The Hewlett Packard HDSP series displays were among the first numerical LED displays that came after the Nixie tube display. These cool little numerical LED displays are unique because they take a binary coded decimal input, and convert it to a human readable 7 segment display with a built in die. The best part is that the BCD to 7 segment converter, the LED dies, and the bond wires are plainly visible under a thin glass window! These cool little displays don’t have many practical applications in the modern world, but they make cool clocks!
I’ve made several clocks in the past with these displays that ran on digital logic chips. This seemed like the way to do it because these TTL logic chips wire the current technology at the time the HDSP displays were common. But I got frustrated with electrostatic damage and other problems, and switched to a micro controller based design that’s proven to be very reliable. My microcontroller of choice is the Atmel 328PB because it’s affordable, it has plenty of GPIO pins, and it does everything I need. I’ll cover how to flash this chip with the Arduino IDE in a different tutorial.
On some of the HDSP displays, pin 4 is a blanking input that can be used as a PWM brightness control. Most of them have a decimal point, and pin 4 is used to turn the decimal point on and off. This circuit includes a 555 timer with a potentiomiter that can be used to adjust the brightness if your displays have a blanking input on pin 4. You can omit this part of the circuit if your HDSP displays don’t have this feature, or you can hook it up and it will dim the decimal points.
Below you will find everything you need to make your own HDSP clock. You can copy my design exactly as it is and order it from your favorite board house, or import it into Easy EDA and modify it to your liking. I will not be offering this clock as a soldering kit. The HDSP displays themselves are hard to find and EXPENSIVE. The rest of the parts on the BOM are not obscure or expensive.
File Download Links:
This file is labeled Nixie clock because I wrote it for my Nixie clock, but it’s really just a binary coded decimal clock.
