Powering 4" - seven segment displays that need high anode voltage

niji99
Powering 4" - seven segment displays that need high anode voltage

Many people want to use their Arduino's or micro's to power 4" high seven segment displays. The chip of choice as it is readily available is the MAX7219 or low slew rate MAX7221 with loads of Arduino code available, but the drawback of these chips is that they can only drive 5V LED Anodes. Many of these 4" displays, due to the fact that they have series LEDs inside them, need anode voltages of 9-13V DC or greater. The board that I have designed, built and tested, uses a 74HC595N shift register to clock the segments via a base limiting resistor and BC547 transistors. So in simple terms you can drive the anodes from a DC supply around 40 volts down, limit the current on each segment via a current limit resistor and display numerals and characters on the LED display. Resistors advised for the circuit are 1/8th watt so at 20mA, you wouldn't want to drop voltages of greater than 6.25 volts before you would need to go to 1/4 watt or 1/2 watt resistors. It will include the working out and setting of these values and supply sample Arduino code. All components are very standard, 8 transistors (7 segment drives and DP), 16 resistors, 1 IC, 2 caps, headers and the display.

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