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Difference between photoresistor and phototransistor | photoresistor vs phototransistor

This page compares photoresistor vs phototransistor and mentions difference between photoresistor and phototransistor. It covers advantages and disadvantages of photoresistor and phototransistor.

What is Photoresistor?

Introduction:
• It is a passive component which varies its resistance in response to the light.
• It is a disc shaped component with two leads as shown in the figure-1.
• When the light falls on the surface of the disc, resistance between the leads diminish (or reduce). In darkness, resistance as high as 10 MOhms and in bright light, resistance as low as 500 Ohms can be achieved.
• It is a passive component with no polarity.
• It represents equal resistance in both the directions and it can be used either with AC or DC.

Photoresistor construction

One of the applications of photoresistor is shown in the following circuit. Here it acts as variable resistor based on light intensity falling on it. The resistance of a photoresistor decreases with increasing incident light intensity. In other words, it exhibits photoconductivity. There are various circuit symbols of the photoresistor. Photoresistors of various sizes are available ranging from smaller one (5 mm in diameter) to larger one (25 mm in diameter). It can be used to generate variable voltage.

What is Phototransistor?

• It is a transistor controlled by exposure to light. It is similar to photodiode controlling an ordinary bipolar transistor.
• It can be either bipolar transistor or field effect transistor (FET).
• Its body is encased in resin or plastic or encased in a metal shell with window in it.

Phototransistor

• Phototransistor has two leads which connect internally with its collector and emitter (or source and drain in FET). The base of transistor (or gate in FET) responds to light and controls the flow of current between the leads.
• In the absence of light, bipolar phototransistor permits leakage between collector and emitter of 100 nA or less. When exposed to light, it conducts upto 50mA. This differentiates it from photodiode which can not pass much current.
• The figure-1 depicts most common variant of phototransistor i.e. Bipolar NPN.
• Examples: Optek/TT Electronics OP506A, Vishay TEKT5400S, Vishay BPW17N etc.

Difference between photoresistor and phototransistor

Following table mentions difference between photoresistor and phototransistor.

Features Photoresistor Phototransistor
Responsive to light Less More
Maximum Resistance in darkness Lower Higher
Minimum resistance in bright light Higher Lower
Current carrying capacity Higher (Twice) Lower
Directional No, Photoresistor is sensitive to incident light from anywhere in front of it. Yes, Phototransistor is sensitive to incident light from particular direction and insensitive from other directions.
Temperature dependent Resistance varies more with temperature Effective resistance varies less with temperature
Resistance change with applied voltage No change in resistance for light intensity regardless of applied voltage i.e. it remains same. Effective resistance varies with applied voltage
Cost More Less
Advantages and disadvantages Photoresistor >> Phototransistor >>


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