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This electronic siren gives out a continuously varying high amplitude sound. Since the supply voltage is not critical, it can be used in cars, motor cycles or at home. It can replace the ordinary call bell. - . The circuit consists of two separate free running multivibrator and an oscillator. More innovative circuits HERE. A free running or astable multivibrator is one which has two quasi-stable states and the output of one stag is connected to the input of the other through a coupling capacitor.Since both the states are quasi-stable, the output attained is continuously varying in nature i.e. high, low high low-.  The output is in the form low pulses, the frequency of which depends on the base biasing resistor and the coupling capacitor, When these resistances and condensers for both the stages are of different values, the output ` wave form is rectangular; this is because the time constant of the two quasi-stable states becomes different. If this time constant of the two, states i...

This is the simple ic based Siren Alarm Circuit Diagram.  This siren alarm circuit may provide the final building block in an alarm circuit using a relay to activate it. This siren circuit is very simple and is constructed using common electronic components .  Siren Alarm Circuit Diagram When the switch is pressed C3 charges up through R4 with a time constant of 0.47 seconds and 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 358 used in this project is set up as a voltage controlled oscillator. When the output of the oscillator (pin 7) switches low there is a charge remaining in 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...

Description This circuit was requested by several correspondents. Its purpose was to obtain more power than the siren circuit already available on this website (One-IC two-tones Siren) and to avoid the use of ICs. A complementary transistor pair (Q2 & Q3) is wired as a high efficiency oscillator, directly driving the loudspeaker. Q1 ensures a full charge of C2 when power is applied to the circuit. Pressing on P1, C2 gradually discharges through R8: the circuit starts oscillating at a low frequency that increases slowly until a high steady tone is reached and kept indefinitely. When P1 is released, the output tone frequency decreases slowly as C2 is charged to the battery positive voltage through R6 and the Base-Emitter junction of Q2. When C2 is fully charged the circuit stops oscillating, reaching a stand-by status. Circuit Diagram:  Parts: P1 = SPST Pushbutton Operating Switch R1 = 1K R2 = 10K R3 = 1K R4 = 220R R5 = 10K R6 = 220K R7 = 22K R8 = 100K C1 = 22uF-25V C2 = 22uF-25V...