Zetex Semiconductors have a siren driver IC Type ZSD100 available that is suitable for use in alarm systems for cars and model craft. With the addition of only a few components as shown in the diagram, the device produces an ear-splitting sound of 120 dB.
The IC contains an a.f. rectangular-wave generator that is driven by a sawtooth generator. The sawtooth sweeps the output frequency range (sweep 2:1) once every second. The frequencies of both oscillators are dependent on an internal 61.5 kΩ resistor and an external resistor, RT=<1MΩ, and capacitors CMOD and COUT. The output driver has an inverting and a non-inverting output. The simplest application of the ZSD100 is shown in Figure 2 in which the sound is produced by a piezo buzzer. If a dynamic 6 Ω loudspeaker is used, it is connected in an H-bridge, that is, driven symmetrically. The value of resistor RT must then be small, or the relevant pin must be strapped to earth.
Pinout of ZSD100
1 – RT Optional resistor to earth for improved frequency control of the modulation and the output oscillator. The RT pin may be used to disable the IC by leaving it open or connecting to Vcc.
2 – SAW Determines the shape of the modulating signal. When SAW remains open, the signal is triangular; if it is linked to CMOD, a true sawtooth is obtained.
3 – CMOD The capacitor to earth (0.1–100 μF) determines the frequency of the modulating signal.
4 – GND Earth.
5 – COUT The capacitor to earth (0.001–0.1 μF) determines the centre frequency of the output oscillator.
6 – Q Non-inverting output driver.
7 – Q Inverting output driver.
8 – VCC Supply voltage, 4–18 V
The IC draws a current of about 25 mA in use, and around 1 μA in the quiescent state. It is suitable for operation over the temperature range of –40 °C to +125 °C. The output can provide 5 mA at a level of 1.4 V and sink 0.5 mA. Provided the specified values of resistors and capacitors are maintained, the circuit is ideal for experimentation. The frequencies are given by:
where the capacitors are in μF and RT is in kilohms.