An interesting circuit
of a bicycle horn based on a popular, low cost telecom ringer chip is
described here. This circuit can be powered using the bicycle dynamo
supply and does not require batteries, which need to be replaced
frequently. The section comprising diodes (D1 and D2) and capacitors (C1
and C2) forms a half-wave voltage-doubler circuit. The output of the
voltage doubler is fed to capacitor C3 via resistor R1. The maximum DC
supply that can be applied to the input terminals of IC1 is 28V.
Therefore zener diode ZD1 is added to the circuit for protection and
voltage regulation. The remainder of the circuit is the tone generator
based on IC1 (KA2411).
The dual-tone output signal from pin 8 of IC1 is fed to the primary
of transformer X1 (same as used in transistor radios) via capacitor C6.
The secondary of X1 is connected to a loudspeaker directly. In case you
are interested in connecting a piezoceramic element in place of the
loudspeaker, remove capacitor C6, transformer X1, and the loudspeaker.
Connect one end of the piezoceramic disk to pin 5 of IC1 and the other
end to pin 8 of IC1 through a 1/4W, 1-kiloohm resistor. IC1 KA2411 is
also available in COB style, with the same pin configuration. Both packages work equally well.
However, to get the best results with the COB
package, change values of resistors R2 through R4 to 330-kilo-ohm,
capacitor C4 to 0.47µF, 63V electrolytic (positive end to pin 3 of IC1),
and C5 to 0.005µF, 63V. This bicycle horn project can also be used as a
telephone extra ringer by just removing all components on the left side
of capacitor C3 and connecting the circuit shown in Fig. 2 to the
terminals of capacitor C3.