Are you asking how this circuit works?
The output of the 555 is roughly form 0 to 12 volts. (slightly less)
The first cap "AC" couples the signal. The right side of the first cap has the same 12 volts peak to peak, but the DC is not the same. The diode going to ground keeps this signal from ever going more positive than 0.6 volts. So the voltage is -11.4 to +0.7.
The next diode and cap peak detect (rectify and filter) the signal. (in the negative direction) The -11.4 volts pull down on the output cap. There is another diode loss so -10.7 volts is about what you should get out.