IC 555 in this circuit is configured as a monostable multivibrator that has one input. because monostable circuits have a certain period, the number of periods is determined by a 47k resistor connected to pin6 IC and 100nF capacitors. In this series a number of Time periods “T” = 1.1 RC or 1.1 X 50,000 X 0.1 X10 -6 = 0.0055 or 5.5 mS (milliseconds) are obtained.
When this circuit works, IC will receive trigger pulses from distributor points. But at this point, it is limited to a maximum voltage of 5V through a zener diode. The milli Ampere meter will show a maximum of 50 mA when IC555 receives some pulses whose sensitivity is adjusted via a 200k potentiometer.
The power supply of this circuit is taken from a 12V car battery. But on IC555 there is only 9V voltage because it is lowered by 9V zener diodes. The thing to note is that the 10uF capacitor must be as close as possible to the IC to avoid interference.