It will not work as well as your eye in detecting light because the human eye is the most sensitive light detector there is.ﾠ But it comes close.ﾠ
For best results have the MEL12 pointed in the direction which light will come. The circuit uses an MEL12 Darlington, or Super-alpha pair of phototransistors.ﾠ It is typically ten times more sensitive than normal phototransistor.ﾠ And a phototransistor is typically 100 times more sensitive than a photodiode (but the frequency of response is different - something we are not concerned with here).
All three devices are optosensors; they are electronic devices which alter their electrical characteristics in the presence of light (which may or may not be in the visible range).ﾠ In dark conditions the resistance of these devices is high; typically several megohms.ﾠ In light conditions the resistance falls to typically 30 ohms in bright sunlight.ﾠ Many practical uses of this property are possible. ﾠ The circuit diagram shows that the circuit consists of three main stages:
ﾠﾠﾠ 1. Front detector: In dark conditions the MEL12 is virtually an open circuit.ﾠ No current flows through the 500K potentiometer into the base of the transistor.ﾠ Thus the transistor is OFF and the logic level on the input line to the 400 is LOW.ﾠ
ﾠﾠﾠ 2. The 4011 and the tank circuit.ﾠ The 4011 is a quad, 2 input, NAND gate.ﾠ It is wired up to oscillate when the input to it goes high, tha is the BC557 transistor turns on after light is detected by the MEL12.ﾠ The oscillating output from the 4011 turns the BC547 on and off.ﾠ When it is ON the tank circuit (the choke and the piezo buzzer) charges up.ﾠ When the transistor turns OFF the piezo (which acts as a capacitor) discharges into the choke and makes a "click" as it does so.ﾠ Once the piezo is discharged the magnetic field around the choke starts to collapse and recharge the piezo, and the cycle repeats.ﾠ This is called "ringing".ﾠ Of course there are some resistive losses and theamplitude of the oscillation is slightly less with every cycle.ﾠ However, the ringing does not decay away completely before the transistor turns on again and recharges the tank circuit fully.
ﾠﾠﾠ 3. After the alarm has been operating and it is put back into dark conditions again then alarm will continue to sound for about 3 seconds.ﾠ This is due to the 1uF capacitor keeping the input to the 4011 HIGH.ﾠ To increase the delay increase the value of this component.
The kit introduces the 4011B integrated circuit.ﾠ Go through the connections of the IC to determine the LOW/HIGH levels as the input is HIGH and LOW.ﾠ Change some of the component values - the 1M, 100nF, 330K and 1nF - and notice how the frequency and tone of the alarm changes.ﾠ Notice how robust the circuit is; the value of the components may vary and still give an acceptable alarm.