This article provides information about phototransistors, a type of
bipolar junction transistor (BJT) that operates as a switch when exposed
to light. The base terminal in a traditional BJT is replaced with a
light-sensitive material, allowing electrons to flow from the emitter to
the collector. The article discusses the similarities between
phototransistors and BJT NPN transistors, as well as photodiodes in
terms of appearance, spectral range, and sensitivity. The PT334-6C is
used as an example of a low-cost phototransistor, with a sensitivity to
light wavelengths between 400-1100 nm. The article also explains how to
wire a phototransistor to measure light intensity using a voltage
divider circuit and a transimpedance amplifier with an op-amp.
A phototransistor is a type of bipolar junction transistor (BJT) that
functions as a switch when exposed to light. The base terminal of a
traditional BJT is replaced with light-sensitive material, which causes
electrons to jump from the p-type silicon to the n-type silicon,
enabling the flow of electrons from the emitter to the collector.
Phototransistors are similar in appearance to conventional LEDs and can
have similar spectral ranges and sensitivities to photodiodes. They can
be used for measuring light intensity and are commonly employed in
light-activated sensor applications.
The phototransistor has similarities with its BJT NPN transistor counterpart, but instead of a base terminal, it uses light as the switching mechanism. Photons excite electrons to jump from p-type to n-type silicon, allowing for the flow of electrons from the emitter to the collector. The symbol for the phototransistor in a circuit diagram resembles that of a conventional transistor. Although it looks similar to an LED, it's important to label the storage bag accurately to distinguish between the two. Phototransistors have spectral ranges and sensitivities comparable to photodiodes. The PT334-6C shown below, is an example of an affordable phototransistor, with two leads and 5mm in size.
It has a sensitivity to light wavelengths between 400-1100 nm and a rise and fall time of 15 μs in typical conditions. To improve the sensor's directionality and reduce the impact of stray light, wrapping the phototransistor body with a loop of black electrical tape can be effective. The phototransistor can be wired using a voltage divider and transimpedance amplifier to measure light intensity.
Following shows circuit diagram for measuring light intensity using voltage divider circuit. The resistor RS is the sense resistor.
And the following shows phototransistor based transimpedance amplifier using op-amp to measure light intensity.
Due to the high demand for dependable and responsive light sensors, several innovative integrated circuits such as TSL235R have been introduced to the market. Such ICs seamlessly incorporate various ideas and do not require additional components, reducing the need for adjustments to gain values.
References:
[2] LM393 light sensor circuit
[3] How to use LM358
