Controlling birdsong playback

Introduction

The goal of this project was to build an interface from two simple switches to a Windows laptop so that the laptop could play one of two bird songs, depending on which switch was closed. The idea is to have a bird peck at the switches to control what song is played.

There are two parts to this project:

• Formatting the switch closures into a form which was easy to interface to a PC. I chose to use a microcontroller to read the switches and format the data into a serial stream. The microcontroller emitted the number of switch closures (debounced) for each switch since the last read request. The PC requested data about 25 times per second. The first version of the circuit used normally-open mechanical switches which connected the input of the MCU to ground when pressed. These switches proved unreliable because the birds could not always push the lever far enough (about 5mm) to actuate the switch. The second version of the circuit used slotted photointerruptors instead of switches, but the program logic is unchanged. When the mechanical switch was closed, the input dropped to logic low. The interruptor we used (Optek OPB625) produced a logic low when the beam was blocked. The birds will push a small lever which will cause an opaque tab to cut off the beam. The total distance change is only around one millimeter. The schematic of each sensor board is shown below. The logic in the OPB625 produces a logic-high when lit and logic-low when dark.
  
• Generating the sounds in a flexible manner on the PC. A matlab program queried the microcontroller for switch positions, then played one of two songs. Multiple switch closures were queued so that each song played once for each switch closure.

The Microcontroller

The microcontroller was built on a generic board designed for ECE476. This board includes the microcontroller clock and RS232 serial interface. Two bits of PortC were attached to switches (or interruptor boards). The main board and two slotted interrupter units are shown below. Three wires (5 volts, ground, and output) connect each sensor board to the main board.

The microcontroller code is written in CodeVision C and consists of two parts. The timer 0 interrupt service routine (ISR) runs a state machine to debounce the switches and blinks an LED to show the system is alive. The main loop sets up the I/O ports and timers, then continuously polls to see if the PC has sent the character "s". When the PC sends the "s", the microcontroller responds with a serial string consisting of two numbers.

The Matlab PC code.

To run this program, you must own Matlab 7.0 or above. The program opens a serial port, then queries the user for two wav file names, which will be the songs played when the switches are pressed. It then opens a window which contains a quit button, the two filenames and two numbers indicating the total number of times each button was pressed. The program then repeats a query to the microcontroller (by sending a "s") and listens for the string of two numbers returning from the microcontroller. If either number is nonzero, the program plays the appropriate song.