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

1 file(s) 33.49 KB

Download

Here the circuit diagram of electronic siren based NE555. This circuit produces a sound like factory siren. It applies a 555 timer IC which is utilized as an astable multivibrator of a center frequency of about 300Hz. The frequency is controlled by the pin 5 of the IC. When the power supply is switched ON, the capacitor charges slowly and this alters the voltage at pin 5 of the IC therefore the frequenct gradually increases.

When the capacitor is totally charged, the frequency no longer increases. Now, when the push button siren control switch is held depressed, the capacitor discharges and the siren frequency also decreases.The presets VR1 and VR2 needs to be adjusted for optimum capabilities.

How the circuit work:

The 555 on the right is wired as an alarm sound generator and the second 555 timer on the left is a 1 Hz astable multivibrator. The output of the left timer is to modulate the frequency of the right timer. This process will cause the right timers frequency to alternate between 440Hz and 550Hz at a 1 Hz cyclic rate. The transistor 2N3055 is used to amplify the sound signal to the loudspeaker, with transistor type of?2N3055, this circuit should be loudy . This circuit should be nice for newbie hobbysts. This circuit works with 12VDC power supply or battery.

This is Electronic Siren circuit diagram which use standard discrete components. The sound produced imitates the rise and fall of an American police siren. When first switched on the 10u capacitors is discharged and both transistors are off. When the push button switch is pressed to 10u capacitor will charge via the 22k resistor. This voltage is applied to the base of the BC108B which will turn on slowly. When the switch is released the capacitor will discharge via the 100k and 47k base resistors and the transistor will slowly turn off. The change in voltage alters the frequency of the siren.

The siren sound generated from the oscillation process. Oscillator action is as follows:
As the BC108B transistor switches on its collector voltage falls and so the 2N3702 transistor is switched on. The process is very fast ( less than 1us). The 22n capacitor will charge very fast as well. As this capacitor is wired between the collector of the 2N3702 and the base of the BC108B, it soon reaches almost full supply voltage. The charging current for the capacitor is then much reduced and the collector emitter voltage of the 2N3072 is therefore increased; the collector potential will fall. This change in voltage is passed through the 22n capacitor to the base of the BC108B causing it to come out of saturation slightly. As this happens its collector voltage will rise and turn off the 2N3072 transistor more. This continues until both transistors are off. The 22n capacitor will then discharge via the 100k, 22k resistor, the closed push button switch, 9V battery, the speaker and 56 ohm resistor. The discharge time takes around 5-6msec. As soon as the 22n capacitor is discharged, the BC108B transistor will switch on again and the cycle repeats. The difference in voltage at the collector of the BC108B (caused by the charging 10u capacitor) causes the tone of the siren to change. As the 10u capacitor is charged, the tone of the siren will rise, and as it is discharged, it will fall. A 64 ohm loudspeaker may be used in place of the 8 ohm and 56 resistor, and the values of the electronic components may be changed to generate different sound effects of the siren.

Current drain is fairly high in this circuit so a suitable power supply is needed. The duration the tone takes to rise and fall is determined by the 10u and 22k resistor. These values may be changed for different effects.