Two Tone Alarm Circuit

Sound alarm generators are essential in several applications, for example fire and burglar alarm techniques.

This specific circuit creates a infiltrating two tone alarm signal possessing an output power of among around 250 mW and 4 W RMS based on the speaker impedance and supply voltage applied.

For low power applications a 9 volt supply and also a speaker impedance of approximately 64 to 80 ohms will provide a nominal output power of 250 to 300 mW for a current utilization of about 55 to 70 mA.

Wherever high volume is required, a 12 volt supply and 8 ohm speaker will offer an output power of around 4 W RMS at a mean current usage of around 700 mA.

The two tone alarm circuit is based on two of the four Norton amplifiers involved in the LM3900N IC.

A Norton amplifier is in several techniques just like a common functional amplifier, however it is the comparative input currents instead of the input voltages which figure out the output voltage.

IC1b is utilized within a form of relaxation oscillator which usually produces the sound tone. At first the bias current flowing into the non inverting input requires the output high, and C2 starts to charge through R9.

This triggers the current flowing into the inverting input via R6 to slowly increase since the voltage on C2 accumulates, till it surpasses the non inverting input bias current.

The output of IC1b then moves lower, triggering C2 to release via R9 prior to the inverting input current gets to be lower than which flowing into the non inverting input.

IC1b output after that will go higher, plus the techniques commences once again right from the start, presenting constant oscillation.

Remember that whenever IC1b output went through the high to the low state this triggered R8 draining away a few of the non inverting input bias current where it seemed to be earlier included in it.

This will make it essential for C2 to substantially release prior to the current into the inverting input falls below that placed on the non inverting input.

This effect is a sort of "hysteresis", and it is necessary in functioning of the two tone alarm circuit. R8 furthermore delivers optimistic feedback which usually helps to ensure that as soon as IC1b output starts off to change polarity it quickly and dependably switches from one state to another.

The squarewave output of IC1b reaches at pretty high impedance, and so the loudspeaker is powered utilizing a common source amplifier making use of VMOS transistor Q1.

IC1a is employed in a second oscillator circuit, but it has component values which often provide oscillation at a frequency of just a few Hertz.

Its output is freely combined to C2 by R5, and it frequency modulates the tone generator to generate a kind of warbling impact.

This provides an infinitely more recognizable and fewer effortlessly masked signal than the usual easy audio tone.