Friday, February 1, 2019

Electric cabinet lock and other small DIY

Every once in a while, we have people (such as cleaners, babysitters or contractors) who have access to our home while we are away or distracted. And I am perfectly aware that petty theft usually does not happen in these scenarios (one case is enough to ruin the perpetrator's career), we also know that from time to time, against all odds, it does happen. So we needed to come up with an idea of protecting my wife's jewelries from an opportunistic snatch.

In other words, I needed a lock on the jewelry drawer, preferably one that would be discreet, and would not involve drilling the front of the (moderately beautiful) dresser cabinet. 

In a previous edition of this scenario, I accomplished this using two magnetic child locks (like these ones, they come in a huge variety) - to open the drawer you'd need to simultaneously place magnetic keys at two unmarked, previously known spots, which makes for an excellent discrete opening mechanism. But this time the cabinet dimensions proved incompatible with the locks. To add to that, even a casual glimpse of the open drawer, sporting the big white child locks, would instantly reveal our trick. Finally, the mounting holes for magnetic locks look bad even on the inside of the cabinet. (Whoever tries to convince you that these locks would hold on an adhesive mount, has not tried it. Adhesive mounting tape can be strong, but is is invariably terrible for dynamic loads such as repeated banging from trying to open a cabinet with the lock engaged but forgotten about.)

So I was searching for a geometrically suitable lock and stumbled upon this part, which just happened to have just the right dimensions to fit between the back wall of the drawer and the back wall of the cabinet. In addition, an electromagnetic lock has the advantage of not requiring submillimeter-precision alignment between the lock and the armature/striker plate - something that would totally plague most mechanical designs (like this one). 

I already had an unused electric key switch, which could be discretely built into the back wall of the cabinet, totally out of sight yet within easy reach. The only missing piece now was how to power the lock. Using a DC power adapter seemed an easy choice, but it is very easily defeated by unplugging it from the wall. Using batteries (and placing them in the same space between the drawer and the cabinet) was more secure, but with the lock's power consumption of 100 mA, batteries would require replacement every 1-2 days. 

Therefore, an ideal trade-off would be a plugged in adapter with a battery back-up that would take over the lock if the adapter is unplugged. After giving it some thought I came up with this circuit:



The central component here is the relay K1 that connects the battery through the normally closed contacts and disconnects it when external DC power is present. The diode D1 is there to ensure that the battery is not getting charged by the adapter (non-rechargeable batteries don't like that). The diode D2 ensures that the battery is not getting discharged through the output circuitry of the adapter. The diodes D3 and D4 are flyback diodes, which prevent arcing and sparking at the key switch (or the power jack).

Finally, the resistor R1 is the current limiting resistor for the relay coil. It actually proved the most problematic component because of the need to mitigate heat dissipation in the enclosed space behind the cabinet. With some experimenting I found that the relay coil current for reliable operation was 40 mA, yielding 0.32 W of dissipated heat, so when I stupidly put a 0.125W resistor, it got pretty charred very soon. Even a 0.5W resistor was getting worryingly hot. Since I totally don't want my lock to start a house fire - that would be the exact opposite of what a "security lock" is supposed to do - I first hooked up four 800 Ohm, 0.5W  resistors in parallel (2W total). This got the resistors slightly warm to the touch but not hot. Here is how it looks like from the inside and outside:




As an upgrade, I later replaced the 2W resistor arrangement with a 5W component on a heat sink. Now operating at 6% capacity, the heat dissipation was small enough for the lock to run for days on end without getting warm. As an additional precaution, I have installed a thermal fuse designed to cut the AC power circuit should the temperature ever exceed 73 degrees C (this was the lowest value I could get off Amazon). So this is the new set-up:



The beauty of the design, aside from it being totally discrete, is its being both fail-secure and fail-safe. It is fail-secure in the short term, meaning that power outage will keep the lock running on battery power long enough for a malefactor to not want to stick around. On the other hand, the rightful owner can wait several days for the batteries to discharge, and have the cabinet open if the key gets misplaced or lost.


BONUS: Here's some more office DIY. At one of my workplaces, the desk phone used too much useful space on my desk, so I wanted it next to my desk instead. Not wanting to drill any holes in the shiny new company property, I came up with a mount out some stuff lying around in the office, namely:

  • an old cardboard small packet from a recent online order
  • some Scotch tape
  • some good supply of cable ties
  • and a jar of spare furniture bits and pieces, apparently mostly from IKEA. 
This is the "before" and "after" image. If interested, I can give you more detail.




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