This is an easy, low cost and easy built circuit of an electronic horn which is designed close to quadruple op-amp IC LM3900 (IC1). IC LM3900 has four independent operational-amplifiers (A1 through A4) having a large output voltage swing. It is able to operate at up to 32V DC.
The 1st op-amp (A1) is designed as being a low-frequency squarewave generator. Op-amp A2 operates as an integrator, whilst op-amp A3 runs as being a comparator. A2 and A3 together with each other perform as being a “wandering voltage generator” op-amp. Op-amp A4 is wired as being a buffer and its output delivers base current to NPN transistor T2. NPN transistor T2 and audio output transformer X1 create a voltage-controlled oscillator.
When power line is switched on, a general/basic tone is produced by transistor T2 and transformer X1, that is frequency-modulated by the wandering voltage generator, which, in turn, is affected by the low-frequency squarewave generator.
The circuit runs off regulated 9V DC supply. To produce some various tones, connect its point A1 to pins 1, 3, 4, 5, 8, 9, 10, 11, 12 and 13 of IC1 and point A2 to pins 1, 2, 3, 6, 8, 11 and 13.
The circuit could possibly be applied as an automobile horn by utilizing about 10W audio amplifier.