Three Fives Kit

Posted on September 22, 2018 in hacks

Some time ago I got a Three Fives Kit from Evil Mad Scientist—but never took the time to actually build it. The Three Fives Kit is "a faithful and functional transistor-scale replica of the classic NE555 timer integrated circuit, one of the most classic, popular, and all-around useful chips of all time."

I have finally taken some time to build the kit, cursing at my poor soldering skills. And then, in order to test it, I have built a basic astable oscillator (Electronics Tutorials have a nice page on 555 oscillators).

A basic astable oscillator can be modeled in LTspice as (download schema):

The various online tutorial all give the magic formulas to calculate the values of R1, R2 and C1 for a given frequency. Or, if you are lazy, All About Circuits has a frequency calculator. The circuit above runs at about 500Hz with a 66% duty cycle:

The equivalent oscillator, but running on the kit, can be modelled as (download schema):

And, although some levels are slightly different, it runs with very similar traces:

The nice thing is that you can the poke at the various parts of the kit with an oscilloscope, and compare with what LTspice calculates. All in all, it helps revisiting your understanding of transistors, including beasts such as Darlington pairs or current mirrors. Evil Mad Scientist's Principles of Operation manual is also very helpful to understand how the 555 works.

Finally, replace C1 with a 100µF capacitor, bringing the frequence to around .5Hz, and you have yet another LED blinker¹!

LED on the right is blinked by an integrated NE555; LED on the left is blinked by the kit