11/27/2023 0 Comments Red alert alarmThis produces a quick rising and fading sawtooth pluses across the capacitor C1. The capacitor C1 is alternately charged through the elements R1 and D1, and is alternately discharged through R2. The IC 2 here is configured as a non symmetrical oscillator circuit. Just like the previous alarm and siren sound circuits, this circuit also keeps repeating the sequence as long as it remains powered. Typically, the red alert alarm sound initiates with a low frequency tone, that rises to a high frequency note within a quick span of around 1.15 seconds, and cuts off for 0.35 seconds, and again rises from a low to high frequency, and the cycle continues giving rise to the star trek red-alert alarm sound. It is the red-alert alarm sound generator, also called the star trek siren due to its frequent use in the popular TV series star trek. The fifth circuit in the list is another very interesting sound effect generator using the IC 555 astable. Due to this pin 3 frequency and PWM also modulates accordingly generating the wailing police siren sound effect. ![]() The slow ramp causes alternate cycles of slow rising voltage for 3 seconds, and slow decaying voltage for the 3 seconds on pin 5. The slow rising and falling ramp at base of Q1 follows at its emitter and modulates pin 5 of IC1. The frequency of IC1 is set at 500 Hz which becomes its center frequency. The slow exponential triangle wave ramp generated across its C1 is fed at the base of Q1 configured as an emitter follower. Here IC2 is connected as a low frequency oscillator with a frequency set at a 6 second ON OFF rate. This fourth circuit is designed to produce the typical wailing sound commonly heard in police sirens. The IC 555 can be also used for making a perfectly imitating police siren circuit as demonstrated above. This causes the speaker to generate the required sharp high volume warble alarm sound at 1 Hz rate. The 1 Hz alternate high low supply at pin 5 of IC2 forces the output pin #3 voltage of IC 1 to generate a symmetrically changing frequency varying from 500 Hz to 440 Hz. We know that pin 5 can be used with an external high/low signal for modulating the pin 3 output with a corresponding widening/narrowing pulse widths. Also known as he-haw alarm, these are commonly used in European emergency vehicles. If you want to convert the previous design into an ear piercing warble alarm, you can do it by simply replacing the D1 diode with a 10 K resistor as revealed in the above diagram. This produces a 1Hz sharp pulsed alarm effect on the loudspeaker. This in turn causes pin 3 800 Hz pulse width to narrow to an extent which almost turns OFF Q1. ![]() Here IC 2 is configured as a 1 Hz oscillator circuit which causes pin 5 of IC 1 to alternately become low at a 1 Hz rate. We have already studied how pin 5 can be used for controlling the pulse width of the IC 555. The previous 800 Hz monotone alarm could be converted into a more interesting pulsed 800 Hz alarm by adding another astable multivibrator with the tone generator circuit as shown in the second concept below. Since the design may produce a high level of ripple voltage, D1 and C3 are included to prevent the ripple interference with the IC 555 functioning.ĭiodes D2 and D3 are included to neutralize the inductive switching spikes generated from the speaker coil, and to safeguard the transistor Q1 against damage. A safe value could be around 70 Ohms 1 watt.įor making a high power continuous tone alarm circuit we cam upgrade the above circuit through a power transistor driver Q1 and a more powerful loudspeaker, as shown below: The speaker can have any impedance value, due to the presence of the current limiting resistance Rx.
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