The Random Doorbell circuit diagram shown in Fig. plays about 17 million tunes, thus employing about one component for every one million tunes played! Low frequency oscillators IC1a to IClc provide eight possible combinations of notes, while timer IC1d plays a sequence of about eight notes every time pushbutton switch S1 is pressed – resulting in 88 tunes. (There are bound to be some world hits in there)! The circuit is based on a HEX Schmitt inverter IC1. Four inverters are wired as RC oscillators, with three of these (IC1a to IC1c) modulating the fourth (IC1e). Gate IC1d provides a timer, which plays a sequence of about eight notes, while the remaining inverter (IC1f) isolates the piezo transducer WD1 from the carefully balanced circuit.
On Balance The circuit strikes a balance between playing too many notes at too fast a tempo (which would approach “white noise”) and playing too few notes at too slow a tempo (which would tend towards monotony). This balance is fixed by resistors R2 to R4. Presets VR1 to VR3, with correct adjustment, ensure that all of the notes are played in tune. These may be used to select major or minor scales, arpeggios, or any other such combination of notes. Switch S1 may be replaced with a tilt switch or a lever-operated microswitch for various applications. Inset diagram Fig.2 shows how the sound may be amplified to a level that should be hard to ignore. Note that since the circuit is carefully balanced, only the HEF40106BP (Philips) should be used for IC1, and the supply voltage and component values should not at first be altered. On Call The author built a unit for friends, who claimed that they often didn’t hear a multitune doorbell they had, since they had grown accustomed to the sound. Since the installation of the Random Doorbell, they said, they had not missed it once. As the circuit plays continuously in the background (that is, without being heard) so that when the pushbutton is pressed, a small slice of its continual activity is played out loud, for this reason the use of a mains adapter is advisable.
