Join Barron Stone for an in-depth discussion in this video Photodiodes, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] A photodiode is basically the opposite of a light emitting diode. Unlike an LED, which converts electrical current passing through it into light, when light shines on the semiconductor junction in a photodiode, the photodiode absorbs energy from the light and converts it into electrical current. The schematic symbol for a photodiode is similar to the similar for an LED, except that the arrow points in the other direction to represent light shining onto the photodiode.
Physically, photodiodes come in a variety of shapes and sizes for different applications, ranging from small photocells that can be used to build light detector circuits to large solar panels that can be used to power devices. Some Through Hole photodiode components come in a package that looks very similar to an LED. So be careful not to get them mixed up in your parts kit. Like LEDs, photodiodes are polarized components, so their orientation in the circuit matters. The Through Hole photodiodes that look similar to LEDs use similar features to indicate orientation.
The lead connected to the anode will be longer than the cathode lead, and there's usually a flat section on the cathode side of the photodiode's body. When light shines on the photodiode, the current it produces will flow from the cathode to the anode, which is the opposite of the direction the current flows through a normal forward biased diode. Remember, photodiodes are just the opposite of LEDs. The amount of current generated by a photodiode will depend on the amount of light shining on it.
The more light it gets, the more current it will produce. I can actually measure the current produced by a photodiode using just my DMM. It'll be a tiny amount of current, so I'll set this DMM to its smallest current measurement range, which is 20 microamps. When I connect the DMM probes to the terminals of this photodiode, I can see that it's producing about four and a half microamps from the light that's currently shining on it. If I move my hand to cast a shadow over the photodiode, the output current drops down to less than a microamp because the photodiode is getting less light energy that it can convert into electricity.
And if I shine a flashlight on the photodiode to give it more light, the current it produces will increase to around 10 microamps. Photodiodes are commonly used in photo detector circuits that sense the amount of light shining on them. These photo detectors show up in a wide range of consumer electronic devices, including CD players, smoke detectors, and infrared TV remotes. I can build a very simple photo detector circuit with just a photodiode and a resistor in series connected to a DC voltage source.
In this configuration, the diode's cathode is connected to the positive side of the voltage source, so it will be reverse biased. As a reverse biased diode, it blocks current from the voltage source from passing through it. But as light shines on the diode, it will allow some of those electrons to flow through and generate a small amount of current that flows out of the anode down through the resistor. According to Ohm's law, when current flows through a resistor, it'll create a voltage across the resistor which we can easily measure with a multimeter, or using the analog to digital convertor on a microcontroller system like the Arduino platform.
So to demonstrate that, I'll use the five volt source from an Arduino Uno microcontroller board to reverse bias of photodiode connected in series with a 330 kiloohm resistor. And I'll measure the output voltage across that resistor using analog input channel zero. I've chosen to use a 330 kiloohm resistor here because the amount of current produced by the photodiode is so small, and I need that large of a resistance to produce a large enough voltage for the Arduino to measure it.
You may need to use a different resistor value based on the photodiode you're using and the light conditions in your room. I've written a simple Arduino program that repeatedly reads the voltage on analog input channel zero and prints the voltage value to the serial port. I've also included a conditional if/else statement within the program to determine whether the detector circuit is in the light or the dark based on whether the measured voltage is above or below a certain threshold value, which I configured in the program.
I've included the code for this program in the exercise files as Photodiode_Arduino_Demo.ino. With my Arduino program running, I can see that the light detector circuit produces around 2.9 volts from the ambient light in this room. And the program says that there is light. When I cover it with my hand to cast a shadow, I can see how much the voltage drops, and the program says that it's dark. It took a bit of testing for me to determine the voltages that my circuit would produce in these two conditions so I could set the voltage values in my software to easily distinguish between light and dark.
It's possible to build a light detector circuit with similar functionality using other types of light sensitive components, such as a photoresistor. And the fact that photodiodes and photoresistors can be both be used for similar purposes often leads to confusion between them. But they are different components that work in different ways. Photodiodes produce a current that is proportional to the amount of light shining on them, whereas photoresistors change their resistance depending on the amount of light shining on them.
There are lots of other light sensitive components that can be used to build specialized light detectors, such as photo transistors and photo multiplier tubes, but deciding which one of those many options is best for a specific application is beyond the scope of this course. To keep things simple, when you need a photo detector, I recommend sticking with either a photoresistor or a photodiode, because they're easy to use and can create light detector circuits that are good enough for most hobbyist projects. Photoresistors can usually give you a more sensitive measurement than photodiodes, but the downside is that photoresistors react much more slowly to changes.
So if you need to measure something that changes slowly, like the ambient light in a room, a photoresistor would be good for that. On the other hand, if you want to measure something that changes quickly, like a fast blinking LED, you would be better off using a photodiode because they're able to react to changes in light much more quickly than photoresistors.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals