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2. Alarms using oscillator driving high impedance earphone
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In the following circuits, the chip used is a 4093 quad NAND Schmitt trigger. These circuits will
sound the alarm when the light goes off but it can easily be changed to give an alarm when the diode is illuminated
simply by changing the places of the diode and resistor, R.
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The components R’ and C determine the frequency of the oscillator and therefore
the "pitch" of the sound produced by the alarm; try 560k and 1·5n.
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If the other two Schmitt triggers in the 4093 are used as shown below, you can have
an alarm which gives a series of "beeps" rather than a continuous tone.
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Vero diagram |
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Alarms using an electro-magnetic buzzer
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