How do we build a driver for a high power siren?

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Most car or home alarm systems use a high-power speaker, controlled (powered) by an alternating, rectangular signal, via an "H" bridge, with transistors, in the capsule. TO126 (type BC139) or TO220 (type BC911).

Electronic circuit operation

The electronic assembly presented in the article uses, at the output ("H" Bridge Output Stage), power transistors, but in the TO92 capsule. Part of the order, Signal Generation Circuitry, uses transistors 2N2222. The diodes are 1N4148, and are very important in the operation of the output stage, especially when switching the "H" bridge, from one arm to another.

Electronic scheme

The speaker, connected to the Output, must have an impedance of approx. 6 Ohm. A lower impedance speaker can be used if the values ​​of R13 and R14 are reduced to ensure a higher base current for transistors; the transistors will be replaced with models ZTX851 / ZTX951 (BD681), so that the current reaches 4… 5A.

Obtained parameters (with 6 Ohm load)

  • 12-14V supply voltage;
  • current consumption 1.2A;
  • 98.5% yield;
  • frequency 870Hz… 1.9kHz;
  • modulation with 1… 9V ramp at 4Hz.

List of required components (with equivalents)

R1 = 680 Ohms;
R2, R3 = 27k;
R4 = 4.7k;
R5 = 10k;
R6 = 47k;
R7 = 680 Ohms;
R8, R9 = 5.6k;
R10 = 560 Ohms;
R11, R12 = 3.3k;
R13, R14 = 330 Ohms;
C1, C3 = 0.1uF;
C2, C4 = 10uF;
C5 = 47uF;
C6 = 470uF;
D1 - D4 = 1N4148;
Q1 - Q6 = 2N2222 (or BC547, BC171);
Q7, Q8 = ZTX690B (or BD237, BD681);
Q9, Q10 = ZTX790A (or BD238, BD 682);
1 x 25W / 6ohm speaker;
2 x terminal contacts for power supply and output.


Conex Club Magazine - no.7-8 - 2006
Zetex Handbook Applications


  1. Easiest driver is a dual 555 timer driving a couple mosfets that drive a step up transformer that runs a resonant pezio speaker. If it is a 3 element one like in a smoke detector then the piezo can provide feedback using a single transistor and an RC circuit to control the frequency with a simple timer to determine burst rate.

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