I have that kit! It was great fun to build it, make some simple circuits, and trace the voltages through the "chip". The kit is really nice quality and thoughtf

I have that kit! It was great fun to build it, make some simple circuits, and trace the voltages through the "chip". The kit is really nice quality and thoughtfully designed. This kit is actually from Evil Mad Scientist [1], the same people who make the EggBot and other nifty stuff. Their own site has a lot more information about it, and they sell it cheaper than Elektor. [2] They also have a similar μA741 op-amp kit. [3] Back in the '70s I used to make blue boxes [4] with a bunch of 555s in them. This was probably not the best way to design a DTMF box - I remember I had a heck of a time keeping the frequencies stable. Woz's blue boxes probably worked a lot better! [1] http://shop.evilmadscientist.com/ http://shop.evilmadscientist.com/ [2] http://shop.evilmadscientist.com/tinykitlist/652 http://shop.evilmadscientist.com/tinykitlist/652 [3] http://shop.evilmadscientist.com/tinykitlist/762 http://shop.evilmadscientist.com/tinykitlist/762 [4] https://en.wikipedia.org/wiki/Blue_box https://en.wikipedia.org/wiki/Blue_box