Circuit Diagram

Circuit Analysis.

Based on Figure above, The signal from the piezo transducer is connected to the low pass filter, comprising IC1a and the associated resistors and capacitors. The 100kΩ resistors and 1µF capacitors set the low pass filter at 2.3Hz, while the 3.3MΩ feedback resistor and .015µF capacitor set the gain at 33 times at or below the 2.3Hz rolloff frequency.

IC1a is biased at ½ supply (+3V) at pin 3 by the 1MΩ voltage divider resistors connected across the supply. This half supply is decoupled with a 100µF capacitor.

The output of IC1a is also at ½ supply and this drives a 2.2kΩ resistor decoupled with a 470µF capacitor.

The voltage across the 470µF capacitor is therefore at 3V (½ supply) and the resistor and capacitor form a low pass filter to reject signals above 0.15Hz.

Hysteresis for the Schmitt trigger (IC1b) is set by the ratio of resistance between the 3V supply and pin 5 and the resistance between pins 5 and 7.

Thus the hysteresis can be varied from about 13mV when VR1 is wound with its wiper closes to the 2.2kΩ resistor and around 300mV when VR1’s wiper is closest to the 1MΩ resistor.
The output of the Schmitt trigger is used to drive a “charge pump” consisting of diodes D1 and D2 and capacitors C1 and C2.

These produce a voltage negative with respect to the +6V line across capacitor C2 whenever IC1b’s output is toggling (in example, the circuit is sensing water disturbance). The voltage across C2 is fed to pin 2, the trigger input of timer IC2.

IC2 is triggered when its pin 2 goes below one third of the supply voltage, or 2V. When triggered, the 47µF capacitor begins charging via the 1MΩ resistor and the pin 3 output goes high and drives transistor Q1’s base via the 2.2kΩ resistor. This transistor drives the siren.

The siren can be driven directly via the 6V supply or via the 10kΩ resistor connecting to the 6V supply for a reduced output level (for testing). This is selected using switch S2.

The output of IC2 (pin 3) stays high until the 47µF capacitor at pin 6 reaches two thirds of the supply voltage. The pin 3 output then goes low and the capacitor is discharged via the pin 7 output and 10kΩ resistor. The time duration for the alarm is around 50 seconds.

When power is first switched on, the reset input of IC2, pin 4, is held low via the 10µF capacitor to prevent the timer from being triggered by IC1b.

After about a second the reset pin voltages reaches about 1V due to the 10µF capacitor being charged via the 560kΩ resistor and then the timer can be triggered.