LDR – LED on as Light Falls

A photoresistor (or light-dependent resistor, LDR, or photo-conductive cell) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increase in the intensity of light (it exhibits photoconductivity).

A photo-resistor can be used in light-sensitive sensor/ detector/ indicator circuits, and light-activated and dark-activated switching circuits, or in other words LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits.

Normally the resistance of an LDR is very high, about as high as 1MΩ (1,000,000 Ω), but when they are illuminated with light, their resistance drops dramatically, and can have a resistance as low as up to a few hundred ohms.

It is a type of resistor in which resistance (either increases or decreases) of the resistor (LDR) depends upon intensity of light.

• It has negative resistance coefficient.
• Resistance of an LDR decreases as light falls on the LDR.
• High Intensity Low resistance and low intensity have High resistance.

Another component we need is Operational Amplifier IC (op-amp) which we discussed earlier those are LM741 and LM358

We will make two circuits, out of which first is discussed here and other in next topic

1. LED ON as Light falls on LDR and Turn OFF in the Dark
2. LED OFF as Light falls on LDR and Turn ON in the Dark

REQUIRED HARDWARE

LED ON AS LIGHT FALLS ON LDR AND TURN OFF IN THE DARK

WORKING

As we can see the output of the bridge is fed into an op-amp. The output of terminal A fed to non-inverting (+) input of op-amp and output of terminal B fed to inverting (–) input of op-amp.

We know as light falls on LDR its resistance starts decreasing corresponding to the intensity of light and in the dark resistance increases, we know current will follow a low resistive path. So, if light falls on LDR it has low resistance means more current will pass through LDR (more at point A) while in the dark more current will flow through another path (at point B).

So in sunrise point A has more current and B has less. In dark B has more current and A has less.

We know op-amp compares inverting (-) and non-inverting (+) voltages and passes which one is higher. So

IN LIGHT

• Voltage at inverting (-) is Lower.
• Voltage at non-inverting (+) is Higher. We know if signal fed into non-inverting (+) of op-amp it does not change the polarity of the signal. So the output of op-amp is positive

That implies LED will get this positive voltage and turned ON as it is forward biased.

IN DARK

• Voltage at inverting (-) is Higher. We know if a signal is fed into the inverting (-) of op-amp it inverts the polarity of the signal so the output of op-amp is negative voltage.
• The voltage at non-inverting (+) is Lower.

That implies LED will get this negative voltage and would not turn ON as it is reversed biased.

OUTPUT

As light falls on the LDR output of the op-amp is positive means LED gets the positive signal that needed by LED to be turned ON and in the dark output of the op-amp is negative LED will not get the positive signal that needed by LED to be turned ON so LED will remain OFF.

2. LED OFF AS LIGHT FALLS ON LDR AND TURN ON IN THE DARK