This is a low cost, simple, yet a surprisingly powerful electronic siren powered by just a 9V battery. The circuit may provide the final circuit block module in an alarm circuit using a relay to activate it.

How The Simple Electronic Siren Work

When the switch is pressed C3 charges up through R4 with a time constant of 0.47 seconds. When the switch is released C3 begins a slower discharge through R7 and R3 with a time constant of about 5 seconds. The op amp is set up as a voltage controlled oscillator. The control voltage in this simple electronic siren circuit is the exponential rise and fall in the voltage of C3 as it charges and discharges.

When the output of the oscillator (pin 7) switches low, there is a charge remaining on C1 which holds pin 5 below the switching point. Current through R7 is proportional to the control voltage on C3. This current discharges C1 causing the voltage on pin 5 to rise towards the switching point at a rate proportional to the voltage on C3. When the switching point is reached pin 7 switches high, and initially pulls pin 6 high via C1. This causes the op amp to temporarily turn on hard. But C3 quickly recharges through D2 causing the voltage on pin 5 to fall below the switching point and causing the op amp to switch off again.

The positive pulse output from the op amp puts a fixed amount of charge into C2 slightly raising the potential of pin 6. This causes the potential on pin 6 to rise and assist the sharp switch off of the op amp. Also R5 & C2 delay the rise on pin 6 long enough to get a good output pulse.

The cycle then repeats. However, during the C3 discharge cycle the rate of charge of C1 is lower with each repetition of the oscillator (because the control voltage is lower) and the output frequency is correspondingly lower. During the C3 charge cycle the reverse applies.

The output pulses are buffered by a second op amp then the current is applied to a driver transistor. The output waveform has a low duty cycle, but gives a surprisingly loud sound.

The kit of this simple electronic siren based LM358 is available. Download the PDF version, part list included there…

PDF Manual Simple Siren Circuit Kit

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This is the diagram of heat detector circuit which already integrated with siren circuit in the output. This circuit applies a complementary pair comprising NPN metallic transistor T1 (BC109) and PNP germanium transistor T2 (AC188) to detect heat (because of outbreak of fire, for example) in the area and activate a siren/alarm. The collector of transistor T1 is joined towards the base of transistor T2, while the collector of transistor T2 is joined to relay RL1.

The 2nd section of the circuit consists of well-known IC UM3561 (a siren and machine-gun sound generator IC), which is able to generate the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are joined towards the +3V supply once the relay is in activated state, whereas pin 2 is grounded. A resistor (R2) joined across pins 7 and 8 is utilized to deal with the frequency of the inbuilt oscillator. The output is obtainable from pin 3.

Two transistors BC147 (T3) and BEL187 (T4) are joined in Darlington configuration to boost the sound from UM3561. Resistor R4 in series with a 3V zener is utilized to deliver the 3V supply to UM3561 when the relay is in activated state. LED1, joined in series with 68-ohm resistor R1 across resistor R4, glows once the siren/alarm is activated.

To check the operating of this circuit, provide a burning matchstick near to transistor T1 (BC109), that causes the resistance of its emitter-collector junction to go low because of an increase in temperature and it will begin conducting. At the same time, transistor T2 also conducts since its base is joined to the collector of transistor T1. Because of this, relay RL1 activates and switches on the siren circuit to generate loud sound of a fire-brigade alarm.

Below table is the possible sound effects generated by UM3561:

This heat detector + siren circuit already tested and it is working.