Many of today’s modern alarm systems employ a moving coil loudspeaker as the siren element. To interface between the alarm system’s processing unit and the speaker, the usual circuits consist of a signal generation IC and a discrete ’H’-bridge. The signal generation circuit is often built around standard 555 timers, although other systems may use discrete transistor circuits, op-amp derived circuits or even small microcontrollers. The 555 configuration will consist of a slow running astable multivibrator, generating a ramp signal, which modulates a second multivibrator causing it to sweep over a range of frequencies.

The drive circuit usually employs TO126 or TO220 packaged power transistors, since the currents involved and the resulting power dissipation are both in excess of common smaller packaged products. The devices used normally require parallel collector-emitter diodes, to divert destructive reverse transients generated by the inductive load, and base emitter resistors to provide a path for any transistor leakage current.

This factor is particularly important with usual TO126/TO220 products as their poor VCE(sat) performance can lead to a significant temperature rise. Some circuits use Darlington TO220’s which can reduce the component count if the devices include integral collectoremitter diodes however, the Darlington configuration produces a high on-state voltage which causes excess dissipation. This type of circuit requires base emitter resistors and also reduces the power delivered to the load.

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