Topics Discussed:
- How an LED works.
- Uses for an LED.
- Common Cathode vs. Common Anode
- How to dim an LED.
- LED’s used as sensors.
1. The term “LED” is actually not word at all, LED is an acronym for “Light Emitting Diode”.
2. The most common use of an LED is for pulsing light on and off.
3. When choosing an LED it is important you know whether you are picking a common cathode or a common anode. The difference is determined by the way the terminals are configured. Look closely at the way this 7-segment LED display has it’s LED’s oriented before applying power to your 7-segment LED display.
4. One common question that arises is “how do I dim an LED?” The key to this question is to first understand what you mean by “dim”. There are 2 different ways to dim an LED.
First, you can physically make the LED use less power by limiting the current flowing into the LED. The would be done by changing the resistor values from your power source to your LED. This method physically dims the LED, however every time you want to change how bright or dim the LED is you have to change the amount of resistance or total power entering the LED. In this manner, you would need to set your hardware up in a way that it would only have a finite number of dimming values. This would lead you towards the binary processes which is used today in many LED applications. Using this method on a microprocessor, using 3 pins in an on or off state with different resistance values all wired to one LED would give you a maximum of 8 different brightness levels and likewise 8 pins could give you 256 values. But as you can see, this would use many pins on a microprocessor. So if this isn’t the best way, then what is a better way?
The second method to dimming an LED is done with eye trickery and can be done with a microprocessor, such as the Parallax Propeller chip. Many applications today that use LED’s don’t actually have the LED constantly at an “on” state. Instead, what is done is to flash the LED at a rate faster than your brain can process the information so the light appears to be constantly on. The relationship between the pulsing rate and percentage of “on” time is called “Pulse Width Modulation” (PWM). PWM is controlled by the “period” (time between start of first pulse and start of second) and “duty cycle” (the percentage of time the pulse is “on” during the period).
The method of pulse width modulation is the preferred method for blinking an LED if you have a microprocessor available such as the Parallax Propeller chip. A microprocessor reduces the amount of hardware you need, and it gives you more flexibility to change blinking rates, or how bright or dim the LED may appear to the eye.
5. An LED can also be used as a photodiode. An LED is a photosensitive semiconductor with a lens when you just hook a meter up to it.
Photodiodes are sometimes used in robots and devices as light sensors. Photodiodes have a spectrum wavelength to which they are most sensitive, usually infrared. However, a reversed LED is most sensitive to the same color of visible light as it normally emits. For example, if a circuit uses a reversed green LED, the most current will flow from exposure to green light.
Links:
Here is a good source to keep from getting your wiring mixed up on a 7-segment display.
7-Degment Display