Light Sensing Using Photodiode: Analytical Essay

Abstract

The main aim of the project is to design a circuit which would allow the detection of light by using a photodiode and to critically assess in details how a light sensing circuit using a photodiode works to obtain a desired output. This is achieved by using a transistor Darlington pair, four resistors (2*330, 1k, 4.7k), one LED and a photodiode. In all processes of design all simulations, breadboard design and printed circuit board yielded the results that were required hence justifying the function of the circuit.

Introduction

Background information

[1]A Photodiode is a semiconductor diode which converts light into electric current. This form of diode is also known as a photo-detector or light sensor. It is also a PN junction type of diode. It works on both the reversed and forward bias mode. The current produced which is of small leakage flows in the reversed direction (opposite normal current flow), even when there’s no light falling on it. The current produced in the diode are directly proportional to the intensity of light absorb by it. The photodiode is used in switching circuit, and in electronics devices like a smoke detector, compact disc players, in light meter, in street lights etc.[2] The working principle of a photodiode is, when a photon of ample energy strikes the diode, it makes a couple of an electron-hole. This mechanism is also called as the inner photoelectric effect. If the absorption arises in the depletion region junction, then the carriers are removed from the junction by the inbuilt electric field of the depletion region. Therefore, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. The entire current through the diode is the sum of the absence of light and the photocurrent. So the absent current must be reduced to maximize the sensitivity of the device.

Significance of project

Findings from this project will be a reflection to the benefit of the society, businesses considering the fact that there will be a friendly technological system which switches on/off lighting systems. Thus, applying the recommended approach derivative from this new technological system the cost of operating lighting systems will be lowered to the maximum and it will function effectively.

Statement of problem

The use of lighting systems is expensive as it requires people to operate, that is switching on and off of lights, and its maintenance is of high expense as well. By so doing means that companies have to pay the system operators which may result in more financial cost and huge budgets.

Objectives

• To design a circuit which would allow the detection of light using a photodiode.
• To make references on how to improve the operational effectiveness of the use of a light sensing circuit using a photodiode.
• To classify and evaluate, understand the concepts involved and the fabrication of a light sensing circuit using a photodiode.
• To critically assess in details how a light sensing circuit using a photodiode works to obtain a desired output.

Literature review

[3]A light sensor is a passive sensor that is used to indicate the intensity of the light by examining the radiant energy that exists in a certain range of frequencies. In the spectrum of electromagnetic waves, the ranges of frequencies that are used to detect using a sensor are between infrared to visible and up to ultraviolet. Light sensors convert the light energy in the form of photons to electrical energy in the form of electrons. Hence, they are also called as Photo Sensors or Photo Detectors or Photo Electric Devices. Light sensors or Photo Sensors can be classed into three types based on the physical quantity that is affected. The main classes are Photo resistors, photo voltaic and photo emitters. Photo emitters generate electricity when exposed to light. Photo resistors change their electrical properties when illuminated.

A light dependent resistor can also be used in a light sensing circuit. The sensitivity of the LDR switch is fairly low. To increase the sensitivity of light sensing, few modifications can be applied. The fixed resistor R1 is replaced with a potentiometer VR1. The transistor is replaced with an Operational Amplifier and the light dependent resistor is incorporated into a Wheatstone bridge. The new and more sensitive light sensing circuit using a light dependent resistor is shown in the figure below

Figure 1

[3]The resistors LDR, VR1, R1 and R2 will form a Wheatstone bridge. The sides of bridge LDR – VR1 and R1¬ – R2 form a potential divider with output voltages V1 and V2. These voltages are connected to non-inverting and inverting inputs of the operational amplifier respectively. The operational amplifier is operated as a differential amplifier whose output is a function of the difference between the two input voltages V1 and V2. This is also known as a voltage comparator with feedback. The feedback resistor Rf is used to provide required voltage gain. The output of the operational amplifier is connected to a relay which can control an external circuit. When the voltage V1 due to light sensing by LDR falls below the voltage V2, which acts as a reference voltage, the output of the amplifier changes its state. This causes the relay to activate and the load is switched on. As the intensity of the light increases, the output switches back and the relay is turned off. Here the relay is turned on when the intensity of light is less. The operation can be reversed by reversing the positions of the light detecting resistor and potentiometer. Now the relay is turned on when the light level increases and exceeds the level set by the reference voltage.

[3]The sensitivity of a photo transistor is dependent on the DC current gain of the transistor. Hence, the overall sensitivity, which is a function of collector current, can be controlled by the resistance between emitter and base. For high sensitivity applications, like Optocouplers , a Darlington Photo Transistor is used. It is generally called a Photo Darlington Transistor and uses a second bipolar NPN junction transistor. This second transistor will provide additional amplification. The circuit of a photo transistor with second transistor is shown below.

Figure 2

Design description and requirement specifications

• Function: the artifact should be able to detect/sense light by using a photodiode
• Material (electronic components): the device should use two NPN transistors of code BC547, four resistors, one photodiode and one light emitting diode (LED).
• Power: the design should use a minimum voltage of 5v as to power
• Size and Shape: It should be having a minimum size of 50mm×25mm as the device is used inside a building and it should not consume more space and also the design fabricated with small electronic components.
• Maintenance requirements: the project should take a minimum of 30 minutes to maintain whenever there is any damage done to the device.

