Flash signal light circuit 2

The circuit is shown in Figure 13-4. It uses triode control.
    The two transistors VTi and VTz are coupled by the capacitor C, so that they are turned on and off in turn, causing the signal light H to flash
. The flash frequency is determined by the penetration current I" of the transistor VTi and the capacity of the capacitor c. According to the component values ​​in the figure,
the flash frequency of the bulb is about 20 times per minute.