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The Demon Razor that Wouldn’t Turn Off

What do you do when a battery-powered appliance won’t turn off? And when it’s a sealed unit, so removing the batteries is impossible? And when its body starts to grow disturbingly warm? That’s the situation I found myself in a few days ago.

 
Riddles in the Dark

I was working at home one night, and gradually became aware of a strange buzzing sound. Initially I thought the sound was outside, but when I went to investigate, I discovered it was coming from the bathroom. My skull shaver, plugged in and recharging, had mysteriously turned itself on and the blades were spinning away. Pressing the on/off button had no effect. Unplugging the charging cable had no effect. The body is a single piece of molded plastic, so there was no non-destructive way of opening it. Nothing could stop the whirrrrrrrrrr of the blades, and the shaver was noticeably warm.

I started to panic that the razor would explode. The internal battery is likely lithium polymer, and from my days with RC cars and aircraft I know that defective or damaged LiPos can fail catastrophically. Like literally go boom and eject flaming molten goo everywhere that burns down your house.

I quickly took the razor outside, and set it on the concrete patio, blades whirring this whole time. A couple of minutes later, I began to fear that it was still too close to the house if it exploded, so I moved it to the street. Thankfully it didn’t explode, and those blades kept whirring for 90 minutes, during which two people stopped to ask what the horrible noise was.

 
A Tale of Two Chargers

So what caused the skull shaver to go crazy? Bad charging. Besides this manly pink skull shaver, I also own a more conventional Norelco cordless shaver. I’d never noticed it before, but the chargers for the two shavers have the same plug at the end of their cords:

A quick check confirmed that yes, I’d accidentally plugged the skull shaver into the Norelco charger. Is that bad? You might think that the plug shape is standardized, and that all charger plugs with this shape are designed for the same voltage. Let’s check. Here’s the skull shaver charger, which is nicely labeled. 5V output, max 1000 mA:

And here’s the Norelco charger. Instead of a label, its specs are molded into the charger body using impossible to read tiny-sized black-on-black lettering. Yuck.

But if it’s tilted at just the right angle to the light, and you get your reading glasses, here’s what emerges:

15 volts! Ouch! I charged a 5 volt device with a 15 volt charger.

I’m suddenly nostalgic for the days when real on/off switches physically disconnected the power. Many of today’s electronic appliances have a soft on/off switch that’s really just an input to some controller circuitry. When soft switches work, they’re great. But when something goes wrong with the control circuit you suddenly have a zombie appliance that can’t be shut off. In the case of this razor, the 15 volts apparently killed the control circuitry before the LiPo battery could be damaged to the point of explosion by over-charging. And the failure mode of the control circuitry was to fail ON.

Have you ever made a similar charging mistake, or exploded a battery through mistreatment? Leave a comment below and tell your story!

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ProDOS Software Collection for Floppy Emu

Craig of apple-2.com has put together a great collection of Apple II software with a Floppy Emu theme. It’s a 32 MB bootable ProDOS disk image with an animated splash screen, and it uses the amazing Bitsy Bye program launcher to select from a variety of included games and utility programs.

Floppy Emu can use this disk image to boot your computer, when the Emu is configured for Smartport hard disk emulation mode. Just rename the disk image to SMART0.PO and copy it to your SD card. It’s compatible with the Apple IIGS, Apple IIc and IIc+, and Apple IIe with Liron card.

Craig cautions that some of the included games like CANNON.BLITZ may not work correctly when launched directly from Bitsy Bye, due to a memory conflict. If you find a game that crashes when run from Bitsy Bye, reboot and select GAMES.CATALOG from within Bitsy Bye. Then type in “-CANNON.BLITZ” or “BRUN CANNON.BLITZ” and the game should run normally.

Download the disk image here: SMART0.PO

Don’t forget to check out apple-2.com’s other vintage download collections.

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Get 10 Give 10 – Floppy Emu and Samaritan House

 

Looking for a discount on a Floppy Emu Deluxe Bundle, and want to spread some holiday cheer at the same time? BMOW is running a holiday promotion called Get 10 Give 10. Use the coupon code GET10GIVE10 during checkout, you’ll save $10 off a Floppy Emu Deluxe Bundle, and I’ll donate a further $10 to Samaritan House of San Mateo. It’s a chance to save money on retro computer hardware and do something good for the world too.

Floppy Emu is a floppy and hard disk emulator for classic Apple II, Macintosh, and Lisa computers. It uses an SD memory card and custom hardware to mimic an Apple floppy disk or hard disk drive. It’s perfect for booting your favorite games, transferring files from vintage to modern machines, and troubleshooting a computer without a working OS. The deluxe bundle includes the Floppy Emu, an acrylic case, and an SD card with a collection of vintage Apple software disk images. Full details are available here.

Samaritan House provides food, shelter, healthcare, housing, financial assistance, and more to low-income and homeless persons in the San Francisco Bay Area. Even in the midst of Silicon Valley’s affluence, there are many people struggling just to meet the basic needs of daily life. A small boost at the right time can help them regain self-sufficiency. Samaritan House operates a broad variety of free services and one-on-one assistance with caring staff. I’ve seen first-hand what they can do, and it’s amazing.


