Unlocking Circuits the Maker Way

Unlocking Circuits the Maker Way


Equal parts woodworking and electronics, The Combinator is a difficult but rewarding Weekend Project. It not only protects your valuable goods, it’s highly gratifying to say you made it yourself.

Rather than using a microcontroller (Arduino) or single-board computer (Raspberry Pi, BeagleBone Black) to handle logic, Jordan Bunker’s design uses integrated circuits (ICs) to test and compare voltage levels. For someone only familiar with software, the transition to testing conditional statements in hardware can seem daunting. Not to worry — this project is meant to address and calm such concerns (and, in the end, to build an awesome project).

Annotated combinator schematic
Annotated combinator schematic

Refer to the schematic and look closely at a and b, the output from two comparators. The two outputs become the inputs for AND gate 1 and are evaluated as follows: 1 is only true if both a and b are true. The truth table below shows in long form the evaluation of a and b in 1.

screen-shot-2014-12-12-at-11-18-49-amThe same truth evaluation done on a and b, in 1 is repeated with c and d, in 2, and also on e and f, in 3.

The next level of AND gate, 4, compares the input from 1 and 2, testing in the same way as 1 tested a and b.

The output of 4 and 3 are evaluated by the final AND gate, 5. If 5 is true, the servo inside the safe is powered and the latch unhooks from the door clasp, granting access to all your goodies stashed away.


Sure, it’s a bit confusing at first, but so too are nested conditional statements in software. Our Weekend Projects are designed to be fun and approachable for all, and are great projects for learning new skills. For full project instructions to build your own Combinator — including all the woodworking — head on over to the project page.


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I love to tinker and write about electronics. My days are spent building projects and working as a Technical Editor for MAKE.

View more articles by David Scheltema


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