 R1 = 10K 1/4W Resistor R2 = Photoresistor (any type) R3 = 2M2 1/4W Resistor R4 = 1M 1/4W Resistor C1= 10µF 25V Electrolytic Capacitor C2 = 100nF 63V Polyester Capacitor D1 = 1N4148 75V 150mA Diode IC1, IC2 = 7555 or TS555CN CMos Timer ICs BZ1 = Piezo sounder (incorporating 3KHz oscillator) B1 = 3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series) When a beam of light enters from the opening, or the fridge lamp lights, the photo resistor lowers its resistance (<2K) stopping C1 charging current. Therefore IC1, wired as an astable multivibrator, starts oscillating at a very low frequency and after a period of about 24 sec. its output pin (#3) goes high, enabling IC2. This chip is also wired as an astable multivibrator, driving the Piezo sounder intermittently at about 5 times per second. The alarm is activated for about 17 sec. then stopped for the same time period and the cycle repeats until the fridge door closes. **So I'm trying to build this circuit and have a question - Does this circuit start sounding the alarm 24 seconds after the light enters or just after? I don't quite understand it** and also how should I calculate to get the values 24 seconds and 17 seconds. Thank you in advance.