555 Contest Entry

Projects 28 Comments

Yes, it’s not really vacuum tube related, but I built an entry for the 555 timer contest. It uses an ICM7555, which is Maxim’s second source of Intersil’s CMOS version of Signetic’s original NE555 timer. Turns out the fact that it is CMOS is important for this particular circuit…

ICM7555 - CMOS Timer IC

My entry is an AM radio. The only active device (silicon, germanium, or otherwise) is the ICM7555. The tuning is accomplished with an inductor and a capacitor, and the ICM7555 acts as an AM demodulator and class-D power amplifier to drive the speaker.

555 Radio

You may be wondering how all this is accomplished with a 555. The schematic is below.

555 Radio - Schematic

Here’s how the circuit works: The AM radio signal is tuned by inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made out of an old toilet paper roll, along with the 100pF variable capacitor. One end of the parallel configuration of L and C connects to an antenna (surprisingly long!) and the other end connects to a ground wire which is tied to the AC outlet ground (old books tell you to ground it to a water pipe). So far this is exactly like an AM crystal radio.

The 555 timer is configured as a pulse width modulator in a non-traditional configuration. If I used the standard approach and connected the input to the CV pin, the low impedance of the pin would prevent the circuit from receiving any radio signals. I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. This is the reason why the CMOS version of the 555 timer performs much better than the standard bipolar, which has higher input bias current.

The pulse width modulator ramp is created by the 0.01uF capacitor and the 10K bias potentiometer which are connected to the Discharge pin. The potentiometer wiper goes to the LC arrangement. With no radio signal coming in, the voltage on Threshold/Trigger ramps up until it hits the threshold, and then Discharge causes the voltage to ramp down again.

When a radio signal comes in, it gets superimposed on the ramp signal, causing the threshold and trigger comparators to trip early or late in a cycle. This variation causes the output duty cycle to vary, which we can hear as sound in the speaker.

Demodulating the signal properly requires adjustment of the bias knob, so that part of the radio signal is “clipped” and ignored by either the threshold or trigger comparators. This ensures that the negative “halves” of the radio wave don’t cancel out the positive “halves”.

Want to hear what it sounds like? Check out the video below:

And of course, I can’t end the post without a gratuitous shot of the ICM7555 in circuit.
555 Radio - The Core

Steampunk Wristwatch

Clocks, Projects 5 Comments

Steampunk Wristwatch
Is that…?
Steampunk Wristwatch
Yes, it’s an LED steampunk wristwatch! It uses the LED wristwatch board (see this previous post). The watch is constructed from a small piece of oak and pieces of brass sheet and tubing. I used hand tools, a Dremel tool and a cordless drill to shape and form each of the pieces.

There are four more LED wristwatch boards left. I wonder what style of watch I should make next…

Here’s one more picture.
Steampunk Wristwatch

LED Wristwatch

Clocks, Projects 12 Comments

Sure, this doesn’t use a vacuum tube, but it’s still a neat way to reuse some old-fashioned 7-segment LED displays. OK, so I can’t wear it yet. It still needs a case and watch band. I am kicking around a few ideas, but feel free to post a comment if you have any suggestions.
LED Wristwatch
The LED displays are quite tiny and would have been used for calculators or similar devices back in the day. They were made by Fairchild as you can see by the original packaging:
LED Wristwatch - Fairchild LED Chips
The unique thing is that each display has a single die inside with each of the segments etched into it. In the picture below, you get a pretty clear look at the bond wires and the top metal layer. If you click the image, I have annotated the Flickr page.
LED Wristwatch - Closeup of LEDs

Back to the watch. It keeps the time with a fairly pedestrian PIC16F628A. It has an internal timer that operates with a separate oscillator (which is the watch crystal in the lower right corner) which can run even during sleep mode. This is critical to keeping the power consumption low. When a timer tick occurs, it generates a wakeup event, and the processor can increment the internal timekeeping registers. The processor can also wake up when one of the buttons is pressed. When that happens, it turns on and starts multiplexing the LEDs so that it can display the time. After a short delay it goes back into sleep mode. I haven’t yet calculated or tested the battery life.

