!!!!!!!!!!!!!!!!!LDR ONLY!!!!!!!!!!!!!!

	LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.

The animation opposite shows that when the torch is turned on, the resistance of the LDR falls, allowing current to pass through it.

	This is an example of a light sensor circuit : When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light. However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor. The LED lights. The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.


1. The circuit above is a light sensor. That means light must shine into the LDR for the circuit to be activated. Draw a circuit composed of the same components that activates when it is DARK (when the LDR is covered). This is a typical examination question. HINT: Simply swap the preset resistor and the LDR.

Light dependent resistor (LDR) is made from cadmium sulphide. Cadmium Sulphide is a II-VI semiconductor. (It is so called because Cadmium is in group II and Sulphide is in group VI.) It is light sensitive. When the light shining on it is stronger, the resistance of the LDR is smaller.

It is important to note that light has dual properties. On the one hand, light is electromagnetic wave, on the other hand, it can be seen as photons (energetic particles). When light shines on the LDR, the photons break the bonds in the cadmium sulphide and release electrons for the conduction. If the light is of a stronger intensity, then more bonds are broken, thus more electrons are freed for the conduction. So LDR's resistance decreases when stronger light shines on it.

(i) In this experiment a 12V, 24W light bulb is held as close as possible to a light dependent resistor.
(ii) The power consumed by the light bulb is gradually increased in order to change its brightness.
(iii) Readings of current and potential difference are recorded both for the bulb and the LDR.

(iv) It is important that the light bulb and the LDR do not move throughout the experiment.
(v) Note that the LDR circuit has a variable resistor in it to limit the current through it, thereby avoiding the risk of damage to it.
(vi) As the light dependent resistor is sensitive to light, a graph is then plotted of Resistance of the LDR on the y-axis against power consumed by the light on the x-axis.

A light dependent resistor is a semi-conducting material (rather like Silicon). By shining a light onto an LDR, the light "injects" energy into the semiconductor which is absorbed by co-valently bonded electrons. This energy breaks the bonds between atoms. The electrons become delocalised and are free to move within the LDR. This leads to a larger current (smaller resistance) flowing in the semiconductor.

Thus the resistance of a Light Dependent Resistor decreases with increasing Illumination. This is used in cricket light metres and burglar alarms etc.

Potential difference across bulb (V)	Current through bulb (A)	Power consumed by bulb (W)	Potential difference across LDR (mV)	Current through LDR (mA)	Resistance of LDR (W)
0.91	0.54		438	108
1.24	0.60		439	110
1.49	0.64		431	110
1.74	0.69		433	120
1.99	0.72		425	138
2.25	0.77		410	168
2.52	0.81		390	214
2.74	0.83		371	263
3.00	0.87		352	323
3.26	0.90		334	390
3.50	0.94		316	461
4.02	1.01		283	603
4.50	1.06		256	726
4.97	1.11		236	839
5.47	1.16		217	930
6.00	1.22		196.2	1048
6.56	1.28		177.7	1162
6.96	1.33		166.6	1228
7.50	1.37		153.8	1302
8.00	1.42		143.6	1363
8.49	1.47		134.6	1430
9.00	1.51		126.1	1482
9.54	1.56		105.6	1518
10.00	1.59		110.7	1559

10.46	1.63	105.6	1595
11.11	1.69	98.7	1640
11.47	1.72	95.3	1656
12.01	1.77	89.3	1670
12.73	1.82	83.4	1704
13.92	1.91	76.1	1775