To study some applications of Light Dependent Resistors (LDR).


      Transistor (BC 107), IC555, resistors (10KΩ, 1KΩ, 100Ω, 1MΩ (Pot), Light Dependent Resistor (LDR), LED, Digital multi-meter, dc power supply, bread board, connecting wires.


     In this experiment, you will learn how to utilize the switching characteristics of a transistors and a LDR for fabricating light intensity monitor for instance to automatically switch on a light at dusk and switch it off at dawn. Additionally you will also learn to convert the light intensity into frequency.

Before proceeding further

  1. Familiarize yourself with transistors (PNP/NPN) and their characteristics.
  2. Revive your memory on LDR and 555 timer.



     This is a simple experiment that will help you visualize the switching characteristics of a transistor.

1 MW Pot


10 kW


10 kW




1 kW


BC 107


Form a circuit as shown in Fig.1, with BC 107 transistor. Decrease the Pot slowly until the LED starts to glow. Alternatively, Connect a 10KΩ in series with the 1MΩ pot. Initially adjust the pot such that it forms a short. Increase the pot until the LED goes off. The circuit works on the fact that, the transistor does everything to pull the collector to ground, once the voltage across the base to emitter goes beyond the forward voltage drop Vbe(which is about 0.7 V). You may also do the following. Connect a 10KΩ and the 1MΩ(pot) in the base to emitter path. Insert a 1MΩ in the collector-base path. Slowly reduce the 1MΩ pot until the LED goes off. Essentially, you are playing with voltage dividers to send the transistor to ON or OFF states.

     What will be the order of switching time for a typical transistor? Can you make a guess? When do you think you need to consider seriously the switching time?





EXPERIMENT 2 [LDR light intensity monitor]




1 kW


10 kW





                                                            Fig. 2

      This is a simple light level (intensity) monitor [Fig. 2]. Form the circuit as shown in the figure. Place the LDR (as you know, it is just a resistor whose value depends on the intensity of light falling on it. Its dark resistance should be of the order of 1 to 2 MΩ. By dark resistance, we mean its resistance when you keep it covered from external light) in the collector base path. Plug in the 10KΩ in the base emitter path. Keep the LDR covered initially. Since most of the voltage falls across the LDR, the transistor will be in the OFF state. Now expose the LDR to ambient light. You will notice the LED to go on. Interchange the LDR and the resistor, i.e keep the LDR in the base emitter path and the 10 KΩ resistor in the base collector. The transistor will go from ON state to OFF state as soon as you expose the LDR to ambience.



      Can you guess a circuit that can help monitor the light intensity? i.e. a circuit that switches perhaps an alarm on if for instance, the visibility (basically light intensity) goes below a certain level.









1 MW



IN 4007



       If you cannot guess, then look at the circuit shown here. Here we have replaced the resistor + LED in the collector path with a relay + diode (both are in parallel). The diode is necessary to prevent the back EMF from destroying the transistor. Note carefully the polarity of the diode. A wrong move can kill the transistor instantly. Check the functioning of the circuit. You will hear a click sound whenever the relay changes its state (you are expected to know what a relay is. If not see the note given at the end or consult your teacher).




Form the circuit shown in Figure 4. this is simply the astable multivibrator that you hade constructed while you studied the 555 timer. Instead of Rb, you have the LDR. Since the resistance of the LDR depends on light intensity, the timer output frequency will also vary accordingly. You must follow the following guidelines if you want to get some sensible results.









1.      Choose Ra to be much less than the resistance value of the LDR, when it is exposed to light. [Because LDR should decide the frequency.] This also ensures 50 % duty cycle.

2.      If you have scope available, then choose Ct according to your convenience. You can measure the frequency easily with the scope.

3.      If you do not have oscilloscope, use 330 W + LED to monitor the frequency. However, you have a constraint in choosing the Ct value, as the frequency should be about a few Hz.  (Note that your eyes have persistence of vision).

4.      Best thing would be to cover the LDR fully first and measure its DARK resistance. Choose Ct now so that the frequency is about 1 Hz (Use LDR’s DARK resistance value for Rb).

5.      Form a circuit and cover and uncover the LDR. You will find a change in frequency. If the LDR resistance changes by a factor of five the frequency will also change by a factor of five.


Note on Relay:


A relay is essentially a switch activated by a small electromagnet. The electromagnet consists of a coil wound over a magnetic core. When powered, the core gets magnetized and attracts a lever, which in turn will establish physical contact (electrical short) between two points, similar to an ordinary switch. When power is removed, the lever goes back to its initial position, disconnecting the electrical short. You will find five pins in the relay given to you. Two pins are used for powering the relay (to be connected to the transistor collector). Among the other three pins, one is common. The remaining two are identified as N/C (normally closed) and N/O (normally opened). This means that common and the N/C terminals are short when power is not applied to the coils (no connection between common and N/O pins). When powered, common pin and N/O will show a short (no connection between common and N/C pins). Play with the relay before you connect it in the circuit. You can use a 6 V supply to power the relay.