Combo lock uses relays and logic gates


Here’s a really fascinating circuit that implements a combination lock using relays and logic gates. Even with the schematic and written explanation of how it works we’re still left somewhat in the dark. We’ll either pull out some paper and do it by hand this weekend, or build it chunk by chunk in a simulator like Atanua. Either way, the project sparked our interest enough that we want to get elbow deep into its inner workings.

From the description we know that it uses a combination of CD4017, CD4030, CD4072, and CD4081 chips. You’re probably familiar with the 4017 which is a decade counter popular in a lot of project. The other chips provide XOR, OR, and AND gates respectively. The relays were chosen for two purposes. One of them activates when a correct combination has been entered, effectively serving as the output for the combo lock. The other two are for activating the clock and affecting a reset if the wrong combination is entered.

It makes us wonder if this would be incredibly simple to brute force the combination by listening for sound of the reset relay activating? It’s hard to tell from the video after the break if you can discern a wrong digit from a right once just based on sound.

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Blinky headgear

This hat has a chasing LED feature thanks to our old friend the 555 timer. [BananaSlug] even built in the option to change the speed at the push of a button.

His design starts out with a costume hat. Each of the 25 LEDs is soldered to a 2×4 hole chunk of protoboard. The LED package is pushed through a slit in the hat, but the protoboard remains on the inside where it can be sewn in place. From there [BananaSlug] soldered one negative bus around the circumference, and an individual positive lead from each module back to the control board. They’re addressed by a set of CD4017 decade counters which are clocked by the 555 timer circuit.

This is a great little analog/logic project and the style is perfect if you’ve got the coat to go along with it.

Challenge: Figure out how this logic-based chaser works

[Andrea] built this LED chaser using one logic chip. It illuminates all but one of the six LEDs, with the dim bit moving back and forth along the row in a chase sequence. This is something like an inverse Larson Scanner without the fading tail. But doing it with a logic chip instead of a microcontroller is a fun challenge.

Which brings us to the point of this feature. [Andrea] didn’t really post an explanation of how the circuit works. Usually missing details mean that we archive the tip and move on to the next one, but we think this provides a fun activity. Can you figure out how the circuit works? We already know that it’s using a CD4017 decade counter/divider chip. This gets its clock signal from a 555 timer circuit. [Andrea's] schematic is a bit hard to read, but grab a copy, blow it up a bit (or use your browser zoom) and study the CD4017 datasheet (PDF) if you need to.

Want proof that it does actually work? It’s embedded after the break.

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