use barcode reader in asp.net or rearranged to nd Rcurrent limiting, the formula becomes Rcurrent limiting (Vapplied VLED)/ILED in Software

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or rearranged to nd Rcurrent limiting, the formula becomes Rcurrent limiting (Vapplied VLED)/ILED
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Using this formula for an LED that has a 1.5-V forward voltage and lights at 5 mA in a system that provides 3.3 V of power, the current-limiting resistor can be calculated: Rcurrent limiting (3.3 V 1.5 V)/5 mA 360
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In this situation, I would use a 330-
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current-limiting resistor.
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Figure 17.2 The seven-segment LED can have a common anode or cathode.
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Probably the easiest way to output numeric (both decimal and hex) data is via sevensegment LED displays. These displays were very popular in the seventies (if you re old enough, you re rst digital watch probably had seven-segment LED displays) but have been largely replaced by LCDs. However, seven segment LED displays (Fig. 17.2) are still useful devices that can be added to a circuit without a lot of software effort. By turning on speci c LEDs (each of which lights up a segment in the display), the display can be used to output decimal numbers. Each one of the LEDs in the display is given an identi er, and a single pin of the LED is brought out of the package. The other LED pins are connected together and wired to a common pin. This common LED pin is used to identify the type of seven-segment display (as either common cathode or common anode). Wiring one display to a microcontroller is quite easy it is typically wired as seven [or eight if the decimal point (DP) is used] LEDs wired to individual pins. The most important piece of work you ll do when setting up seven-segment LED displays is matching and documenting the microcontroller bits to the LEDs. Spending a few moments at the start of a project will simplify wiring and debug of the display later. The typical method of wiring multiple seven-segment LED displays together is to wire them all in parallel and then control the current ow through the common pin. Because the current is generally too high for a single microcontroller pin, a transistor is used to pass the current to the common power signal. This transistor selects which display is active. In Fig. 17.3, four common-cathode seven-segment displays are shown connected to a microcontroller. In this circuit, the microcontroller will shift between the displays, showing each digit in a very short time slice. This is usually done in a timer interrupt handler. The basis for the interrupt handler s code is listed below:
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Int: - Save Context Registers - Reset Timer and Interrupt
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Figure 17.3 Multiple seven-segment LED displays can have common segment wiring with their common pins individually controlled, and the application scan through them faster than the human eye can perceive.
- LED_Display = 0 - LED_Output = Display[ Cur ] - Cur = (Cur + 1) mod #Displays Sequence Display - LED_Display = 1 << Cur Current Display - Restore Context Registers - Return from Interrupt
; ; ;
Turn Off all the LEDs Point to Next Display LED for
This code will cycle through each of the digits (and displays), having current go through the transistors for each one. To avoid icker, I generally run the code so that each digit is turned on/off at least 50 times per second. The more digits you have, the faster you have to cycle the interrupt handler (i.e., eight seven-segment displays must cycle at least 400 digits per second, which is twice as fast as four displays). You may feel that assigning a microcontroller bit to select each display LED to be somewhat wasteful (at least I do). I have used high-current TTL demultiplexor (i.e., 74S138) outputs as the cathode path to ground (instead of discrete transistors). When the output is selected from the demultiplexor, it goes low, allowing current to ow through the LEDs of that display (and turning them on). This actually simpli es the wiring of the nal application as well. The only issue is to make sure that the demultiplexor output can sink the maximum of 140 mA of current that will come through the common-cathode connection. Along with seven-segment displays, there are 14- and 16-segment LED displays available that can be used to display alphanumeric characters (A Z and 0 9). By following the same rules as used when wiring up a seven-segment display, you shouldn t
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