The batteries are ZnAir number 10 (a common hearing aid battery). This is a zinc-air cell that uses oxygen as part of the electrochemical reaction, which is why there is a tiny hole in the top of each cell. Any battery holder has to allow air to make contact with the hole. In many states, these are the only batteries you’re allowed to throw away in the regular garbage. California is one of the exceptions, and the state considers zinc to be hazardous waste, so these batteries have to be collected separately in a category called “universal waste”. To me this seems foolish because I suspect that a lot more zinc is released into landfills as bits of scrap galvanized metal. Things like galvanized flashing, nails, and deck screws. Regardless of legislation, zinc is a lot less harmful than lithium, so remember to dispose of your burned-out LED throwies properly (and not in the regular garbage).

Here’s a puzzle for you: the PIC has 13 I/O pins. The LED displays use 8 (7 segments plus 1 decimal point) anodes and 4 cathodes. That leaves a total of 12 I/O pins, and I am not using the 13th I/O pin (RA4) because it is open drain and not useful in this circuit. So how are the two pushbuttons wired to the PIC? In fact, how can either button cause the PIC to wake up from sleep? Post a comment with your theory. I’ll give you one hint: there is a dual diode connected in some fashion to both switches.

And here’s one last photo to give you an idea of how small this thing is:
LED Wristwatch - Coin for Scale

Electroluminescent Display Panel

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You’ve probably seen LCD displays that use electroluminescent panels for the backlight. They’re not so common since the advent of the white LED, but they were used all the time for those 5×7 character displays. Wikipedia has a nice article about electroluminescent technology.

Well, it turns out they can be used to make displays:
EL Display Panel

Here’s a closeup of the pixels:
EL Display Panel - Closeup

The “ghostly” numbers are letters are the result of the screen displaying the same thing for years and years. Looks like it was from a machine used for rapid thermal processing of semiconductor wafers.

This display panel was made by Finlux. There’s a very thorough history of this company and EL displays here: http://www.indiana.edu/~hightech/fpd/papers/ELDs.html.

It’ll be fun coming up with a project for this one. The display is 640×200, “high resolution” CGA. The video inputs are TTL-level video, hsync, vsync, and pixel clock. It will probably work at other frequencies too since the panel just uses high-voltage shift registers, not some special video ASIC. The tricky part will be getting the pixel clock to the device since video card connectors don’t provide that. Maybe I’ll just drive it with a microcontroller…

IEE Clock Internals

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Let’s take a look inside the previous posted IEE clock:

IEE Clock - Laced Wiring

Here you can see the laced wiring harness for the displays. This is not a multiplexed display–each of the 40 light bulbs has its own transistor driving it. All these wires come off the displays and into a very large DIN-style connector that plugs into the main circuit board. The ICs are all on the other side of the board.

IEE Clock - Disassembled

This is a closeup of the main PCB. It’s quite small. You can see the PIC’s ICD header off on the right, along with a header that allows access to the I2C bus for troubleshooting.

IEE Clock - PCB

And looking at the other side, we can see the driver ICs, the PIC, and the RTC device, along with some passives. You can also see the spare pads I put next to the PIC so that I can easily solder wires to the unused GPIOs if I ever need them.

IEE Clock - PCB Top

Some people commented about the switch on the front. I have a decent collection of switches of all colors, types, and vintages. It’s disconcerting for me to see what would be a really neat project marred by boring switches. Here’s a small sampling of my collection:

Switch Collection

People really liked the photos of the disassembled IEE module from the other website. Here are a few gratuitous “artsy” shots:

IEE Clock - Display Modules

IEE Clock - Bulbs

Modifying an AC Adapter’s Output Voltage

Cleverness, Projects No Comments

My IEE clock runs on about 6.5VDC. Why the odd voltage? There are sundry voltage drops throughout the circuit that require the power supply voltage to be higher than the 6V rating for the light bulbs. The problem is that you just can’t find 6.5V AC adapters.

Fortunately it’s quite simple to modify an existing AC adapter to run at the new voltage. I’m sure there are other people who might want to get their own custom output voltages, so here’s a short tutorial.

