The sensor for this alarm is an LDR and it consists of an oscillator that goes into action when the sensor stops receiving light. Adjusting the sensitivity and therefore the trigger point and the frequency of the generated signal is done in P1. Capacitor C1 also influences the frequency of the generated signal and can be changed over a wide range of values. In the figure we have the complete diagram of this experimental alarm. Note that the power supply used does not have to be symmetrical. As a low impedance speaker is used, a 120-ohm limiting resistor is provided to limit the current at the output of the operational amplifier. The circuit can be easily modified to use a loudspeaker or higher impedance transducer or to excite an amplifier stage with one or two transistors. With a BD135 transistor, for example, you can directly excite a 4 or 8-ohm speaker. It is also interesting to note that other types of resistive sensors can be used at the input of this circuit. With an NTC, for example, we can have a temperature alarm. It is also possible to use a pair of wires as water level sensors or touch on the same circuit.