If you’ve had your eye on a new Floppy Emu, or need a second or third unit for your growing computer collection, here’s your opportunity. Thank you for supporting the good work of Samaritan House!

Enter the coupon code GET10GIVE10 during checkout to take advantage of this offer.

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ADB-USB Wombat Firmware Update

  

Version 0.3.1 firmware for the ADB-USB Wombat is now available. The Wombat is a bidirectional ADB-to-USB and USB-to-ADB converter for keyboards and mice. It enables the use of modern USB peripherals on vintage ADB-based Apple or NeXT computers, or ADB peripherals on modern machines equipped with USB. The Wombat is great for breathing new life into your vintage hardware collection.

Firmware 0.3.1 fixes a small but important bug that affected a small number of USB-equipped Macs that were built between 1998-2003, including some G3/G4 models of iMac, iBook, Powerbook, and Power Macintosh. ADB-to-USB conversion worked normally on these computers when running OS X, but they were unable to detect the keyboard when running MacOS 9. The reverse USB-to-ADB keyboard conversion worked normally under both MacOS 9 and OS X. If you weren’t affected by this bug, there’s no need to update your Wombat and you can safely stay with firmware version 0.3.0.

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Apple IIc Drive Switcher Back in Stock

The Internal/External Drive Switcher for Apple IIc is once again available in the BMOW Store. The Switcher is a convenience option for Floppy Emu owners with a IIc, and makes everything easier when when emulating a 5.25 inch floppy disk drive. It provides a simple way to select whether the internal floppy drive or external Floppy Emu will appear as 5.25 inch Drive 1, which is the only bootable drive on the IIc. More details are here.

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Halloween LED Matrix and PSU Death

In preparation for Halloween this year, I built a large and colorful LED matrix. After programming it with monster-themed animations, it looked fantastic! Months passed, and when October 31st finally arrived, at dusk I hung the LED display outside by the street. It was to be the perfect lure for neighborhood kids.

But when I checked back an hour later, the LED display was dark and dead. Halloween passed in sad form, with no display of animated monsters. What happened?

 
LED Matrix ‘Hello World’

This all started last May, when I bought a generic 64 x 32 LED matrix from eBay. These matrix displays are designed to be controlled by an Arduino, Raspberry Pi, or other microcontroller or FPGA. The smallest displays are the simplest: the control unit selects one row of LEDs to illuminate, and then sends a stream of 0’s and 1’s to turn off or on the individual LEDs in that row. In effect, each row of the display is like a large shift register, with each bit corresponding to a separate LED. By cycling rapidly through all the rows, and providing different data for each row, the control unit can create the appearance of the whole LED matrix being lit. For larger displays, two rows can be illuminated at once, using two separate streams of bits, but otherwise the interface is the same.

A blog post from May describes my experiences with that first 64 x 32 LED matrix, using custom software I wrote for an Arduino. It worked well, and the matrix was impressively bright and colorful – the photos can’t do it justice. My custom software was limited to displaying static images, with only 8 colors, because the red, green, and blue LEDs were simply on or off with no in-between state.

 
Matrix Upgrade

If one matrix is good, two matrixes must be better! I bought a second identical matrix and connected it to the first. These matrixes are designed to be daisy-chained, with the shift-out from one matrix connected to the shift-in of the next. Logically this results in rows that are twice as long as before, creating a 128 x 32 matrix. But physically I arranged the displays to create a 64 x 64 matrix. As a result, the control software became more complicated with mappings between physical and logical lines.

I quickly abandoned my custom Arduino solution, and adopted the excellent rpi-rgb-led-matrix Raspberry Pi library by Henner Zeller. It’s incredibly rich, supporting many different physical to logical mappings, thousands of colors using PWM, video playback, and many other advanced features. Really, if you’re experimenting with one of these LED matrixes, this is the software you want.

For the Raspberry Pi, I selected a Zero W thanks to its built-in WiFi, small size, and rock-bottom price of $10. The OS is a default Raspbian image configured to run in terminal mode. It’s easy to connect to the Pi over WiFi using ssh, and then use rpi-rgb-led-matrix command-line utilities to display images on the LED matrix. It’s a powerful solution, and the only downsides compared to the Arduino are the few seconds required for the Pi to boot up, and the need to perform a clean shutdown instead of just pulling the power plug.

It’s possible to connect the LED matrix directly to the Pi’s GPIO pins, if you don’t mind a squid-like mass of wires. I chose an easier route and bought the Adafruit RGB Matrix Bonnet, which has the same footprint as the Pi Zero and makes the LED matrix connections a breeze. I performed a simple mod to the Adafruit bonnet in order enable hardware PWM to reduce flickering, as described further here. After that it was just plug and play, using the --led-gpio-mapping=adafruit-hat-pwm command-line switch for the rpi-rgb-led-matrix software.

Using the advanced search tools from Google Images, I looked for 64 x 64 animated GIFs with monster-related keywords. In short order I was able to locate several dozen. I was in business!

