7 Segment Common Anode Display

[Christine Thompson]

Hi Folks,
Hoping the group can help. I built an eight 8 clock using 2 savaged 4 digit bedside alarm clocks. I am using a multiplex method to support the display. The multiplex is based on a design provided by David one of this group's members and consists of a 74HC595 SPI 16 shift register and a 74HC245 Octal tri-state transceiver register set in pairs, with 22 ohm resistors in series with the 7 segments plus decimal point, the other pair powers the 8 anode power lines. The system is currently running at 5V from a dedicated DC-DC step down 12V-5V transformer.

While the clock is working I feel that the intensity of the digits varies from one end to the other and the overall brightness could be more intense. So is there a way of increasing the brightness of each LED with a change to the circuit?

Thanks for any help you can offer.

[gregebert]

Reduce the resistors to increase the LED segment current. I assume you are doing 8:1 multiplexing, so each digit is only on for 1/8 of the time therefore you need more current.

Many years ago when I built my first computer, the HEX display that I wire-wrapped used 10:1 multiplexing and the digits were pretty dim. Mine were high-efficiency HP 5082 displays.

None of the TTL parts available at Radio Shack had a 7-segment decoder to display 0-9 and A-F (HEX), so I built my own using discrete diodes.

[chuckrr]

It's a 9368

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