This is a simple, low-cost and easy to build electronic speaker circuit which is designed close to the quad operational amplifier IC LM3900 (IC1). IC LM3900 has four independent operational amplifiers (A1 to A4) with large output voltage swing. It is capable of operating at DC voltage up to 32V.
The first op amp (A1) is designed as a low frequency square wave generator. Op amp A2 operates as an integrator, while op amp A3 operates as a comparator. A2 and A3 together act as a "drift voltage generator" op amp. Op amp A4 is connected as a buffer, and its output provides base current to NPN transistor T2. NPN transistor T2 and audio output transformer X1 form a voltage controlled oscillator.
When the power line is turned on, transistor T2 and transformer X1 produce the common/fundamental tone, which is frequency modulated by the drift voltage generator, which in turn is influenced by the low frequency square wave generator.
The circuit is powered from a regulated 9V DC supply. To produce some different tones, connect its A1 point to pins 1, 3, 4, 5, 8, 9, 10, 11, 12 and 13 of IC1 and connect point A2 to pins 1, 2, 3, 6, 8, 11 and 13.
This circuit can be used as a car speaker using an audio amplifier of about 10W.
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