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Using a potential divider with a logic gate

Setting up the circuit

In this simplified example we use a light dependent resistor (LDR) to turn a night-light on when it gets dark.  The darker it is the higher the resistance of the LDR.

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The circuit uses two potential dividers.  Each potential divider controls one of the two inputs to a voltage comparator.  If input 1 is greater than input 2 the comparator's output changes so that the night-light is switched on.  It needs a relay to do this.

The potential divider controlling input 2 uses two equal fixed resistors to divide the 12 V in half.  This means input 2 is always at 6 V.

The potential divider controlling input 1 uses an LDR and a variable resistor to divide the voltage.  If the LDR resistance is greater than the variable resistance then the mid-point is at more than 6 V and the light comes on.

As it gets darker the resistance of the LDR increases.  This means it takes a bigger and bigger share of the 12 V.  So the potential of the middle point gradually increases.  Eventually it's greater than 6 V and the light switches on.  This happens when the resistance of the LDR is greater than the resistance of the variable resistor

Changing when the light comes on

The variable resistor is used to control when the light comes on.

If you wanted the night-light to come on when it was quite light you'd need to make it's resistance quite low.  This means that it could still be quite light but the LDR would still have a higher resistance than the variable resistor.

If you wanted it to be very dark before it came on you'd need to make the resistance higher.  This means that it would need to be very dark before the resistance of the LDR was higher than the variable resistor.

back to Lesson 9: Series Circuits