idea: Elex Team

Roll-o-Phone

From an electronics perspective this is actually a very simple toy, as is demonstrated by the schematic below. Two NAND Schmitt-trigger gates from a type 4093 CMOS IC (certainly familiar to the more senior among us), are used to build two astable multivibrators (oscillators); the other two gates in the IC are used to build a kind of bridge circuit, which drives the AC piezo buzzer.

The frequency-determining components of oscillator IC1a are capacitor C1 and resistors R2 and R3, and capacitor C2 plus resistors R5 and R6 for oscillator IC1c. Nothing special thus far – the neat part are the four switches S1 through S4. These are so-called reed switches – metal contacts in a small glass tube. Such a contact closes only when a magnet is held in the vicinity.

Those reed switches can (for example) be attached to the outside of a small plastic box; inside that box a magnet can move around freely. When the box is moved or shaken, different tones will be produced in a more or less random order. With the component values as shown in the schematic, the generated tones are all in the audible range. The volume can be adjusted with (trim) potentiometer P1.

A comment about resistors R1 and R4: these have the important task of ensuring that the inputs of CMOS ICs IC1a and/or IC1c will never ‘float’ when both of the reed switches happen to be open at the same time.

How you actually realize this toy we leave confidently to your imagination; and of course you do not need to limit yourself to the two oscillators with four reed switches as shown in the schematic; with an additional 4093 you can easily add four more oscillators. Here too: connect the inputs of the unused pins to ground or the power supply rail; never leave them unconnected!