• This 555 astable oscillator has a very short duty cycle square wave output.
• When the trigger input drops below 1/3 of the supply voltage, the output goes high.
• The discharge transistor turns off and the 1nF capacitor charges rapidly through the 1K resistor.
• When the threshold input rises above 2/3 of the supply voltage, the output goes low.
• The discharge transistor turns on and the 1nF capacitor discharges slowly through the 100K resistor.
• During this period, the reverse biased diode blocks rapid discharging through the 1K resistor.

This circuit can be used with different component values to generate > 10 microsecond pulses and 40 millisecond spaces for the SRF05 ultrasound range finder. Finding the correct resistor and capacitor values is interesting and pushes the circuit to the limits of the available component values.