Advanced Electronic Siren

Posted on Nov 25, 2012

Voltage is applied to the chip via pins 14 and 7 so that when power is applied to the project, the high and low frequency oscillators will come into operation to produce a wailing sound from the piezo. The high frequency oscillator is the only tone sent to the piezo. The low frequency oscillator merely modifies the high frequency by putting a varying `set on the 1n capacitor via the 1M so the frequency of the oscillator changes. The low-frequency oscillator charges the 4u7 via the 1k and this voltage is passed to the 1n via the 1M. The best place to put the piezo is in the room you are protecting.

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The output stage of the oscillator consists of a BD 679 Darlington transistor with a piezo tweeter and 10mH choke in the collector lead. These two components form a resonant circuit at the frequency of operation and without the correct value of choke, the output would not be nearly as high. The choke works by becoming a resistance path for the transistor when it is turned on and energy flows through the coil to create magnetic flux in the core. The transistor is driven by a square wave oscillator and it is switched off very fast by this type of waveform. The magnetic flux produced by the current flowing through the turns in the coil collapses very quickly when the current stops and produces a voltage across the coil that is of opposite polarity and very high in amplitude. We have measured voltages of 70 - 120v and this voltage is passed to the piezo diaphragm to make it bend or "dish." The piezo diaphragm behaves exactly like a 30n capacitor and the energy flows out of it after a short period of time to feed the choke. The action of voltage flowing out of a capacitor is exactly the same as reversing the voltage direction and a short time later the transistor turns on again to start the cycle again. All the components fit on a small PC board and you must use modern, small-size parts so that they fit on the board. All the resistors are 1/4 watt and the electrolytics are 16v or 25v. Don't use old or large parts as they...

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