Beard trimmer from cordless to cordful

Finished trimmer with power supply

Some people are not men (it’s true). Some men don’t shave (or trim their beards in any way). Some don’t use electric shavers (or beard trimmers). All those people may not find this article the least interesting.

Those who do use electric shavers (and I will use that word to include ‘trimmers’ henceforth for simplicity), however, are likely familiar with the dominant shaver design paradigm: the cordless shaver.

Cordless shavers are powered by a rechargeable battery, so you can shave anywhere - on the beach, while driving and texting and drinking a latté, on a camel, etc. As you would expect, this is an extremely useful feature that no one wants to be without.

It does come with a cord, of course, so it’s not really ‘cordless.’ You plug in this cord in order to charge the shaver up, which you have to do every so often. Which is maybe once a week when the battery is brand new. And then after, oh, a week or so of that delicious freedom, you have to plug it in pretty much once a day - but you can still unplug it while you’re shaving, so you can dance around in your bathroom instead of shaving while tethered to the wall. And then after a year at most, the battery doesn’t hold a charge very well and you pretty much have to leave it plugged in all the time, as it gets weaker and weaker and eventually doesn’t have enough oomph to slice through even a single arm hair, let alone the wires that grow out of your chin, and you throw it in the garbage and buy a new one.

So clearly, cordless shavers are a vast improvement over the old ‘corded’ shavers, which you could use for years and years, only replacing the blades every Presidential administration or so, and which cut through those wires quickly and cleanly, year after year. Because, dammit, a lot of us apparently do shave while riding camels, and are willing to buy as many shavers per year as it takes to do so; and because Norelco has hungry stockholders to feed.

So what’s my problem then?

My problem is that I am a skinflint. It galls me to throw anything out. And it galls me particularly painfully to throw out such an amazing pinnacle of technological achievement as the electric shaver simply because its battery is old.

So, over the years I’ve tinkered with several shavers that have gone out with the usual whimper. I’ve replaced batteries three or four times now. At first, I searched the web for the exact manufacturer’s part number to buy replacement batteries. I think they were something like $20-$30 plus shipping, but I was happy not to be throwing out the shaver itself. And when I heard the sweet sound of my old shaver whizzing happily again, it made me happy.

Then I tried using good-quality but generic nickel-metal hydride (NiMH) AA batteries, which are a few bucks each. First I did it in the one beard trimmer I had that actually made it simple to replace the battery - just three screws to open the case, and a regular AA form factor with the button on the end. The manual urged me never to use anything but the official manufacturer’s replacement part, but it worked just great.

Then I tried it with a harder model, that was a bit trickier to get into and used a battery with solder tabs on the ends. These are tough to deal with, because if you’re not very careful with the soldering iron, you can damage the cell internally. But I got it working with a button-end I made from the little metal blade insert in a package of dental floss. Victory again.

When the going gets easy, the engineer gets new problems

But I soon acclimated to these thrills, intoxicating though they were at first. I needed more.

In particular, it was starting to annoy me that I had the battery in the picture at all. It’s not a big thing to throw away once every couple of years, but it’s something. And even when the battery still had some useful life to it, I was tempted to bring the shaver on trips without the cord - and when I did, the battery invariably died. So what’s the point of having to replace the battery AND still have to use a cord all the time?

When the beard trimmer’s battery died last Spring, I was struck by another puzzle: When the battery is dead, but the cord is plugged in, why can’t it work? I suspected planned obsolescence, and that gets my back up.

An absurd amount of work later

I reverse-engineered the trimmer - drew out the circuit schematic from the circuit board, and studied it. I concluded that the key issue was cost, and possibly size. To wit:

  1. If you want to run a motor off of a single AA cell, it has to be happy working at about 1.3 volts. That’s pretty low for most small electronic devices.

  2. The motors I tested pulled about 0.8 amps @ 1.2 V. That’s pretty high for most small electronic devices.

  3. A NiMH cell can discharge happily at 0.8 A. But you generally charge it at a much slower rate, unless you have relatively sophisticated smarts in place to do “rapid charging” safely.

  4. The step-down transformer and other components that you need to get the right voltage to charge a NiMH cell from mains current can be pretty small, IF you don’t need much current. But sourcing nearly an amp from them will require them to be much larger, or more expensive, or both.

  5. SO: The designers generally settle on a power supply circuit that can deliver a very small charging current to the NiMH cell, and then rely solely on the NiMH cell to run the motor.

And there you have it: works with a fresh battery, but without it, you can’t run it even if it’s plugged in.

But I don’t care

That’s all relevant if you know that being able to run off an AA-sized battery is a real design requirement. But since I end up plugging the damn thing in all the time anyway, I just don’t care.

So: I designed a new circuit board that used a generic 5V switching power supply for safety (this one happened to be from my old Treo 270 phone), and then stepped this 5V down to about 1.75 V efficiently, without overheating. I used a simple “buck” topology in the switcher, run by a PIC microcontroller.

Some photos are in my gallery, and include some more details. I was particularly satisfied that I was able to get the switch on the body of the trimmer to work - it was tricky getting it to mate directly with the circuit board.

The growing season begins

It works. It’s zippy. Tomorrow is the Fall Equinox, traditional beginning of the winter beard. I look forward to the day, probably some weeks off, when I can plug my new/old baby in and savor that cordful, consistent, dependable buzz, from a power supply that won’t get tired out by the end of the season or maybe even the end of the decade.

They say that knowledge is power. I say, power is power.