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The Three Fives Kit: A Discrete 555 Timer

Three Fives' Discrete 555 Timer Kit - RF CafeWhen I first laid my hands on a 555 timer IC back in the early 1980s, it was Nirvana. Having recently separated from the USAF, working as an electronics technician at Westinghouse Electric, and working on my electrical engineering degree in night school, I was eager to learn about and adopt every bit of technology within reach. No opportunity was missed to integrate the 555 into my designs for test fixtures and experimental circuits. One such application was a custom timer that controlled a UV light source for curing a particular adhesive we were using in a DoD classified project. Those were exciting times.

Signetics (now NXP) had introduced its NE555 in a decade earlier (1971) to provide a simple, reliable means of generating a periodic squarewave signal with an adjustable duty cycle. Astable, retriggerable monostable, and bistable multivibrator configurations were accommodated. The 555 faithful still design them into products, but for the most part other resources have replaced it. For those who still salivate at the mere sight of those three fives strung together, a company named Evil Mad Scientist Laboratories has, with the assistance of Eric Schlaepfer (, created the Three Fives Kit: A 555 Timer, which is a macro size (3-1/8" x 5-1/4") 555 timer kit for you to build out of discrete parts. It even has a supersize DIP pins - four on each side.

Depending on which company's schematic you find, there can be between 24 and 26 transistors in the circuit, with a sprinkling of biasing resistors. This one uses 24 transistors and 15 resistors. According to EMSL the model can be patched into circuits in place of the original 555 timer and it will work in an equivalent manner. The Three Fives Kit would definitely make a great conversation piece, and it might also be useful during an interview to probe a candidate's basic knowledge of transistor circuits. $35 (+ as little as $3.22 shipping) seem like a reasonable price for such a piece of nostalgia.

LM555 Timer Schematic (TI) - RF Cafe      NE555 Timer Block Diagram (Wikipedia) - RF Cafe
                   LM555 Timer Schematic (TI)                                                  LM555 Block Diagram

Here is an hour-long video where Dave Jones (not Davy Jones of the Monkees, but British nonetheless) of the EEV Blog website takes you through the entire assembly process. You can skip past a lot of the soldering if you are not the type to watch grass grow or paint dry.

Posted  March 11, 2014

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