These LED’s or LCD’s are used to display the required numeral or alphabet.įor an LED to work it needs to be connected to a voltage source with the correct side. The binary information is displayed using these seven segments. These are arranged in a way to form numbers and characters by displaying different combinations of segments. How does the 7 segment display work?Ī 7 segment display is made of seven different illuminating segments. Seven-segment displays are widely used in digital clocks, electronic meters, basic calculators, and other electronic devices that display numerical information. What are 7 segment displays used for?Ī seven-segment display is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot matrix displays. So driving one of these means running a current from the particular anode (positive) pin for the desired segment to the common cathode pin. Is anode negative or positive? How does a common cathode work?Ĭommon cathode means that the cathodes of all of the LEDs are common and connected to a single pin. A positively charged platinum electrode where H2 gas is oxidized to H+ ions is the anode. A simple example is electrolysis of water. In this case you need active-high GPIOs for the a-g anodes.An anode is any electrode where oxidation takes place. You can also use a decoder chip such as a 74xxx138 for the common cathodes. If you use active-low GPIOs for a-g cathodes, you'll need PFETs or PNPs for the common anodes. If you use active-high GPIOs for a-g anodes, you'll need NFETs or NPNs for the common cathodes. With a multiplexed array of LEDs you can probably drive each of the seven segments a-g with a GPIO, but you will probably need external transistors for each digit to switch the fairly large current if all segments are on simultaneously. With today's high-efficiency LEDs that only need a few mA it probably doesn't make much difference. This was the common wisdom back when each LED segment needed 10 mA. What this means is that you usually want to use active-low GPIOs connected to LED cathodes rather than active-high GPIOs connected to LED anodes. Since the NFETs are more efficient, they'll heat up your chip less than similar PFETs with the same amount of current. I think the same is true of NPN transistors versus PNP transistors, but I don't think the difference is as dramatic. I think it's a factor of 4, but that's relying on memory. The main consideration is that NFETs a lot more efficient than PFETs because electrons move more easily than holes. Special Note: Remember: The I2C bus you define from the uC to the expander(s) CAN control more that one!! The 3 bit address of each expander chip is defined by how its pins 1-3 are connected to Vcc or Vdd individually. Note 3: One of the displays will require that its bit pattern xxx-xxxx be sent out as 1's complement representation. This approach solves a problem with unequal brightness between different segments and/or characters. Note2: The toggle bit needs to control the correct N (2N3904) or P (2N3906) type transistors to connect the "common" pin of each display to the appropriate power rail. Required refresh rate is 60 Hz (cycles per second). The 7 pins of the latch/expander form a "bus" with the two segment led displays listening but only changing state when directed to by the toggle bit. I think it is correct to say that you will need 1 resistor for each of the output pins of each buffer chip. Likewise, loading the latch with 1xxx-xxxx will light up the other display's segments corresponding to the bit pattern xxx-xxxx (see note 3). For instance, loading the latch with 0xxx-xxxx (binary) will light up one display's segments corresponding to the bit pattern xxx-xxxx.
A single bit (see note 2) will let you toggle between the two 7-segment leds (the hex alpha numeric ones, right?). If you understand multiplexing and I hope you do, you can use one of each of the common types (cathode/anode). It seems to me that one of those would be very useful in your project unless you are looking for a total low current circuit.
COMMON ANODE VS COMMON CATHODE SERIAL
There is an interesting 8 pin latch/buffer chip (see note 1) used by the 2x16 parallel LCD vendors on ebay so that it becomes a I2C Serial LCD. The specs for the uC will tell you the max.
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