Two Tones (Ambulance) Alarm Circuit
Using very popular multivibrator 555 IC, we can build an alarm circuit which is similar to old ambulance siren. There are two sections inside the circuit, first is a low frequency oscillator for two level frequency modulation, and the second is the higher frequency oscillator for sound generation. Here is the schematic diagram of the circuit:
The upper part is the low frequency oscillator, and the output of this section is a square wave around 1Hz. This signal is fed to the pin 5 of the lower IC, to control the frequency of the lower part. The frequency of hi-lo frequency switching is controllable via the left potentiometer, and the frequency of both high and low sound is controlled by the right potentiometer. The power supply voltage of this circuit can be around 4.5V to 12V, but for best performance an reliability 9V is recommended. Some 555 IC produce excessive heat when operated at 12V, although it should be OK according to its specification.