Disclaimer: if you go ahead with this project, you’re doing it at your own risk. AC line voltage is quite dangerous and you could be injured or even killed. If you get shocked, get medical attention right away: there have been people (often with a previously undiagnosed heart condition) who have received a “small shock” only to drop dead a couple of hours later.

Here is what you need: A switching AC adapter, a flat screwdriver, a soldering iron, an assortment of surface mount resistors, and some time. It is important that the AC adapter is of the new switching style rather than the old transformer style–usually you can tell if the AC adapter feels light and is small enough not to block adjacent outlets, yet has a relatively high current rating for its size. I am starting off with a fairly generic 5V 2 amp AC adapter. Yours will likely use a similar circuit inside.

Quick side note: Why are the new AC adapters so much smaller? First, to provide a given amount of power, a transformer (copper windings around some type of core material) has a physical size that decreases as the frequency goes up. A transformer designed to operate at 60Hz or 50Hz  is going to be much larger than a transformer that is designed to operate at 15KHz. Since the frequency of the AC line is fixed, these new AC adapters work by converting the input AC to a much higher frequency. There is also a little circuit that looks at the output voltage and tweaks the converter circuit to maintain a constant output voltage. Thus, the new AC adapters have much better regulation than the old styles. As an added bonus, this type of design can operate at 50Hz, 60Hz, 120V, or 240V! To get into more detail, there is a feedback circuit that compares the output voltage to a voltage reference, then sends an error signal to the converter circuit at the primary side of the transformer. The error signal is isolated (for your safety) using an optocoupler.

To take apart the AC adapter, crowbar the plastic halves of the case where they meet in the middle. Try to find the little plastic catches that keep the halves together. You might need two screwdrivers to get them to come apart. Inside you can see a little circuit board with components and some wires.

Modifying an AC Adapter

To modify the output circuit, the feedback circuit must be modified. I sketched out the circuit and learned that this particular AC adapter uses a clone of the TL431 reference+error amplifier circuit (which is the weird zener-diode looking thing in the photo above). This particular device, the AM431, has an internal 2.495V reference. The output voltage is divided down so it can be compared with 2.495V, and this is done with a resistive divider formed by R1 (4.87K) and R10 (5.23K). R10 goes to the output voltage, and R1 goes to ground. In this design, I can only modify R1 since R10 (acting in combination with the feedback capacitor) also determines the loop compensation pole (no need to mess with this). Based on my reverse-engineered schematic of the secondary side, the formula for the output voltage is Vout = (1 + R10/R1) * 2.495. Replacing R1 with a 3.3K resistor should bump up the output voltage.

It’s easier to remove a surface mount resistor using two soldering irons, but if you only have one, you can blob a bunch of solder on top of the resistor to wet both sides, and then flick it off with the iron. Be careful to get rid of any splattered solder since it could cause some dangerous short circuits.

You can test it before putting the case back on by plugging it into a power strip that has a circuit breaker and a switch–turn it off first! Connect a multimeter to the outputs with alligator clips, and use one hand to switch on the power and check the voltage. If you want to be a little extra safe you could plug the power strip into a GFI outlet. The safest approach (especially if you want to take measurements on the AC line side of the isolation barrier) is to connect the whole rig to an isolation transformer. Once you’ve confirmed that it works, put the case back together and mark the label with the new output voltage. If the output voltage is higher, you will need to decrease the current rating proportionally. If you’re decreasing the output voltage, you shouldn’t increase the current rating since internal components may have a pretty low maximum current capability.

If you want more information about the TL431 (which is a really neat little device that could be useful for lots of other circuits), check out this application note: Designing with the TL431.

IEE Clock

Clocks, Projects 7 Comments

Update: more photos and information in the followup post.

Wow, things have been very busy lately–I moved to a new apartment, traveled to China, attended my brother’s wedding, and still had time to finish another clock!

IEE Clock - Front View

This one actually does have filaments. There are 12 light bulbs per digit; one for each numeral, and two for decimal points (left and right). These are neat little display modules that were made by a company called IEE (Industrial Electronic Engineers) way back before 7-segment LEDs were invented. Each light bulb sits behind a clear printed slide with the corresponding number printed on it in negative (the number is clear and the background is black), and in front of the slide is an array of tiny lenses. When a light bulb turns on, it projects an image of the number through the lens and onto the back of the lightly frosted plastic at the front of the display module. The Vintage Technology Association has a great exploded view so you can get a better idea of how this works.

