Circuit is very simple. The generator uses a PIC12F629 microcontroller with clock frequency set by an external RC. Output frequency can be set trimmer P1 in the range of about 2 to 170 Hz. Oscillator frequency can be adjusted if you change C1 capacitance. Pulses are generated with a period of 200 Tcy. All pulses are of equal length. Output frequency is 800 times lower than the frequency of the oscillator.
For later use save calibration data before programming device. This is essentially OSCCAL constant, which is stored on the production at the end of the program memory at 3FFh. Programmer before programming erase program memory and calibration data would be lost. OSCCAL settings is different for each piece of IC.
By changing the constants in the program can be set any number of pulses up to about 65 000 (2^16). If you need to generate a different number of pulses, just change the constants in the program. I think that everything is clear from figure 2. This segment of the program there with a small variation twice for each key separately.
