This circuit is based on the first but has an astable connected to the output of the timer (monostable) to produce a pulsed output. So all the timing and function of the monostable are as before.
To make a monostable into an astable just keep repeating monstable pulses. To make this happen, the trigger pin 2, is connected to the threshold input, pin 6. When C3 charges to 6v, the threshold detects this and turns off the output (as in the monstable), but this time when the discharge pin, pin 7 goes low, C3 discharges through R6 until it reaches 3v. This triggers the IC to start timing sending the output high again and the process repeats.
The astable will not work unless pin4 on IC1b is held high. When timing starts, the output of the timer, pin 3, goes high. This turns on Q1 and illuminates the LED, showing it is timing, This means pin 4 of the astable will be low while the timing cycle is running, disabling it. When the timing is over, pin 3 goes low, turning off Q1 and the LED, and sending pin 4 high, thus activating the astable.
The output of the astable turns on Q2 and drives the buzzer. The frequency of the astable is about 5Hz and can be calculated as shown right.
Instead of using 2 555 Timers, I used a 556 dual timer, which is basically 2 timers in one 14pin DIL package, but with different pin numbers - obviously.
Download circuit simulation - Livewire
Graph showing C3 charging up to 6v (2/3 x 9) and then discharging back to 3v (1/3 x 9) and repeating.
Blue trace= C3
Red trace = Output (pin 3) of astable
The frequency can be calculated using
f = 1.44 / (R1 + 2R2) x C
In this case R1 is R5 = 10K and R2 is R6 = 120K, so the frequency is:
= 1.44 / (10000 + 240000) x 0.000001
= 1.44 / 250000 x 0.000001
= 1.44 / 0.25