This circuit started as a sinewave oscillator and I was having trouble taming the little beast . The original circuit made use of a thermistor as feed back controlling element . I was going to discard the circuit when I decided to see what happens when I substitute a photocell for the thermistor . With the ambient surrounding light the circuit was not even oscillating since the photocell resistance is very low it was effectively shorting the R/C feed back circuit but when I passed my hand over the cell there was an instantaneous oscillation of a nice square wave. I coupled the output to an amplifier(LM386) and at low volume setting I obscured the photocell and was immediately blasted by the sound output . Adjust RV1 for oscillation .