What’s included

• 1x 1W 8Ω 13mm speaker, Multicomp MCKP1330SP1-4723

• 1x Phototransistor, Everlight PT333-3C

• 1x Hex Schmitt trigger inverter, ON Semi MC74HC14ADR2G

• 1x 47µF electrolytic capacitor, Multicomp MCUMR6V3476M4X5

• 1x 1µF ceramic capacitor, Suntan TS170R1H105MSBFA0R

• 2 x 0.1µF ceramic capacitor, Multicomp MC0805B104K500A2.54MM

• 1x 0.01µF ceramic capacitor, Suntan TS170R2A103KSBBA0R

• 12 x 4.7KΩ resistor, Multicomp MCF 0.25W 4K7

• 1x 2032 battery holder, Linx BAT-HLD-001

• 1x Small signal diode, Diotech 1N4148

• 1x SPST switch, Apem IKD0103101

• 1x 16cm of 20AWG wire

• 1x Lovely PCB

Assembly

The circuit uses a mix of surface-mount and through hole components. The surface-mount components are the switch and the main chip at the centre of the board. All the other components are larger through hole parts.

To assemble, use the assembly guide on the right, we recommended to do it in the order shown as it may be harder to solder some components when the larger ones are in place. When cutting components to length first measure against their placement on the PCB and before starting break the panel in three and smooth out the break points.

Note: Use a tool to separate the panel into the individual boards. Carefully and gently slightly bend the pieces at the snap points (the bits that hold the individual boards together) until they separate. Continue doing so until the boards separate completely. Use small pliers or cutters to remove the pieces that remain attached and smooth the area of the fastener that's left on the board.

TAKING IT FURTHER

The design uses three oscillators. They are constructed from three parts, a resistor and capacitor to create the rising and falling slope with a digital inverter as the charge and discharge control. Taking advantage of the inverters hysteretic input to determine the charge and discharge point, the frequency is simply determined by the time it takes for the capacitor to charge between the threshold levels of the inverters input.

This circuit is simple to modify. All the frequencies can be changed by playing with the appropriate resistor or capacitor values. The relevant capacitors are: C_gate for the on and off sound, C_mod is for modulating the frequency, and C_carrier is for the base frequency.

The concept behind this circuit has also been modelled in Pure Data which is an open-source audio synthesis program that uses a graphic programming style. See link for details, it demonstrates the general idea of how the sound is synthesized. Play with the falstad simulator of the circuit to see and hear the effects of changing the capacitances. As you could see the sound is not strictly modulator and carrier style FM synthesis as the capacitor labels suggest. This is because both oscillators affect each other equally as they are connected by a resistor together at the same point. But the effect is similar due to the fact that the slower oscillator has a more noticeable influence over the frequency of faster one.

Additional information

• Community contributions for this project

• Open source circuit board design files