Nuts and Volts - June 2017

Asynchronous and Pulse Inputs

Two headers (J7 and J8) are provided for asynchronous inputs. Voltage to J7 is selectable at 5V or 3.3V using jumper JP2. J8 is fixed at 3.3V. We usually use J7 for a GPS antenna module such as the Byonics GPS5HAOEM.

If 5V is selected, be sure to use a 3.9K resistor for R19 to limit current to the Propeller input pin. If 3.3V is selected, a resistor is not needed and you can use a wire jumper instead of a resistor at R19.

Either of these inputs can be used for many other devices. On several flights, we used J8 to provide power to and count pulses from the “MightyOhm” Geiger counter (see Resources sidebar).

It has also hosted a prototype gamma ray scintillator designed and built at UNH. J7 could be configured as an output in the code, and used to drive an LCD.

Flight Enable Tab

The flight enable tab at JP1 is a jumper that is pulled shortly before launch. It essentially means “committed to launching.” Pulling the tab starts the main program loop and initializes the mission clock. We attach a long piece of brightly colored tape to the jumper which adds visibility, grip, and more importantly some drama.

Relay Outputs

The relay outputs are both unpowered, and simply provide a switch closure that can be used to trigger various external devices such as cameras. Or, they could be wired into a pull-up on another microcontroller such as might be used for an independent student science experiment.

BCD Input

For several years, we have been thinking about a way to transmit commands up to the microcontroller from the ground during flight. Suggested by our colleague, Lou Broad at Timberlane Regional High School, a dual tone multi-frequency (DTMF) signal can be generated by a handheld radio on the ground and transmitted to the balloon payload over a suitable amateur radio frequency.