A few sensor works on the bridge concept but every sensor has its own sensing element with some different phenomena such as physical principle, chemical principles etc. so let we understand the working principle of sensors.

Physical principles are

  1. Ampere’s Law
    • A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer)
  2. Curie-Weiss Law
    • There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behavior
  3. Faraday’s Law of Induction
    • A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer)
  4. Photoconductive Effect
    • When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor)


In this circuit we have a battery of 12V and total current ITotal is flowing.

  • A bridge circuit or two branches in parallel are connected at two terminals A and B of the battery end.
  • Bridge circuit contains four resistors R1, R2, R3 and R4. And there are four points A,B,C and D.

Total current ITotal is being divided at point A in current I1 and I2 and at point B  total current is I1 + I2 = ITotal. Current I1 follows path ACB and current I2 follows path ADB.

Now, we know current follows low resistive path. If the resistance R1 = R3 and R2 = R4 that means value of resistance is same in both path so I1 will equal to I2 (i.e. I1 = I2).

That means Voltage drop at R1 and R3 are same, so the voltage at point C and D are same which means potential difference (voltage) between point C and D is zero (VC – VD = 0) this is known as balanced bridge network.

If the value of resistance changes in any path can make change in potential difference which will be measured and this is known as unbalanced bridge network.

So if we replace any resistor with any sensor for example replace R2 with LDR (Light depending resistor) or any variable resistor. Then as value of resistor changes either increase or decreases,

  • if resistance increases in path ACB, voltage drop is higher in path ACB and lesser current will pass through path ACB and
  • if resistance increases in path ADB, voltage drop is higher in path ADB and lesser current will pass through path ADB.

So we can have both values of result either positive or negative to detect changes.


See bridge circuit below


As both the resistors are in series, current (I) flows through both of them is same. Therefore, the current (I) is given as V/RT

I = V ÷ R => 12V ÷ (10Ω + 20Ω) => 0.4A

The voltage at point C (which is remaining voltage left after dropped at R1 i.e. 12 – 4 = 8), which is also the voltage drop across the lower resistor, R2 is calculated as:

VR2 = I × R2 = 0.4A × 20Ω = 8 V

Thus we can see that the source voltage VS is divided among the two series resistors in direct proportion to their resistances as VR1 = 4V and VR2 = 8V.

This is the principle of voltage division, also known as a voltage divider circuit or network.


Now if we add another series resistor circuit using the same resistor values in parallel with the first (Voltage in parallel branch is same), we  have the resultant circuit like this

As the second series circuit has the same resistance values of the first, the voltage at point D, which is also the voltage drop across resistor, R4 will be the same 8 volts, with respect to ground (battery negative), as the voltage is common and the two resistive networks are the same.

But something is also important, that the voltage (potential difference) between point C and point D will be zero volts as both points are at the same value of 8 volts as: C = D = 8 volts, then the voltage (potential difference) is: 0 volts

When the voltage at point C is the same value as the voltage at point D or their potential difference is zero then bridge network are said to be balanced.


Now let’s consider what happen if we invert the position of resistors R3 and R4 in the second parallel branch with respect to R1 and R2.

The current flows through the series resistor combination is same but the voltage at point D, which is also the voltage drop across resistor, R4 will be:

VR4 = 0.4A × 10Ω = 4 volts

Now with VR4 having 4 volts dropped across it, the voltage (potential difference) between points C and D is 4 volts as: C = 8 volts and D = 4 volts. Now the potential difference this time is: 8 – 4 = 4 volts

The result of changing resistance in any parallel branch gives us positive or negative potential difference between point C and D which is said to be unbalanced network.

The resistance ratio of these two parallel arms, ACB and ADB, results in a potential difference lies between 0 volts (balanced) and the maximum supply voltage (unbalanced), and this is the basic principal of a bridge circuit.

As we learn working principle of sensors now let add on with LDR in this circuit for making darkness sensor circuit.

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