardware) serial port), radio transmitter (simple to “bit bang” at the low baud rate), and the GSM phone for messaging. The latter will need to run from a software UART. The computer will have some logic such as “if height below 3 miles, send position every 10 minutes. If below 1 mile send every minute”, or something like that.
I have 2 options for the GSM – a Wavecom modem and a T39m mobile phone. The T39m has 3.3V logic levels which are simple to interface to the Arduino flight computer’s 5V levels, so that’s what I did first. I downloaded an updated software UART that runs on interrupts and is therefore less disruptive to other operations the computer will need to do. I then set up a simple program to echo PC keyboard characters on to the phone, and to display anything coming back. Then I wired the Arduino to the phone using a phone pinout I found on the web.
Nothing. I could see the Arduino was transmitting, but the phone didn’t respond. I tested the voltage levels and made sure things were as per the diagram, but got nothing.
Researching more, I found another pinout reference that had different pins for the transmit and receive lines. After quickly rewiring, everything worked! I hate it when people post rubbish on the web …
I then set up some messages to send to the phone so it would send me a message to my main phone. Again, nothing. Checking the phone’s menus the message was sat there in the outbox, but wasn’t being sent. No credit! The SIM card is one that sits in my netbook PC and hardly gets used, and T-Mobile had kindly timed it out! Will try with a SIM card that has credit soon!