This is my latest micro-project. I turned a joule thief into a light sensitive sound/noise generator. The joule thief circuit already is a generator, but to drive a speaker I would need a some sort of preamp circuit, unless... I could use a high impedence speaker. I found one 400 Ohm 0.1W headphone speaker and attached it so joule thief's output. Yeah... this high pitched noise will cause headache quite quickly. Then I took another transistor, resistor and LDR to make it nyctophobic. I am not sure if this is done before (probably is), but it works well for a very simple circuit.

The second transistor Q2 controls the base current of Q1 which is the switching transistor, so when light hits the LDR, its resistence drops opening the Q2 and allowing the oscillator to generate. It is possible to adjust the frequency a little bit by changing the value of R2.

When left in dark room, this will make a high pitched noise or screeching sound. I see a potential for pranks.