A 9V battery is connected to the circuit and the anode of the photodiode is connected to the base of a transistor, in series with a 330ohm resistor and parallel to a 4.7k ohm. The collector of the first transistor is connected in series with a 1k ohm resistor whereas the collector of the second resistor is connected in series with a 330ohm and a LED. The circuit uses transistors connected in a Darlington pair mode as the switch which turns the LED on and off automatically. This happens by the aid of the voltage divider rule, that is when there is no light the resistance of the photodiode is increased due to current produced by the photodiode known as dark-current therefore there will be a low voltage drop and by so doing the second transistor is switched on hence allowing current to flow through the LED and making it lit.

When there is light the resistance of the photodiode is low, therefore allowing current to flow through it and now the second transistor is not triggered on as a result there will be no current flowing to the LED, and thus it will be switched off.

Figure 3 shows circuit design diagram

Components list

• Photodiode, 1
• Transistors (BC547), 2
• LED, 1
• Resistors- 2*330 ohm, 1*1k ohm, 1*4.7k ohm

Component's theory

• Light Emitting Diode

Use: It emits light when the electric current passes through it.

Figure 4: Shows LEDS

• Transistors

Use: To switch or amplify electric signals

Figure 5: Shows a Transistor

• Resistors

Use: To control current flowing through the electric circuit

Figure 6: shows Resistors

• Photodiode

Use: It converts light energy to electric energy

Figure 7: Shows a photodiode

Design and implementation

NPN Darlington Pair Transistor

A Darlington Pair acts as one transistor but with a current gain that equals:

Total current gain (hFE total) = current gain of transistor 1 (hFE t1) x current gain of transistor 2 (hFE t2) for example if you had two transistors with a current gain (hFE) = 100:(hFE total) = 100 x 100(hFE total) = 10,000

This gives a vastly increased current gain when compared to a single transistor. Therefore this will allow a very low input current to switch a much bigger load current.

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Figure 8: shows Darlington pair

Power supply unit is a unit which converts the AC power to the DC power so as to be used by an external circuit.

Figure 9: shows power supply unit

Photodiode is connected parallel with a 4.7k ohm resistor and controls the circuit via the voltage drop concept.

Figure 10 :shows photodiode which control the circuit

A red light emitting diode is used to show the out, proving the function of the circuit.

Figure 11 shows output

Block diagram

(INPUT Photodiode)

(SWITCHING Transistor Darlington Pair)

(Power Supply 9V)

(OUTPUT Red Light Emitting Diode (LED))

(CONTROL Photodiode, Transistors)

Testing, results and discussion

After a profound joint into the design processes there is now a need to implement the design, validate and test out the solutions.

SIMULATION

Figure 12:shows proteus simulation

BREADBOARD

Figure 13: shows Breadboard testing of the circuit

Figure 15 shows Breadboard testing in the presence of light

Figure 17 shows the final circuit

The above circuit from figure 17 shows a complete circuit (artifact),the results obtained are the expected as the LED was glowing in the absence of light and it was switched off in the presence of light hence detecting light just as specified by the project topic and objectives.

From all the processes of design which were simulations in proteus, multisim, real circuit on breadboard and printed circuit board, the circuit turned out a success as it performed as expected and gave expected results. It also showed that a photodiode indeed is a light sensing component which senses infrared light/rays and direct sun rays as the circuit was kept lit in the presence of general light in the lab but when test using a cell phone torch it was turned off. This showed and proved that in the presence of light photodiodes have low resistances.

In multism simulation the circuit connected and tested, it was a success but the main problem was that there was no how one could adjust the light intensity on the photodiode to really justify that indeed it works perfectly. That being the case it was a disadvantage as there was no assurance. Again in proteus simulation there was no photodiode component, but there was an alternative to use an LDR (Light Dependent Resistor) as it behaves in a similar way to the photodiode and because it had an option to adjust the light intensity on it, by so doing it performed well and hence gave assurance that the circuit is perfect.

The outcomes that were obtained in both simulations, breadboard design and on the printed circuit board (PCB) were the expected, which is the circuit was able to sense the presence of light with the aid of a photodiode. With all the above mentioned it proved that the circuit can save energy as the system will be automatic and can also safe time.

The project can further be modified by using a comparator instead of transistors therefore a comparator will now amplify the currents produced by the photodiode. With the use of a comparator variable resistors will be connected with the photodiode in order to adjust the light intensity of the photodiode.

The project can be modified such that it’s scope is changed but it is still used as a light sensor. This happens when it is used hand in hand with IR rays in factories as stock counter.

References

1. Circuit Globe. [Online] Available at: https://circuitglobe.com/difference-between-photodiode-and-phototransistor.html [Accessed 9 July 2019].
2. Electronics hub. [Online] Available at: https://www.electronicshub.org/light-sensors/ [Accessed 8 July 2019].
3. Elprocus. [Online] Available at: https://www.elprocus.com/photodiode-working-principle-applications/ [Accessed 9 July 2019].
4. Slide share. [Online] Available at: https://www.slideshare.net/DhananjaysinhJhala/dark-sensor-using-photodiode [Accessed 7 July 2019].