What is LDR & Photo Diode?

LDR & Photo Diode

We have discussed about sensors in the first post while going through some basic science concepts. Let us today learn about two light sensors. Just to recap, sensors are components which can sense physical environment around them and convert them in some type of electrical parameter which can be then measured.

LDR is a cheap light sensor which is read as “Light Dependent Resistor” or “Light Decreasing Resistor” or even as “Photo Conductive Cell”. When light falls on LDR, the resistance across the terminals is in order of some hundreds of Ohms. And in dark condition the resistance is in order of some megaOhms (10⁶ Ohms). It is of two types i.e. intrinsic and extrinsic. Intrinsic one has its own charge carriers and is not efficient. Extrinsic ones are made by doping semiconductor material like Phosphorous in Silicon. 

LDR

A commonly available LDR looks like this having a small disc like shape, the two brown portions are very thing metallic film acting as contacts and are connected to the two terminals. There is a small gap between these two metallic films in snake like structure which contains Cadmium Sulphide (CdS).

LDR Symbol

 LDR is normally connected in series of a resistance, and either the voltage across LDR is measured by circuit or voltage across the series. As the resistance of LDR changes due to variation of light, the current through the series combination also changes. This variation in current changes the voltage across LDR or the series resistor, this variation is proportional to light falling on LDR hence by measuring this voltage we can know the status of light around the LDR. This sensor is commonly used in automatically switching on lights when it is dark and switch off when its day time and sufficient light falls on LDR. In such application, speed of switching operation is not much concerned as they have a response time of some 10-100 milli seconds.

Photo Diode

There is another light sensor (photo sensor) called photo diode. It is a bulky component as it consists of a casing with lens/light filter in front and a P-N junction diode inside casing. Light is allowed to pass through lens or filter and falls on P-N junction diode activating the diode to operate. Materials used for diode here is Silicon, Germanium, Indium Galium Arsenide or Lead Sulphide.

Photo Diode Symbol

In absence of light the photo diode is inactive. Its working can be compared to that of LED, which produce light in visible range when forward biased and the holes and electrons collide with each other. On the other hand, a photo diode is always used in reversed biased, hence no current flows through diode in normal condition. When light falls at the junction of P-type and N-type material, the electrons are exited and start moving and combining with holes near the junction. Reverse current through diode directly proportional to light intensity falling on junction. More the light intensity more is the reverse current and vise-e-versa.

Photo Diode Circuit

Photo diodes can be operated in two modes, one is reversed biased as we saw above and the other is in zero biased condition, in which it acts like a small photovoltaic cell commonly known as Solar Cell. Zero biased means no voltage is applied across the photo diode. But when light falls on it, a forward current is generated inside it flowing from Anode to Cathode. This flow of current generates a potential difference across it with positive at anode and negative at cathode.

That’s all about basics of light sensor or photo sensor, we will see their use in future posts when we will use it to turn on and turn off light automatically based on surrounding light. Further when we advance ahead, we will collect input with light sensor and accumulate in micro-controller and also sending it to website over internet when we will learn about making IoT (Internet of Things) project. 

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