Ca Horn for 6 -12V dc supply

Circuit diagram

 

 parts list

R1______________68K  1/4W Resistor
R2_______________2K2 1/4W Resistor
R3______________56K  1/4W Resistor
R4_______________3K3 1/4W Resistor
R5,R6____________4K7 1/4W Resistors
R7______________10K  1/2W Trimmer Cermet or carbon

C1,C2___________22nF  63V Polyester Capacitors
C3,C5__________100nF  63V Polyester Capacitors
C4_______________1nF  63V Polyester or Ceramic Capacitor
C6_____________220µF  25V Electrolytic Capacitor

IC1,IC2_________7555 or TS555CN CMos Timer ICs
IC3__________TDA7052  Audio power amplifier IC (See Notes)

SPKR___________8 Ohm  Loudspeaker (See Notes)

 

This circuit reproduces the sound of modern car horns. It was mainly designed for models and toys but, using a high output power audio amplifier IC, it can be used also in more demanding projects.

Circuit operation

To obtain a realistic car horn sound, two different tones mixed together are necessary and the interval they will form should be a so called minor third (in musical terms).
This is implemented by two 7555 CMos Timer ICs wired as astable multivibrators and generating a square wave of about 440Hz and 523Hz respectively. These frequencies should be quite precise, so the uncommon values necessary for the timing resistors are obtained by wiring two standard value resistors in series.
The square wave frequencies generated by IC1 (440Hz) and IC2 (523Hz) are mixed through R5 and R6 and shaped by C3, C4 and R7 in order to obtain a more realistic tone. The resulting composite audio signal is finally sent to the audio power amplifier IC3 which, in turn, drives the loudspeaker. R7 should be adjusted to obtain a satisfactory output level.

Notes:

• The loudspeaker can be of any shape and diameter. Obviously it must withstand a power of at least 1 Watt if the TDA7052 Audio power amplifier IC is used.
  In any case, try a good sized device, in order to obtain best results.