Building a נר תמיד (ner tamid) from Scratch
Recently I decided on a נר תמיד (ner tamid) as my צדקה (tzedakah) project. If you’re curious about what these things are, you can click on the inline links to the corresponding Wikipedia articles. The cliff notes version is that a ner tamid is a light that is always supposed to be on.
For my project, I wanted to make one that would always be on, and not require the use of a wall outlet. A solar powered ner tamid wouldn’t go out in a thunderstorm when the power fails and wouldn’t go out when you change batteries. Initially, it seemed like this would be a piece of cake, because surely this kind of technology is available for sale from Amazon or some other retailer.
For my first attempt, I got a solar panel that is used to charge cell phones (so it has a USB cable), plugged it into a phone charging battery pack, and then plugged an LED USB reading light into the battery pack. This was a dismal failure, because the battery pack is “smart” and can control the power output to optimize for chargeable devices. This meant that my LED light went out after 5 seconds because it wasn’t drawing enough power from the battery.
Next, I found one of those solar-powered walkway lights and ripped it apart and tried to convert that to my purposes. Unfortunately, the lights are only designed to stay lit for a couple hours every night and, most importantly, they cannot charge the batteries while the LEDs are lit.
This turned out to be a pretty nasty sticking point. There are a million ways to plug a rechargeable battery into an LED, but making it so that battery can be recharged by some power source (in my case, solar) while the battery has an active draw on it to power an LED was far from obvious. This kind of charging happens when you’re using your smartphone while it’s plugged in, but the circuit that enables it is embedded directly in the phone’s board, so I couldn’t rip apart a phone to scavenge its charging circuit.
Thanks to a good friend of mine who has far more hardware experience than I do (I’m a software guy), I was able to find an MCP73871 chip. This is a demo chip (meaning it’s big and not already integrated into another board like inside a smartphone), and it has terminals for a power source (solar), a draw (the LED), and a battery. It has enough smarts to power the draw directly if the source is adequate, can switch to battery power if the source isn’t supplying enough power, and will charge the battery whether the draw is active or not. This was perfect! Again, I figured things would be all rainbows and unicorns, and again I was wrong.
So I spent the next couple of days making my own cables, with my own dupont connectors. The vast majority of these attempts failed, as the resulting cable wasn’t able to carry current for some bizarre reason (by bizarre, I mean it’s obvious I screwed the cable up). This wasn’t like software where things are deterministic and logical and neat and clean. Hardware is surprisingly fussy. Again, with advice from my friend and colleague, I was finally able to produce enough working cables to solder most of the connections on the MCP73781.
I then had a female barrel connector with two raw wire leads that I soldered to the source terminals on the chip. The result was a chip with 6 dupont-terminated cables dangling from it (there’s 6 because both the LED and the battery have to share the same ground, so I soldered a pair of ground wires to the ground terminal on the right side of the chip).
While I could’ve just soldered everything together without going through the trouble of making the cables, the extra work of making all of the various components modular gave me the flexibility to quickly and easily replace any component. In fact, I can swap out the solar panel without even dimming the LED, I can swap out the battery pack as long as I do it while the solar panel is giving a good 6V. If something goes really wrong, I can make a new one pretty easily and swap it out.
In this picture you can see the MCP inside the plastic box. There are female connectors coming out of the box to accept connections from the battery pack, male connectors to be plugged into the LED, and the female barrel connector for the solar panel. The battery pack is attached to the box with velcro so it’s easily removable.
For the LED, I had to solder a 220Ω resistor to one of the LED poles, then heat shrink the poles and the resistor to avoid shorts and the super-thin poles snapping.
At the beginning of this project, I thought it would be easy and require no custom hardware. At the end, it required 100% custom hardware and a specialized microchip. There were times where it made me feel so stupid and clumsy that I wanted to quit. Feeling stupid isn’t something I’m used to, but I think it’s something I need to do more, if that makes any sense.
I eventually prevailed, with help and encouragement from friends and family. The process of going through the design, prototyping, and final delivery was immensely satisfying. Knowing that people will enjoy something I built is rewarding, but I think even more rewarding is the personal growth both through introspection and education that I experienced during the project.
I strongly encourage anyone reading this to pick a project that requires you to develop a completely foreign skill. It’ll do your brain and your soul a pile of good.
