SIMPLE WARNING SIGNAL

Published:2013/5/7 21:25:00 Author:muriel | Keyword: SIMPLE WARNING SIGNAL | From:SeekIC

This power audio oscillator could be used as a warning signal for alarm systems or to attract attention if something is wrong with an equipment. The oscillator, about 750Hz, exploits the characteristic of certain NPN transistors, in this case a BC337, to oscillate if reverse biased and with the base open. Other equivalent transistors might not work. Despite its simplicity the circuit is quite flexible: the 390Ω resistor, normally connected to negative could be switched in through a logic circuit, so it can be driven directly by the circuit to be monitored; the base is normally not used but frequency modulation of the circuit is possible by connecting a modulating signal to the base via a high value resistor, typically 2.2MΩ. A 3W loudspeaker is adequate for the circuit and it can be either an 8 or 4Ω loudspeaker, although in the latter case a small heatsink is necessary for the BD436. Peak current for an 8Ω loudspeaker can be as high as 1.2A but because the duty cycle is relatively small, the average current was measured at 0.2A hence the overall power requirement is only 2.4W despite the high volume the circuit is capable of. The feed line must be well filtered and can be anything between 9 and 15V although adjustment of the resistor might be required as the oscillation frequency is sensitive to the supply voltage.



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