IC1 astable gives a fixed square wave at pin 3, C1 and R1 derive uS trigger pulses from IC1 and this will trigger IC2 monostable or single shot, the voltage at pin 5 of IC2 will change the pulse width output of IC2, to get it working all the three RC combinations have to be figured out.
Optical Obstacle Switch.
You can even build a small SMPS with this or even control the temperature of your soldering iron using the SSR solid state relay circuits in power section, then you need to think and design the cycle time of a soldering iron heat control system, it will be in seconds but then above circuit is running at audio frequencies, then you have to work that out yourself..
CD4538 is a dual Monostable Multivibrator. When you trigger the chip the output sends off one single pulse or one high-low event.
Mixed Circuits Analog with Digital
The T+ pin 4 of U1a is the positive edge trigger or raising edge trigger input, the T- pin 5 is falling edge or negative edge trigger input. Now see the image of the single pulse above which shows both the edges, If this is the input pulse at pin 5 then the falling edge turns the output pin 6 from low to high, this output remains high for time T = R2 * C1 and then goes low again, The output Q at pin 6 also looks like the image of pulse above.
The Output pin 7 is the complementary state of pin 6, it is the reverse state or inverted form of pin 6 output.
Now why is a slope shown in the edges, this i have exaggerated a bit so that it can be explained. But then there is a slight slope due to gate input and output capacitance.
In fact if you had a wire or twisted track coming to the input and the R2C1 was in nano seconds, then you would see a ringing at the edges, a tiny peak or spike, which will have giga hertz frequency components, in fact a square way may be many sine waves put together, this you know from a spectrum analyzer.
Two Stage Sequential Timer
CD4538B can give an output with pulse width of 1uS and above. 74HC4538 gives 120nS to 60 Seconds pulses. The above circuit produces a pulse of width T = R3 * C2 after a delay of T = R2 * C1. Some Chips formula is T = 0.7 * R * C .