This is a simple pedal switch converted to transmit MIDI Note On commands.
The pedal's 1/4" phone plug is replaced with a MIDI plug and the circuitry requires only a switch, a 3volt coin cell battery, and an 18 pin PIC16F87. No other components are necessary.
The programming makes use of the PIC16F87's internal 8MHz clock, and its internal UART serial transmitter set to the MIDI baud rate. An interrupt based program allows the processor to sleep when there is no activity so there is hardly any drain on the battery. A pdf paper below describes how interrupts work on the PIC16F87.