Tube Time

Digging through my junk box today, I unearthed the scanning mirror from a laser printer, otherwise known as the Heart of the LaserJet. It’s got the infrared laser that generates the image as well as the scanning mirror that creates the raster. It’d be fun to get it up and running for nefarious purposes…

The scanning mirror and motor uses a “custom” (undocumented) Panasonic motor driver, the AN8247SB. As usual, Google returns a million hits for grey market parts brokers who spam the keywords with things like “PDF” and “datasheet” without offering any actual information.
So my usual plan of attack does not succeed.

The second step is to examine the single 5-pin connector to see what I could figure out. Pin 3 is obviously ground because it is the only pin connecting to any large ground planes. What I suspect to be pin 5 appears to be the power supply since it connects to two very low valued resistors (0.75 total) which probably perform a current sense function. Most of the other pins disappear inside the undocumented chip.

Digging around in my junk box produced the power supply board for the laser printer. I was able to find the other side of the connector and quickly verify that pin 3 is indeed ground. What I thought was pin 5 is actually pin 1, and it is indeed power. Tracing back through the power board I notice that it connects to a filter capacitor with a 25V rating. Based on that I conclude that it is very likely a 12V rail. I soldered some jumper wires onto the board and began experimentation in earnest.

Connecting the board to 5V didn’t result in any excessive current, so I slowly ramped up the voltage to 12V. Nothing happened. Not even anything bad.

Looking carefully at the laser printer’s power supply board, I traced the other three connections. They all went into a big microcontroller, but the wiring connections were different. Pin 2 had a 10K pullup to some low voltage supply, pin 4 went straight into the microcontroller, and pin 5 came from an RC filter from the microcontroller.

First I tried connecting a 10K pullup resistor to pin 2 on the motor driver board to 3.3V, and I hung a scope probe on it. It was a logic low. I spun the mirror assembly, and I saw pulses! This must be the tach output. By rotating the mirror very slowly by hand, I counted 6 pulses per revolution.

Next I probed the voltage on the other two pins, which were both weakly pulled up to about 3.6V on the motor driver board. I pulled pin 4 low, and the tiny mirror spun up with a whine to about 13,000 RPM (as measured by the tach output)!

That was really great because I was worried that those two pins were I2C control lines which would have made reverse engineering a lot more difficult. It’s not impossible because you can hook it up to a microcontroller and scan all possible I2C address to see if any slave devices respond, then randomly try to access registers… It gets pretty messy anyway.

The last pin gave me a bit of a headache because grounding it didn’t really do anything. I tried grounding it through an ammeter and noticed that the current, although it started at a few hundred microamps, tapered off quite rapidly. There must be a capacitor in series somewhere on the motor board, and that means the pin is designed for AC signals. Since no signal came out of the pin, it must be an input. I connected a function generator at a few kilohertz with a 3.3Vp-p square wave, and when I turned on the motor, I noticed that it “cogged” a lot and generally had a hard time. On impulse, I dramatically increased the frequency. Suddenly the motor slowed down and settled at a constant speed. By changing the frequency, I could manipulate the motor speed.

So pin 5 is a synchronization input. I guess the RC filter on the microcontroller side was designed to help reduce EMI in the cable. The next step was to figure out the relationship of input frequency to output speed, so I connected my trusty old Nixie frequency counter to the output of my function generator and my multimeter (set to frequency) to the tach output. The ratio appears to be fixed: divide the input frequency by 136.6 and you’ll arrive at the RPM of the mirror.

Here’s the complete pinout:

1 – +12V
2 – Tach output (open drain, 6 pulses per revolution)
3 – Ground
4 – Enable (active low, so drive it low to turn on the motor)
5 – Synchronization input

Now it’s time to come up with projects…

Just to give you a hint, I have something in mind involving a photomultiplier tube.

Drop me a line in the comments if you think you can guess what my idea is, or to post your own ideas, or even if you find this information useful for your own project.

Somebody at Maker Faire was interested in Nixie tube jewelry that actually lit up. I decided to take up the challenge.

The hard part is not making everything small, but making it last a long time on a single battery. In this case, the battery is a CR2032 lithium coin cell. A small circuit takes the 3 volts and steps it up to about 150V which is barely enough to light the Nixie tube. Theoretically it should last around 10 hours or so.

The socket was constructed using my custom-made Dremel drill press. To figure out where to drill the holes, I put some clay on top of the wood and pressed the pins of the Nixie tube into the clay. Then it was a simple matter to mark the pin holes, remove the clay, and drill. The pins are actually from a DB25 solder tail socket, since they fit the Nixie pins perfectly.

The power supply circuit has its problems, and I am trying to improve on it.

Recently I obtained a vintage Clough-Brengle oscillograph. Yes, you read that correctly–the oscillograph is the predecessor of the oscilloscope. Oscillographs lack the trigger function and the calibrated vertical and horizontal scales, along with many other features now ubiquitous on the modern instrument.

First thing I did was pull the cover off to get a look at the innards.

Click the image to jump to the Flickr page which includes some notes describing the various parts of the oscillograph. There’s a lot of rust and grime from years of neglect.

There is more circuitry underneath the instrument, as shown in this photo:

A cluster of components forms the sweep oscillator of the Clough-Brengle oscillograph. The radial-leaded resistors are essentially carbon rods attached to wires and painted with colors indicating their resistance.

Color code for these resistors works as follows: The body color is the most significant digit, the end cap color is the second digit, and the dot on the body is the multiplier. The colors themselves have the same meaning as today.

These resistors are probably of the +/-20% tolerance variety. They are actually trimmed; a single gash in the side indicates where resistive material was removed during production to dial in the value.

For some reason this picture reminds me of a Frank Lloyd Wright building…

I’ll post some more pictures showing the restoration in progress. If you really must look ahead and see them, take a look at my Flickr photostream.

Here’s a genuine antique carbon filament light bulb. It’s 375 watts and was originally meant for 110 volts (currently in the USA we use 120 volts AC). In the photo, the bulb is running from 50 volts.

Perhaps someone out there has more information on the age of this bulb. I think it’s around 60-70 years old. It’s not older since it doesn’t have the glass seal on the top of the bulb.

This graph shows the resistance of the filament in two types of light bulbs. The blue curve shows that a carbon filament decreases in resistance as the bulb heats up, and the pink curve shows that a tungsten filament bulb increases in resistance as it heats up.

Thus, carbon filament bulbs have a negative temperature coefficient and tungsten filament light bulbs have a positive temperature coefficient.

Tungsten is the filament material most commonly used in household light bulbs.

Incidentally the curve for the carbon filament bulb stops short at 90V because I don’t want to damage it. It runs very, very hot!