They also provide on/off and brightness control and multiple ICs can be daisy-chained together for even larger displays. Communication between the MAX7219 and the uC is made over the I2C bus and the MAX7219 takes care of updating and multiplexing up to 8 of the 7-Segment modules using only 3 pins on the microcontroller. If there are multiple digits, the number of required output pins on a uC increases quickly.įortunately, there are ICs like the MAX7219 that are made to interface with these modules and make life a whole lot easier. The downside to 7-Segment LEDs is that they require 7 or 8 drive pins per digit in the module. However, that means that only 1 segment is on which will not be the case usually. If you are building an outdoor race timer or something like that, they tend to be the go-to display to use. When you calculated the Rb, you assumed Ic to be 25mA. Being encapsulated, they tend to be fairly rugged devices and they have good brightness that is visible even in sunlight. We like 7-Segment LED displays because they display numbers in a very clear format and they are available in everything from small 0.3″ to very large 5″ or larger up to 12″ tall characters. Having the pins on the ends of the module make them more breadboard friendly that modules that have the pins on the sides. Values in the range of 75-91 generally work pretty well when max brightness is not required. For that application, a resistor value of 50 ohms will provide max brightness. In that case, you generally want to drive the display with more current since each digit in the display is on only a portion of the time and will therefore appear dimmer than if the digit was being driven constantly on. When you are using displays with multiple digits in a multiplexed mode where each digit is only lit for a portion of the time, the maximum allowable current can be greater than 20mA and can go up to as high as 60 mA. These displays are logic compatible with a max drive current of 20mA and a forward voltage drop of about 2V.Īt an operating voltage of 5V, a series current limiting resistor of 150 ohm will provide max brightness if driving these displays in a static mode rather than in a a more typical multiplexing mode. The pin-out shown here is as if you are looking at the face of the display. This is repeated for each of the digits very rapidly so that all of the digits appear to be operating continuously. Multiplexing means that the ground connection (DIG1-DIG3 )for each of the displays is enabled sequentially by driving it LOW while the data for that particular digit is driven HIGH on the 8 LED lines. The connections for each of the individual segments such a ‘A’ are connected together in all 3 digits, so the displays are designed to be used in a multiplexed fashion unless you want all 3 digits to always show the same number or character which generally isn’t all that useful. Instead of the control pins, current limiting resistors can be put on the pins connected to ground so that fewer are needed, but in that case the brightness of the display will vary depending on how many segments are lit at one time so that practice is not really recommended. These 8 control pins should each have series current limiting resistors to prevent damage to the LED module or the uC. To light a particular segment, the control (anode) pin for that segment is driven HIGH. These connections are labeled DIG1, DIG2 and DIG3 in the drawing. The displays are common cathode (CC) type which means that all 8 of the LEDs in each of the 3 digits have their cathodes tied together and these are pulled to ground to enable that digit. The LEDs are labeled segment A thru G and the decimal point is DP. Theory of OperationĮach digit of these displays have 7 separate LEDs comprising the 7 segments of the digit plus an 8th LED that functions as a decimal point.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |