Before I tackle the design of the payload, I am wanting to design a transmitter and receiver set up that will work over the distances that will be occurring during the flight. I’ve had some fun with this part of the build. It’s been good to see what works and what doesn’t.
I’ve noticed that most people that send up HABs, that transmit their data on the 433MHz band, usually use the Radiometrix modules. I am going to try my luck at the cheaper modules I got from my work. This is a rough circuit diagram of the set up I am using:
Since the Arduinos digitial I/O pins operate at TTL logic levels, and the transmitter module works on 3.3V logic levels, I had to use a buffer IC to act as logic level converter so that they could operate together safely. The 4050 chip has input protection built in so that the inputs can be safely driven higher then its supply voltage. Because of this, all that is required is that both the module and the 4050 are connected to the 3.3V power output from the Arduino for safe logic conversion between them. I am using the Freetronics Eleven as my ‘duino of choice for this test. The main advantage of this is that is has a beefed up 3.3V power rail, allowing for 200mA of current draw. This means that the logic IC and the module can run off it without a problem and also means I don’t have to design a separate power supply to run them. Oh, by the way, all of my designs are done in KiCAD, since I am an open source kind of guy. In the actual flight, I will most probably swap to either an Arduino Fio, or just load the arduino bootloader onto a 3.3V Atmel chip with a custom board. This way I will be able to reduce the weight of the payload.
For the receiver, I am using this Freetronics 433MHz receiver shield I got from work on a Arduino Duemilanove.
Next up I will talking about all of the “fun” adventures I have been having with testing different antenna combinations.