# LDR Darkness sensor – LED off as Light Falls

We will discuss LDR darkness sensor – LED OFF as light falls on LDR and Turn ON in the dark here, it has a previous part which is LED ON as Light falls on LDR and Turn OFF in the Dark

### REQUIRED HARDWARE

 S.No. Item Quantity 1 LDR 1 2 Resistor 1K 3 3 Resistor 220 1 4 LED 1 5 OP-AMP LM358 1 6 Battery 5V / 9V 1 7 Male to Male jumper 8

## LDR DARKNESS SENSOR – LED OFF AS LIGHT FALLS

### WORKING

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

#### IN LIGHT

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

That implies LED will get this negative voltage and remains OFF as it is reversed biased.

#### IN DARK

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

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

### OUTPUT

As light falls on the LDR output of the op-amp is negative voltage means LED gets the negative signal that makes LED to be turned OFF and in the dark output of the op-amp is positive LED will get the positive signal that needed by LED to be turned ON.

IR SENSOR AND ARDUINO

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