A vintage compressor from 1977 housed in a black box(hence the name!) The circuit smooths out the peaks generated by the guitar. It uses a red-led(5mm) and a LDR(light depending resistor) The LDR is a penny-size thing with two legs! They are placed close to each other and are isolated against external light by black cloth or paper (photopaper?)
Uses high gain transistors-BC 209C(=BC109C)