Schalldämpfer (Stereo-Lautstärke)

Although not exactly revolutionary, the circuit shown here is both very cheap and reliable. The well-known 555 timer IC is connected as an astable multivibrator, and delivers a regular train of pulses which are rendered audible via the transistor and loudspeaker. The frequency of the metronome can be varied with potentiometer P1. A 9 V supply voltage means that the circuit can easily be powered by batteries.