Multi-mode Electronic Siren Alarm

alarmMulti mode siren electronic schematic
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This electronic siren alarm is a perfect complement to your custom designed security system. How the alarm is triggered is totally up to you. Regardless of the detection device used – trip switches, fire and smoke detectors, ultrasonic detectors, etc.- the interface to the alarm is a simple switch or relay. Wire the alarm to an existing power source and detector, add a suitable speaker, and your system is all set to operate. The circuit is based on the 556 -dual 555 timer IC.

How it works

The siren alarm circuit is shown in Fig. 1. The IC1-a timer, half of the 556, is connected as a low frequency astable multivibrator. In that configuration, the square wave output at pin 5 remains fixed at approximately 50% of the duty cycle as its frequency is varied by the timing resistor R1. With the exception of the Two-Tone and Pulse modes, the astable multivibrator’s square wave output is integrated by R-C integrators R2 and C4, producing a triangle wave at pin 11 of IC1-b. The timer IC1-b is configured as an audio-frequency voltage controlled multivibrator. Its frequency is controlled by the voltage on the control-voltage input pin 11 and by the trimming resistor R3. The triangular waveform at this pin frequency-modulates the IC1-b multivibrator so that the squarewave output at pin 9 rises and falls in frequency to duplicate the familiar wail of a siren.

Simple multi-mode siren

The multi-mode electronic siren schematic. Typical values for R1-R3 and C4 are listed in Table 1.

The Two-Tone mode uses the same squarewave output from the IC1-a multivibrator except that this is not integrated before it is used to frequency-modulate the next stage. The squarewave, attenuated by R2, causes the voltage controlled multivibrator to shift frequency abruptly at the rate determined by the low frequency astable multivibrator (IC1-a). The resulting sound is adistinctive “twee-dell” similar to that of a European police-car siren.

The Pulse mode also uses the IC1-a’s squarewave output, but not to frequency-modulate the next stage. In this mode, the squarewave is routed to the Reset input (pin 10) of the IC1-b. As long as this pin is held high, the voltage controlled multivibrator, will operate normally but when it is brought low, the IC1-b will stop running. The squarewave, alternately high and low, will gate the voltage controlled multivibrator on an off at the frequency of the low frequency multivibrator (IC1-a). The pitch of the sound will be constant since the frequency of the voltage-controlled multivibrator is determined only by the value of the timing resistor R3.

The Yowl mode is a combination of  the Siren and Pulse modes. The squarewave from the first multivibrator (IC1-a) is both routed to the Reset input of IC1-b and integrated by C4 and R2 at the control voltage input of the voltage-controlled multivibrator.  The voltage-controlled multivibrator is gated on and off as in the Pulse mode; but every time it is gated on, it sees a rising half of the triangle wave at its control-voltage input. Consequently, the pitch of the sound is no longer constant during the on intervals but falls in frequency until the off interval begins.

The Q1 transistor is used as an amplifier and boosts the output to peak power level of over 10 watts. If Vcc is increased to 15 volts from the normal 12 volts value, the output power will be even greater. 

The squarewave output may sound harsh at some frequencies. The addition of a the filter capacitor C5 (from 0.1 to 1μF) will mellow the tone somewhat. The C5 capacitor and the R4 resistor, together form a lowpass filter to remove some of the high frequency components from the squarewave.

How to build it

The alarm is meant to be part of a bigger system and may be built on the same PCB as the detection circuit. The circuit layout is not at all critical. Normally, no heat-sink is needed for the Q1 transistor unless it is used in ambient temperatures greater than about 30 degrees Celsius.  

All resistors are of 1/4W, 5% tolerance type unless otherwise noted.


Modes of operation

The sounds that can be produced are almost limitless. On Table 1, we provide a starting guide for some typical component values for different sounds. Off course, you are free to vary those values until you create a sound you like.

Table 1. Typical component values for different sounds

Mechanical siren 5.6ΜΩ 1K 150K 1000μF Off On
Electronic siren 470K 10K 120K 100μF Off On
Warble 100K 100K 100K 10μF Off On
Two-tone 560Κ 10Κ 150Κ None Off On
Pulse 680K Open 150K None On Off
Yowl 2.2MΩ 1K 100K 500μF On Off

The rate at which the sound of the siren varies is determined by the value of the timing resistor R1; increasing its value will decrease the rate and vice-versa. The pitch of the output is determined by both the amplitude of the modulating control voltage and the value of the timing resistor R3. As the control voltage increases in amplitude, and/or the value of R3 is increased, the pitch decreases and vice-versa.

The range of the modulation, i.e., the difference between the high and low frequencies of the voltage controlled multivibrator, is set by the value of R2. A small value of R2 permits a large range while a large value restricts it. About 1Kohm is the practical minimum value for R2 as well for R1 and R3.

The range is also controlled by the value of the integrating capacitor C4. The product of the capacitor’s value and that of R2 is the time constant of the integrator. The time constant establishes the linearity of each half of the triangle wave and also limits the amplitude that the triangle wave can rise to. Because of that, there will be some interactions among R1, R2, and C4 as the range is set. 

To form a complete alarm system, the electronic siren is best activated by switching the Vcc line. So there is no standby current. The switching may be done directly by any detection device or by a relay on any alarm control unit.

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