Photo Detector Circuit

Photo Detector Circuit

Circuit Project Description

The circuit was designed to create a detection scheme with reference to light energy sources.

Terminology Photo Detector – a device that detects and senses light and radiant energy which uses the principle of photoconductivity where the material exhibiting this characteristic changes their electrical conductivity when exposed to lightCA3130 – a BiMOS operational amplifier that combines the advantage of both bipolar transistors and CMOS, which can be used in photodiode sensor amplifiers, peak detectors, single-supply full wave precision rectifiers, voltage regulators and followers, high input impedance comparators and wideband amplifiers, long duration timers, fast sample-hold amplifiers, and ground referenced single supply amplifiers74HCT13 – high speed Silicon gate CMOS device that are pin compatible with low power Schottky TTL, contains four 2-input NAND gates which accept standard inputs signals, and are capable of transforming slowly changing input signals into sharply defined jitter-free output signals BP103 – a Silicon NPN phototransistor used for control and drive circuits, industrial electronics, photointerrupters, and computer-controlled flashes due to its features such as high linearity, suitability for applications from 420nm to 1130nm, and packaged in a TO-18 base plate transparent epoxy resin lens with base connection Circuit Explanation

During the operation, the circuit is producing a low output when sufficient lighting is received by the light detector. Even in the dark, a source of light can still be produced from many ways. The compatibility between the output and TTL level produces a low logic level. This occurs as the light is detected by the phototransistor BP103 Q1. This model of transistor is manufactured by Siemens. A phototransistor has a photodiode with internal gain and is nothing more than a bipolar transistor in a transparent case so that the base-collector junction can be reached by the light. The photons in the base-collector junction are generating electrons to be fed into the base. The current gain of the transistor amplifies the photodiode current.

The sensitivity of the circuit is regulated by the 10K ohm adjustable resistor RV1. The circuit is being powered by a 5 V supply that can be derived from a battery or from an external power source.

Part List

C1-2=100nF 100V ceramic
RV1=10Kohm trimmer
Q1=BP103 Phototransistor Siemens


The photodetector comes in several varieties and applications in the market today. Some of them are photointerrupters and photosensors having light sensors that are used in mobile devices such as cell phones and other consumer electronic products. They can be used as optical sensor, thermoelectric cooling assembly, calibration of measuring system on the basis of colour targets, and gas analyzer where they are placed at absorption lines of measuring gases.

The photodetector may come in different varieties like the cryogenic detectors used to measure the energy of near infrared photons, visible, and x-ray. When solar or photovoltaic cells are illuminated, an electric current is supplied and a voltage is produced. To record images in digital cinematography, digital photography, and astronomy, charged-coupled devices are used where glass-backed plates were utilized rather than film.

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