Phototransistors are ideal photodetectors and can be used in a host of different applications. Phototransistor circuits are normally relatively straightforward, especially of the detector is only required to detect the presence of absence of a particular light source.
As a result of their ease of use and their applications, phototransistors are used in many applications.
- Opto-isolators - here the phototransistor is used as the light sensor, the light emitter being relatively close, but at a different potential. The physical gap between the light emitter and detector provides a considerable degree of electrical isolation.
- Position sensing - in this application the optoisolator can be used to detect the position of a moving element, often the moving element has a light or interrupts a beam of light which the phototransistor detects.
- Security systems - phototransistor can be used in many ways in security systems, often detecting whether a beam of light is present or has been broken by an intruder.
- Coin counters - phototransistor can be used in coin and other counting applications. A beam of light is interrupted each time a coin or other item passes a given point. The number of times the beam is interrupted equals the number of coins or objects to be counted.
- Many more . . .
The phototransistor can be used in a variety of circuits and in a number of ways dependent upon the application. Being a low cost device the phototransistor is widely used in electronic circuits and it is also easy to incorporate.
Phototransistor circuit configurations
The phototransistor can be used in a variety of different circuit configurations. Like more conventional transistors, the phototransistor can be used in common emitter and common collector circuits. Common base circuits are not normally used because the base connection is often left floating internally and may not be available. If the base connection is required, then it is necessary to buy a phototransistor with a base connection available.
The choice of common emitter or common collector phototransistor circuit configuration depends upon the requirements for the circuit. The two phototransistor circuit configurations have slightly different operating characteristics and these may determine the circuit used.
Common emitter phototransistor circuit
The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.
The circuit actually acts as an amplifier. The current generated by the light affects the base region. This is amplified by the current gain of the transistor in the normal way.
Common collector phototransistor circuit
The common collector, or emitter follower phototransistor circuit configuration has effectively the same topology as the normal common emitter transistor circuit - the emitter is taken to ground via a load resistor, and the output for the circuit being taken from the emitter connection of the device.
The circuit generates an output that moves from the low state to a high state when light is detected.
Phototransistor circuit operation
The phototransistor circuits can be used on one of two basic modes of operation. They are called active or linear mode and a switch mode.
Operation in the "linear" or active mode provides a response that is very broadly proportional to the light stimulus. In reality the phototransistor does not give a particularly linear output to the input stimulus and it is for this reason that this mode of operation is more correctly termed the active mode.
The operation of the phototransistor circuit in the switch mode is more widely used in view of the non-linear response of the phototransistor to light. When there is little or no light, virtually no current will flow in the transistor, and it can be said to be in the "off" state. However as the level of light increases, current starts to flow. Eventually a point is reached where the phototransistor becomes saturated and the level of current cannot increase. In this situation the phototransistor is said to be saturated. The switch mode, therefore has two levels: - "on" and "off" as in a digital or logic system. This type of phototransistor mode is useful for detecting objects, sending data or reading encoders, etc.
With most circuits not using the base connection (even if it is available), the only way to change the mode of operation of the circuit is to change the value of the load resistor. This is set by estimating the maximum current anticipated from the light levels encountered.
Using this assumption, the following equations can be used:
RL = load resistor (i.e. Rc or Re in the diagrams above).
IC = maximum anticipated current.
VCC = supply voltage.
Use of base connection in phototransistor circuits
On some phototransistors, the base connection is available. Access to the base connection allows the phototransistor circuit conditions to be set more appropriately for some applications.
High values of base resistor Rb prevent low levels of light from raising the current levels in the collector emitter circuit and in this way ensuring a more reliable digital output. All other aspects of the circuit function remain the same.
The basic concepts for the phototransistor circuits are quite straightforward. Typically they require little design, although some optimisation may be required to ensure residual current is minimised and in switching applications that the “off” current is small. However the circuits are normally reliable and can easily be designed.
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