Phototransistor

-



A photo-transistor is an electronic component capable of reacting to light by letting current flow.

Like transistors, the photo-transistor is designed according to a base, emitter, a collector. But the base is sensitive to light and therefore controls the flow of current through the collector.

The photo-transistor is much more sensitive than a photo-diode. A classic photo-transistor-based assembly is the Darlington: a photo-transistor controls a classic transistor, which is very efficient for light barriers or obstacle detection.

Phototransistor Symbol

The phototransistor is manufactured most of the time in silicone, but also in other materials (ceramic, and others). The phototransistors are enclosed (generally) in a small opaque or transparent shell, in order to strengthen the light that passes through it and allows light to reach sensitive areas of the phototransistors.

How a phototransistor works ?

A phototransistor is a transistor whose base B is controlled by the lighting:

• If the base is not lit, the transistor is blocked and the current Ic = 0A,

• If the base is lit, the transistor is in linear mode and Ic = Ie is not zero, the intensity Ic then depends on the lighting applied to the base.

When light hits the semiconductor material, the electrons/free holes in the semiconductor material cause the current to flow through the base region. The base would only be used to bias the transistor.

 In the case of an NPN transistor, the collector becomes positive with respect to the emitter and in PNP, the collector is held negative.

The light enters the base region, The phototransistor generates the electron-hole pairs. The generation of electron-hole pairs mainly occurs in the reverse bias. The movement of electrons under the influence of the electric field causes the current in the base region. 

The base current injected electrons into the emitter region. The major drawback of the phototransistor is that it has a low frequency response.

The illumination current of the phototransistor is the photocurrent of the collector-base photodiode multiplied by the amplification gain β of the transistor.

Its photosensitive reaction is therefore significantly higher than that of a photodiode (100 to 400 times greater). On the other hand, the dark current without illumination is greater.

Basic operation of a phototransistor

In this circuit, when the phototransistor receives strong light, it turns on and the LED turns on. When the incoming light is low, the LED has a brightness proportional to the incoming light.

Slowly move your hand over the phototransistor. You should have seen the LED dim until it finally went out. Phototransistors are applied as photo-sensors. 

They detect the ON / OFF switching of the light in non-contact mode, without requiring mechanical contact.

 Amplifier using a phototransistor:

To amplify the current coming from the phototransitor, we can use the diagram below:

The current from the emitter of the phototransistor controls the base current of the transistor. the photocurrent is amplified and the output Vout will be proportional to the intensity of the light.




sources:

https://www.abcelectronique.com/divers/dossiers/photodetecteurs/chap4.phtml
https://illustrationprize.com/fr/304-phototransistor.html
https://pobot.org/+-phototransistor-+.html
http://electrobot.e-monsite.com/pages/les-composants/les-phototransistors.html

Comments

Post a Comment

Popular posts from this blog

7 segments display

-
Image
The 7-segment displays are displays that allow you to view a figure or a number using luminous segments. The light sources are internal. It is almost always this type of display which is used on radio clocks when the color of the digits is green or red.  The 7-segment displays mostly emit a red or green light, but there are also some that glow in the blue. to display a number between 0 and 9, at least seven segments are needed, and these segments have been conventionally assigned the first seven letters of the alphabet: A, B, C, D, E, F and G. Common anode: Here is the principle of common anode displays: (Diagram of the inside of the box) To use its displays, you must always follow the instructions for supplying the LEDs. the (+) of the power supply on the anode and the (-) on the cathode. Just like the LEDs, you will need to add a resistor (220 ohms if the power supply is equal to 5v) to reduce the current on each segment of the display. Otherwise you risk toasting the segment...
Read more

Mosfet

-
Image
Metal-oxide semiconductor field-effect transistor (mosfet) is the second type of field-effect transistor. The mosfet gate is a channel isolated by a layer of silicon dioxide (Si02). The drain and the source are made from substrate materials. they are connected to the gate by a narrow channel. Transistor has type of metals. The first is called the mosfet at depletion and the second is called the   enhancement mosfet (E). The MOSFET transistor is different from the JFET in that the gate is isolated from the channel. Like diodes, these elements are most often made of silicon. They are used to make amplifiers. The enhancement MOSFET In the absence of a control signal at the Gate (Vgs = 0V), the Mosfet is blocked regardless of the type of channel (N or P).Conduction only begins to manifest from a threshold:+ Vth for Drain an N-channel MOS and –Vth for a P-channel MOS. N-channel   enhancement mosfet is a positive voltage applied to the input. The positive charge sticks...
Read more

LM35 Temperature Sensor

-
Image
  The LM35 temperature sensor is an analog temperature sensor .This integrated circuit has 3 pins and generates an output voltage in linear relation with the temperature expressed in degrees Celsius The LM35 sensor is able to measure temperatures ranging from (-55 ° C to + 150 ° C) in its most precise version and with the right mounting, enough to measure any temperature. The analog output of the sensor is proportional to the temperature. It suffices to measure the voltage at the output of the sensor to deduce the temperature. Each degree Celsius corresponds to a voltage of + 10mV. The LM35 does not require any external calibration or size and maintains an accuracy of (+ / -0.4 ° C) at room temperature and (+ / - 0.8 ° C) over a range of 0-100° C. The temperature measured by the sensor is given by the relation: T = Vout / 10 with: T: Temperature [° C] Vout: Output voltage [mV]   DHT22 sensor The DHT22  temperature and humidity sensor communicates with ...
Read more