When Sl is off, Cl charges to within 0.5 V of the battery voltage through diode Dl and resistor R4. When Sl is closed, tbe anode of tbe PUT rises to the positive supply voltage. The PUT does not conduct, because battery voltage appears in series with tbe charge stored on Cl, which raises the gate of the PUT to a level positive with respect to the anode.
The timer relies on the discharge of capacitor Cl through resistors Rl, R2, R3,and R4. Once Cl is at zero volts, the PUT will turn on battery voltage to the Sonalert and cause it to sound.