 
Building the Frame

To mount the two LED matrix panels together and create an eye-catching display, I built a custom frame. The design was loosely based on this Instructable by Al Linke. It uses several layers of laser-cut acrylic with a pile of carefully-sized spacers and machine screws. The frame took a substantial amount of work, but the end result looks great.

First the LED panels were mounted on a black acrylic piece, with pre-cut holes for mounting screws, wiring, and the Raspberry Pi. The rear of that piece was an ugly and bumpy tangle of cables that wouldn’t hang flat against the wall, so a second black acrylic piece was mounted behind the first to contain the mess. This piece also has integrated mounting holes for a wall hook or picture hanging wire. A third semi-frosted acrylic panel was then mounted on top, to give the LEDs a more diffused look. This is matter of taste, but I found that the LED images looked much nicer with the diffuser panel than without.

 
Powering the LEDs

In a 64 x 64 matrix, there are 4096 elements. Each element contains separate red, green, and blue LEDs, so the grand total is 12288 LEDs. Assuming that each LED draws 15 mA (a typical number for a single discrete LED), naive math calculates the total current as a whopping 184 amps! Ouch! But this calculation overlooks the fact that only a few rows are actually illuminated at the same time. This particular matrix uses 1:16 multiplexing, so the maximum current is a much more manageable 11.5 amps.

Armed with this information, I purchased the 5V 10A power supply shown here. Why only 10A instead of 11.5A or more? Enclosed “brick” supplies that can provide more than 10 amps are difficult to find, and I expected I’d never need 11.5 amps anyway for real-world images. 11.5A is a worst-case number for a solid white image where every LED is on. My halloween monster images are much darker, typically with many black pixels, so the required current should be much less.

 
Failure Analysis

When the LED display died on Halloween night, the Raspberry Pi was unresponsive to WiFi connections and the LEDs were dark. The power indicator on the Pi was blinking on and off. Later I noticed that the blue power indicator on the power supply brick was also blinking on and off. Strange.

I brought everything inside and plugged it back in, with the same result. Nothing worked, and the power indicators blinked. At first I thought there must be a short-circuit somewhere in the LED matrix or the Adafruit bonnet, which was repeatedly tripping some protection circuitry in the power supply. But when I disconnected the Pi and the LED matrix, and tried the power supply alone, I observed the same blinking power indicator. The problem was clearly with the power supply itself. Did I exceed its maximum current rating and kill it?

Confused, I left the hardware unplugged for a few days. Later, when I plugged it in again to begin more troubleshooting, I was surprised that it worked! I reconnected the LED display, and everything was great for about an hour. Then it died a second time, with the same blinking power indicator symptom. Uplugging and replugging didn’t help. But if I left it unplugged for a few hours, it would work again the next time it was plugged in.

By this point I was fairly sure I must be over-taxing the power supply and drawing too much current. I guessed that the supply must contain a thermal fuse, and it was overheating and shutting down. Only after a few hours of cooling would it work again, for a short while. 10 amps was simply not enough, it seemed.

To solve my power needs I purchased this 5V 30A “cage” type supply. I was reluctant, because this type of supply isn’t meant for outdoor use, and because it requires wiring to screw terminals instead of using standard power plugs. I’m a reasonably careful person, but I still get nervous playing with bare wires that carry mains voltages. Unfortunately I didn’t see a good alternative.

Before connecting the 30A supply, I decided to do one more test to see how much current the 10A supply was really using. I don’t have an easy way to directly measure the DC current, so I used a Kill-a-Watt meter to measure the power delivered from the wall outlet to the supply. I cycled the LED matrix through its collection of monster images several times, and the highest power measured by the Kill-a-Watt was 19 watts. Hmmm. If I assume the power supply is 80% efficient, then that means it was supplying about 15 watts to the LED matrix. It’s a 5V supply, so that’s a current of only 3 amps maximum – far below the supply’s claimed max of 10 amps. So why did the supply keep shutting off after an hour of use?

While still connected to the Kill-a-Watt, I let the hardware run for a while. This time it took several hours before it shut down, but the end result was the same as before, with dark LEDs and a blinking power indicator. The Kill-a-Watt showed 1 watt. The power supply didn’t feel hot to the touch, but was only slightly warm. This seemed to rule out my “overheating” theory, but didn’t suggest anything else.

Because I’d measured the max current at only about 3 watts, I decided to try a new approach. I pulled out another brick-type power supply from a different project, this one rated at 5 volts and 4 amps max. I connected it to the LED display, and started everything running. It worked just fine, and 18 hours later it’s still running smoothly. Success! At least for this series of Halloween images, it’s all I need.

I’m still curious what caused the first power supply to fail the way it did. Clearly it’s defective or broken somehow, but I’d like to understand more. A 10 amp power supply shouldn’t have any trouble delivering 3 amps continuously. And if it were actually something like a 2 amp power supply, mislabeled as 10 amp, I would expect it to get obviously hot after extended use. But it was never more than slightly warm. Could there be another explanation?

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