SPI Interface Big 7-Seg LED

The circuit uses a serial-in-parallel out shift register, 74HC595 for receiving serial data from uController board. See example of U5 in the schematic, SER is for data input, SRCLK is shift clock and RCLK is Latch clock. Each data bit is shifted into the register on rising edge of the shift clock. When all data bits are shifted into the 8-bit register, the rising edge of RCLK will clock the data to be latched at each output bit, i.e. QA - QH.

The Big LED is made from cheap dot LED. Each segment has five dot LED connected in series with a limiting resistor tied to +12V. The logic high at the input of ULN2003 makes the output active low, thus sinks the LED current into the chip. The driver has 7-bit for segment a, b, c, d, e, f, and g. Q1 is for optional point display.

Multiple digits can easily be made by connecting the QH to the next digit serial input bit, see the circuit below. Please note that, the shift clock and latch signal are

tied to every 74HC595.

Below is exemplary display board with four digits LED for temperature displaying. The control board may be attached to the display board by 10-pin header J2, on the back panel, say.