phototransistor

THE PHOTOTRANSISTOR

The photo-transistor is a photo-conductive device.

It has the same construction as a normal transistor plus a window and lens which focusses light upon the NPN junction.

Any change in current is directly proportional to the change in light falling upon the junction.

Some are designed to operate without a base connection.

As light levels increase fig A gives a fall in collector voltage.

Fig B gives a rise in emitter voltage.

The phototransistor can be operated in class A.
The circuit shown below gives an audio signal at the collector produced by a varying density optical sound track on a movie film.

In the circuit below an increase in brightness causes an increase in emitter current.

This produces an increase in voltage across the resistor thereby triggering the thyristor to its ON state.

The circuit can be reset by breaking the supply voltage by means of the "push to break" switch.

The next diagram shows a liquid level sensor.

As the liquid level rises it eventually breaks the light beam.

The loss of the output signal can trigger an alarm, switch off a valve controlling the flow of the liquid etc.

The phototransistor can be assembled together with an LED to form an opto-isolator.

An input signal varies the LED brightness which is transmitted to the photo-transistor.

There can be a low voltage circuit on the input side isolated from a high voltage circuit on the output.

There can be an isolation of several kilovolts between the two circuits.

The LED and transistor can be assembled as a slotted opto-switch sensor.

As items pass through the gap they produce pulses which can be counted.

Another device is the reflective object sensor.

The LED produces a beam of light which is detected by the photo-transistor if it is reflected off an object.