Such a LED will still light quite visibly with 7.5 mA thru it, even if it could have handled 20 mA or more. "White" LEDs are usually really UV LEDs with phosphors that re-emits in the visible spectrum. Common green LEDs drop about 2.1 V, for example. LED forward voltage is dependent on color. I said 180 Ω minimum for a little margin and because that is a common value. Just about any LED can handle 20 mA forward current. That leaves (5 V)-(1.8 V) = 3.2 V across the resistor. Most LEDs can stand at least 5 V reverse across them.Ī visible-light LED will drop a minimum of 1.8 V. The reason to use a 5 V supply is to limit the reverse voltage across the LED when connected backwards. Even with 1 kΩ in series, you will still be able to see any visible-light LED light up indoors. Using a higher resistance works, but will light the LED more dimly. To safely experiment with pretty much any LED, use a 5 V supply with at least 180 Ω in series. If the LED doesn't light one way, flip it around and try again. Unlike a ordinary light bulb, orientation matters. LEDs are diodes, so conduct only in one direction. Here is what you can do to light your LED and see what you've got. I see Lorenzo has already answered your question directly (+1). The Wikipedia article on LEDs only scrapes the surface of the internal workings of the LEDs and still is fairly complex. The physical explanation lies in quantum mechanics and solid-state physics, really tough subjects. You also ask why all this happens, but the answer is fairly complex, since it depends on the physical structure of the semiconductor crystal inside the diode. After that limit has been reached, the device goes POOF! This up to the maximum current limit of the LED. Once conducting, the light intensity emitted is roughly proportional to the current (not voltage) that flows in the diode (so you get a brighter LED if you decrease the value of the limiting resistor). There are other questions/answers on this site explaining how to calculate the value of the limiting resistor. Hence you need a resistor in series to limit that current to a safe limit. a Joule thief or a boost DC-DC converter) to power the LED.Īfter the threshold voltage is reached, any very slight increase in voltage makes the LED conduct heavily, i.e. Therefore, if you have a battery whose voltage is below the threshold voltage of the LED, you are out of luck, unless you use a more complicated circuit (e.g. Light emission begins only if a certain voltage is reached (threshold voltage), under that voltage the emission is negligible. You may also damage them if you apply more than ~4V-5V in the reverse direction (these are safe values the exact maximum tolerable value depends on the specific device). Reverse the polarity and they won't work. They have a polarity, hence they must be powered using DC respecting that polarity. Main differences (a bit simplified for very beginners): This requires only two pins from the Arduino (data & clock), and they can be chained to have multiple 7-segments driven from those same 2 pins.LEDs don't work like ordinary (incandescence) light bulbs. This limits you to being able to display only numerical digits, but uses only 4 pins of the Arduino per 7-segment.Īnother, more versatile way is to add a serial-in to parallel out chip (74HC595, for example) to drive the 7-segment. One is to add a 7-segment driver, like the CD4511. There are a couple of ways to address this. You will run out of digital pins if you try to add a second 7-segment. The way that you are diving the 7-segment involves using a separate pin to drive each segment, so you use 8 pins of the Arduino to drive the display. So driving one of these means running a current from the particular anode (positive) pin for the desired segment to the common cathode pin. So turning on any particular segment will involve running a current from this common anode (positive) pin to the particular cathode (negative) pin for the desired segment.Ĭommon cathode means that the cathodes of all of the LEDs are common and connected to a single pin. Common anode means that the anode (positive) side of all of the LEDs are electrically connected at one pin, and each LED cathode has its own pin. PinMode(LED8, OUTPUT) // led 8 is outputĪ 7-segment is a packaged set of 8 LEDs (7 number-segments & 1 decimal point). PinMode(LED1, OUTPUT) // led are output for low will be on PinMode(ANODE, OUTPUT) // common anode is obviously an output How can you change the code to have a counter in backwards (from 0 to 9)? const int ANODE = 2 What is the difference in using a common anode and common cathode of the 7 Segment in interfacing with the Arduino? How can you change the code if we need to add another 7 Segment display?
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