From an eighties edition of Radio Shack ScienceFair 130 in one electronic project lab.
When the switch is open, the capacitor slowly charges and the transistor is off.
When the switch is closed, faster path through the capacitor drives the transistor base on, which creates a low-resistance discharge path that causes the capacitor to discharge rapidly through the transistor.
This produces a burst of current through A–C, which continues until the capacitor voltage drops enough to turn the transistor off again.