IEE Clock - Front Panel

The clock itself is fairly pedestrian although this design uses a quadrature encoder (the black knob on the upper right) to set the time. Instead of pushing on a button and waiting while the numbers slowly tick by, it’s much easier to just spin a knob.

Keeping time is the function of a DS3231 IC. The display is not multiplexed. A multiplexed display would involve a lot of diodes which would dissipate quite a bit of heat, and for this design, I use 6 ULN2003 driver ICs connected to 2 MAX7300 GPIO expanders. Technically I used devices that are pin compatible with the ULN2003 since nobody seemed to have any in stock. The microprocessor is a PIC18F2420 which communicates with the DS3231 and both MAX7300 devices using I2C.

Here is the back of the clock. You can clearly see the vents which allow the heat from the light bulbs to dissipate.

IEE Clock - Rear Panel

This clock will be at Maker Faire, so if you plan to attend, feel free to stop by my booth and take a look in person.

Yes, Still Alive

Projects No Comments

OK, it’s been a while since I’ve posted–a long summer involving friends, BBQs, weddings, hiking, and other social activities. Now that the “cold dark California winter” is setting in, I’m spending more time on projects. Here’s a peek at the latest one.

My messy workbench:
What's This? - Part 2

What’s this? It looks like the beginning of a wiring harness.
What's This? - Part 1

Can anyone guess what this is?

2-Axis Joystick From VCR Parts

Projects 7 Comments

One of my CRT clocks has a small joystick on the front which is used to set the time. I built it a while ago using an idler wheel from an old VCR.

Second Scope Clock - Front Panel

Well, the other day I found another idler wheel from the same VCR, and I decided to share the construction technique so that you can make one too. It’s pretty simple and takes an hour or two. You will need a VCR idler wheel that looks like the one in the picture below, four microswitches (you can scavenge these from an old computer mouse), a spring, and a piece of sheet steel to mount the whole arrangement on. The sheet metal must be steel for reasons I will discuss later.

Joystick - Parts

First, take apart the idler wheel by gripping the top washer with a pair of vise-grips, taking care not too dent it too much. Grab the metal shaft with another pair of pliers and pull the assembly apart. Slide off the screw thread and the metal washer, and take the top washer (which has a convex side and a flat side), and slide it back on to the metal shaft about halfway.

Joystick - Parts Ready to Go

Drill a hole in the sheet metal, and make the hole somewhat larger than the shaft. For my joystick I used a 3/32″ diameter drill. Insert the metal shaft into the hole so that the convex part of the washer faces down against the sheet metal. Turn the piece of sheet metal over and drop a spring over the shaft, then press the remaining metal washer onto the shaft so it captures the spring and tensions it slightly. If you’ve done things correctly, when you push the shaft to the side, it will spring back to the middle. You may need to adjust the spring tension if the shaft doesn’t return all the way. If the shaft can’t move very much in any direction, you’ll need to enlarge the hole.

Joystick - Spring Return Mechanism

In the previous picture, you can see how the convex side of the washer allows the joystick to “roll” against the surface of the sheet metal. When you let go of the shaft, the spring tension pushes the shaft back to the center. The joystick action puts a lot of wear on the sheet metal which is why it’s important to use steel instead of a softer metal like brass or aluminum.

Now it’s time to begin mounting the switches to the sheet metal. You may want to lay them out by hand first just to make sure there is room for them all, and that the actuators end up in the right position. Skip ahead a few photos to see how I arranged my switches.

Joystick - Switch Positioning

Position the switch so that the washer on the reverse side of the sheet metal will hit the actuator just right. You don’t want the washer pushing in too hard–the hole in the sheet metal should stop the shaft from moving to the side rather than the actuator in the switch itself. At this point you may need to adjust the bottom washer so that it lines up with the switch actuator. In my joystick, I had to use a different spring to get the correct tension when the bottom washer was lined up correctly.

