Signals is a free and open source app that provides countdown and start signal sounds for sailing races. When connected to the USB cable of a phone or tablet it can send simple commands to a microcontroller (an Arduino for example) or a relay so that an electric horn can be triggered. If not connected, Signals just plays the sound of an air horn.
Standard sailing start sequences are available:
plus a training/utility sequence
Multiple starts can be set up with a 5 minute interval between starts (3 minutes for the training sequence).
The sound signal can also be triggered manually by a button in the app for testing or for extra sailing race signals such as boat-over-the-line, general recall, shorten course or postponement.
For general recalls, a button in the app adds an extra start to the sequence during the countdown.
The USB signals sent are "1" to start the sound signal and "0" to stop it at 9600 baud. For automatic signals, the length of the signal is set to 1.5 seconds. For manual signals, the signal is as long as the button held down.
The micro-controller must be set up to respond to these serial messages. There are example Arduino programs at https://www.rotor-rig.com/apps/signals
The HW-667 relay is also supported
Source code is available at https://bitbucket.org/Rotor-Rig/signals-mobile
The app was made with App Inventor http://appinventor.mit.edu/