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# Electronic Circuit Designing: Functional Block Designing (Part 3)

Step: 5) Design circuit for each functional block

In this step we will be designing the circuit for individual blocks. The circuits required are simple and very basic circuit known to all; hence it won’t be such a big task. We are not designing anything new, but modifying the well-known basic circuits to suit our needs.

Let us begin with the IR sensing part itself as we have done in the case of the block diagram. In the IR sensing part we have two blocks namely the IR PHOTODIODE and the COMPARATOR.

IR PHOTODIODE:

Figure: functional block to be designed – IR PHOTODIODE

We can use any commonly available IR photodiode to realize this block. All we need is a pull-down resistor with its output. The pull-down resistor is used to set a default output when there is no IR light falling on the photodiode.

IR photodiode is a PN junction diode whose depletion region width (kindly refer any basic electronic text book) is sensitive to the incoming IR light rays. It acts very much like a normal diode in the forward biased condition. In reversed biased condition when there is no IR light the depletion region width increases and the conduction decreases as in the case of the normal diodes. But when the IR light falls on the depletion region, reverse breakdown occurs, depletion region width decreases and the conduction increases considerably. The conduction current in certain ranges is proportional to the incoming IR light.

We can make use of this phenomenon in our design. Connect the IR photodiode in reverse biased configuration and take the output from one of its legs. Such a circuit is shown in the following figure

Figure: IR PHOTODIODE circuit

As you can see from the above circuit that the ‘D1’ which is the IR photodiode is connected in reverse biased. Output is taken at the positive lead of the photodiode. Whenever light falls on the IR photodiode it conducts and we get a positive voltage at the output which depends on the intensity of the IR light falling on it.

The resistor R1 is used as a pull down resistor. It is used to set a default output voltage when there is no light falling on the photodiode. What will be the voltage at the output if there is no IR light falling on the photodiode and when there is no resistor R1? The answer is there is no voltage, not even zero or ground. But we are expecting a zero voltage when there is no IR light, hence if we connect a resistor to ground and take the output from the point where the photodiode is connected with the resistor we will get a zero voltage.

The resistor R1 is used not only as a pull down resistor but it biases the photodiode in such a way that the photodiode operates only in a limited current when the conduction occurs in reverse biased condition due to the IR light.

Since we will be using the TV remote from a distance the IR light intensity falling on the photodiode will be very small. We must get a strong output from this block even at this less intense IR light. Since for less intense IR light the photodiode current will be less also, we need to amplify the output using this current.

We all know Ohm’s law; it states that “at a given temperature the voltage across a current conducting material is directly proportional to the current flowing through it”. The proportionality constant is nothing but the resistance of the material.

V = IR                                     (Equation: 4)

Where;

‘V’ is the voltage across the current conducting material

‘I’ is the current flowing through the material

‘R’ is the resistance of the material

We can make use of the Equation: 4 in our design. We are taking the output across the resistor R1 and according to Ohm’s law as we increase the resistance we get more voltage across the resistor for a given current flowing through it.

Since the current flowing through the photodiode depends on the intensity of the IR light falls on it we have values ranging between zero and five volts at the output of this circuit.

 SUPPLY VOLTAGE 5V DC REGULATED OUTPUT VOLTAGE BETWEEN 0 and 5 VOLTS

Table: IR PHOTODIODE voltages

For a significant output voltage even for the weakest signal, we must use a resistor with the high resistance. We can use a 100K resistance as the resistor R1.

The photodiode D1 can be any common photodiode and I’ve used “glass type Directional IR Photodiode” which is commonly called “IR LED receiver”. I don’t have the part number, but the image of the same is given below.

Figure: IR photodiode

The component used in the IR PHOTODIODE circuit and their functions are given in the following table

 COMPONENT FUNCTION D1 The photodiode for sensing the incoming IR light rays R1 The resistor acts as a pull down resistor with the photodiode and is used for biasing the photodiode. Increases the resistance will increase the sensitivity up to certain extend.

Table: IR PHOTODIODE component functions

The components designed for this particular block of circuit is given in the table below.

 Serial no. COMPONENT TYPE VALUE 1 D1 IR Photodiode directional IR photodiode 2 R1 Resistor 100KE

Table: IR PHOTODIODE component values

We have done with the IR PHOTODIODE block and now let us take the COMPARATOR block and try to design the circuit for it.