All Transistor Digital Clock Is a Work of Engineering Art
Hand soldered using only discrete components.
In a class called "Technology for Artists," Leo Fernekes and his students designed and built an All Transistor Digital Clock. Reminiscent of something you would find from the 1960s, there are no integrated circuits, only discrete components! You can, however, find some very innovative circuit designs. For example, instead of an LM7812, the linear 12 volt regulator uses a Darlington transistor as the pass element with a Zener diode in the feedback network.
Other modules in the All Transistor Digital Clock include one for setting the time, a Schmitt trigger for the pulse-per-second signal, 100 volt power for the 7-segment display, divide-by-3000 analog divider, and counter modules for the minutes and hours. Tieing all of the modules together is an enclosure, Fernekes, and his students assembled laser-cut acrylic pieces.
The 7-segment displays do make use of one modern-day component. Filaments from decorative retro-styled light bulbs made up each of the segments. Even though such a component was not available in 1960, the modern alternative reminds us of nixie tube elements because of their high (> 90V) dc voltage requirement.
Like many electronic clocks, the timekeeping signal comes from AC mains. This design only counts minutes, so it needs one pulse per minute. That ratio is 3000 to 1. Traditional approaches might use a series of flip-flops. While the students could build flip-flops with discrete transistors, it would mean at least 24 of them. Instead, this design uses an ingenious multi-stage analog divider. Fernekes does a fantastic job explaining how the circuit works, so we'll leave the detailed explanation to him. (Jump to 9:08 in the video.)
The All Transistor Clock is a beautiful piece of art and a marvel in simple circuits. It belongs both on the wall and in a textbook. For more information on the design and to see a demonstration, check out this video on Fernekes' Bag of Tricks on YouTube.