Voltage Divider Rule

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A voltage divider consists of two resistances R1 and R2 connected in series across a supply Voltage Vs. The supply Voltage is divided up between the two resistances to give an output Voltage Vo which is the Voltage across R2. This depends on the value of R2 relative to R1:

  • If R2 is much smaller than R1, Vo is small (low, almost 0V)
    (because most of the Voltage is across R1)
  • If R2 is about the same as R1, Vo is about half Vs
    (because the Voltage is shared about equally between R1 and R2)
  • If R2 is much larger than R1, Vo is large (high, almost Vs)
    (because most of the Voltage is across R2)

Vo =(VsxRs)/R1+R2…………………………………….(i)

If you need a precise value for the output Voltage Vo you can use Ohm’s Law and a little algebra to work out the formula for Vo shown on the equation (i). The formula and the approximate rules given above assume that negligible current flows from the output. This is true if Vo is connected to a device with a high resistance such as Voltmeter or an IC input. If the output is connected to a transistor Vo cannot become much greater than 0.7V because the transistor’s base-emitter junction behaves like a diode.

Voltage dividers are also called potential dividers, a name which comes from potential difference (the proper name for Voltage).

Using an input transducer (sensor) in a Voltage divider

Most input transducers (sensors, suppose LDR) vary their resistance and usually a Voltage divider is used to convert this to a varying Voltage which is more useful. The Voltage signal can be fed to other parts of the circuit, such as the input to an IC or a transistor switch.

The sensor is one of the resistances in the Voltage divider. It can be at the top (R1) or at the bottom (R2), the choice is determined by when you want a large value for the output Voltage Vo:

  • Put the sensor at the top (R1) if you want a large Vo when the sensor has a small resistance.
  • Put the sensor at the bottom (R2) if you want a large Vo when the sensor has a large resistance.

The above points play important role while making dark/light sensor.

Choosing a resistor value

OR

The value of the resistor R will determine the range of the output Voltage Vo. For best results you need a large ‘swing’ (range) for Vo and this is achieved if the resistor is much larger than the sensor’s minimum resistance Rmin, but much smaller than the sensor’s maximum resistance Rmax.

You can use a multimer to help you find the minimum and maximum values of the sensor’s resistance (Rmin and Rmax). There is no need to be precise, approximate values will do.

Then choose resistor value:  R = square root of (Rmin × Rmax)
Choose a standard value which is close to this calculated value.

For example:
An LDR: Rmin = 100R, Rmax = 1M, so R = square root of (100 × 1M) = 10K.

Swapping over the resistor and sensor

The resistor and sensor can be swapped over to invert the action of the Voltage divider. For example an LDR has a high resistance when dark and a low resistance when brightly lit, so:

  • If the LDR is at the top (near +Vs),
    Vo will be low in the dark and high in bright light.
  • If the LDR is at the bottom (near 0V),
    Vo will be high in the dark and low in bright light.



Using a variable resistor

A variable resistor may be used in place of the fixed resistor R. It will enable you to adjust the output voltage Vo for a given resistance of the sensor. For example, you can use a variable resistor to set the exact brightness level in  dark and light sensor circuits.

The variable resistor value should be larger than the fixed resistor value. For finer control you can use a fixed resistor in series with the variable resistor. For example if a 10k fixed resistor is suitable you could replace it with a fixed 4.7k resistor in series with a 10k variable resistor, allowing you to adjust the resistance from 4.7K to 14.7K.

If you are planning to use a variable resistor connected between the +Vs supply and the base of a transistor you must include a resistor in series with the variable resistor. This is to prevent excessive base current destroying the transistor when the variable resistor is reduced to zero.


Examples

Suppose the LDR has a resistance of 100R in bright light, and 200K  in the shade.

Case I- 10K at the top and LDR at the bottom.IT SHOULD WORK AS DARK SENSOR, THAT MEANS, LED OFF IN LIGHT AND LED ON IN DARK.

When LDR is in light, resistance of LDR-100R.

Vo= (0.1*9)/(10+0.1)

= 0.0891V(NOT ENOUGH TO TURN ON THE TRANSISTOR)

When the LDR is in shade, resistance of LDR- 200K

Vo= (200*9)/(10+200)

= 8.57V(ENOUGH TO TURN ON THE TRANSISTOR)
We can clearly see that the output voltage is large when the resistance of LDR is high.This voltage would enough for a transistor to turn on a LED. Hence, this circuit works as Automatic Dark sensor.

Try this circuit:

Case II- 10K at the bottom and LDR at the top.

IT SHOULD WORK AS LIGHT SENSOR, THAT MEANS, LED ON IN LIGHT AND LED OFF IN DARK.

When LDR is in light, resistance of LDR-100R.

Vo= (10*9)/(10+0.1)

= 8.91V(ENOUGH TO TURN ON THE TRANSISTOR)

When the LDR is in shade, resistance of LDR- 200K

Vo= (10*9)/(10+200)

= 0.428V(NOT ENOUGH TO TURN ON THE TRANSISTOR)

We can clearly see that the output voltage is large when the resistance of LDR is low. This voltage would enough for a transistor to turn on a LED. Hence, this circuit works as Automatic Light sensor.

Try this circuit:

 

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5 COMMENTS

  1. Hello i made this exact circuit but i have a problem.If i put the positive wire to the positive pole of the battery and the negative wire to the negative pole, the circuit doesn’t work at all.If i do the reverse procedure the LED just lights.There is nothing happening if i put me hand in the photoresistor.What is going wrong?Thanks in advance

  2. Hi, I would like to ask why transistor is needed for lighting up the LED? Since LDR has almost similar characteristics as a variable resistor, through voltage divider rules, LED still turned on, right? Hope to receive your feedback. Thanks.

  3. Hi
    I have a question: is it possible, to use the BC547C with the TCRT 5000 as sensor? I tried so much times, but it don’t works. I can’t use the TCRT5000 as Sensor with the BC547C, to let an LED lighting up or down. Need really your help.

    best regards

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