Simple sound & light alarm
This simple alarm circuit is triggered when the resistance of the sensor R1 goes below a certain limit. The sensor can be any type which changes its resistance such as an LDR, switch contact, pressure mat etc.
Trimpots VR1 and VR2 set the threshold level and depending on the resistance range of the sensor, it may be possible to omit VR1.
The circuit itself is an oscillator based on one CMOS Schmitt trigger inverter, IC1b. This is enabled or disabled, depending on the resistance of the sensor.
If the sensor resistance is high, pin 1 of IC1a will be low and its output at pin 2 will be high. This will pull pin 3 of IC1b high via diode D1 and this disables the oscillator.
The remaining inverters in the 74C14 are paralleled to drive a piezo transducer and LED2.
The frequency of the alarm is set by the 100kΩ feedback resistor and 10nF capacitor associated with oscillator IC1b.
Sofia, Bulgaria. ($35)