When you are finished lining it up, mark the sheet metal where the hole should go (I just dropped a drill bit in the switch’s mounting hole and twirled it between my thumb and forefinger, holding the switch down with my other hand). After you drill the first hole, put a small machine screw into the hole and secure it with a nut on the other side. You can rotate the switch to fine tune its position so the washer hits it in the right place. When it’s dialed in, mark and drill the next hole.

Joystick - One Switch Mounted

In this photo most of the switches are mounted, and you can see that I’ve drilled one mounting hole for the last switch.

Joystick - 3/4 Done

Once the last switch is mounted, take a look at the following photo and make sure that your washer hits every switch actuator correctly.

Joystick - Pushing One Button

At this point I drilled some mounting holes in the corners and took the whole thing apart, and cut the mounting plate out of the larger piece of sheet steel. It’s easier to work with a larger piece of sheet metal when drilling holes and such rather than a tiny piece that is harder to clamp. After putting everything back together, the joystick now looks like this:

Joystick - Completed

The joystick is finished, and it’s ready to be mounted in a front panel. For this joystick, I’ll probably mount it behind a 1/4″ thick piece of oak, so I will drill a big round hole for the joystick shaft and then use wood screws to mount the metal plate to the back of the wood. To dress up the front, I could use a piece of brass pierced with a plus-sign-shaped cutout to cover the hole in the wood like I did on my CRT clock in the first picture. As a finishing touch, I will mount a small wooden knob or perhaps a brass bead on the end of the joystick shaft.

Neon Lamp Leakage Current

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For another project in progress, I needed to test the “off” leakage current of neon bulbs. Along the way I discovered some interesting things. First, let’s take a look at the test subjects.

Neon Lamps Used for Leakage Tests

The lamp on the left side is a brand new miniature-type, the middle lamp pretty much represents the average neon lamp, and the lamp on the right is a special “frosted” lamp that I pulled out of some old equipment. Sorry, I do not have type numbers for these lamps, and they are not marked. Some older neon lamps were marked with the type number when the seal was crimped.

The testing used a power supply adjustable from 0V to 40V and a Fluke 87 multimeter. Any good multimeter with a 10MΩ input impedance can be used to measure extremely low currents by wiring the meter in series (like an ammeter) while it is in volts mode. Tests of the tiny neon lamp and the “average” neon lamp used the meter in mV mode, while tests of the frosted lamp used the volts mode. The tests were conducted at room temperature at normal indoor lighting conditions. Before testing, each lamp was washed with 90% isopropyl alcohol and dried with canned air.

Neon Lamp Leakage Current

There are some very interesting observations to be made with this data. The tiny lamp exhibits very low leakage current, peaking at 200pA. The “average” neon lamp peaks at 2nA, and the frosted lamp peaks at 263nA. I tried a few more lamps of each type and although they vary quite a bit, each type of lamp approximates the same current as the data above. It is due to the construction of the lamps themselves. According to Techlib.com, the increased leakage current is due to the radioactive thorium present in the lamp electrodes.

If it’s possible for radiation to increase the leakage current, I surmised that a strong light source could increase it as well. Taking another “average” neon lamp, I measured a leakage of 2.8nA at 40V. When exposed to a very bright white LED flashlight, the leakage current increased to 11.0nA, and when exposed to an ultraviolet LED flashlight, the leakage current climbed to 16.6nA. It may be that the photons impinging on the electrodes cause electrons to “leap” from the outer electron shell of the metal atoms and drift (due to the electric field between the two electrodes) across to the opposing electrode. This shows up as the increased drift current.

By increasing the temperature of a neon bulb, I was able to increase the leakage current as well. At the same time, I discovered that some small amount of water vapor still remained as a film on the surface of the bulb. For the case of the tiny neon bulb, the leakage current of 200pA fell to 50pA after a few seconds of applied heat, invalidating my previous experiments. As the temperature of the bulb increased, the leakage current increased to 1100pA before I removed the heat. This principle is the same one that enables vacuum tubes to operate–the heated cathode generates a space charge “cloud” of electrons that drift depending on the applied potential.

For my application, I would rather not have any thorium in the lamp since I want the leakage current to be as low as possible.

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