Basics: Base Resistors on Transistors

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Our reader Jon wrote in with a question about our open collector tutorial:

I really appreciated the tutorial, and I was able to follow along and understand it very well. One question I had was – what is the purpose of the 1 kilo-ohm resistor that is connected to the base of the PNP transistor? Because when the open collector is ‘high’ then the base of the transistor is at 12 V and it appears the 1 kohm resistor didn’t affect anything, and then when the open collector goes ‘low’ then the base is connected to ground through the output of the SN7407. So basically, what would the difference be if there was no 1 kilo-ohm resistor at all?

And, that’s actually an excellent question, about something that we usually gloss over.

The short answer is that this is a “base resistor” that we use to limit the maximum current that flows through the base of the PNP transistor.  But, let’s take a look in a little more detail, and see what would happen if we didn’t have that there.

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Inside the ULN2003

Over at ZeptoBars, they have an incredibly detailed “take-apart” post on what’s inside the ULN2003 seven channel Darlington driver chip. The ULN2003 is commonly used for driving LED displays—you can find it, for example, in our Mignonette game.

We often receive comments that while out microchip photos are beautiful and interesting, it is completely unclear how integrated circuit implements basic elements and form larger circuit. Of course it is impossible to do a detailed review of an 1’000’000 transistor chip, so we’ve found simpler example: ULN2003 – array of Darlington transistors.

They’ve stripped off the outer housing and put it under the microscope. They then analyzed the photos to show you what parts make up the individual transistors, resistors and diodes inside the chip.

The Mug Marker

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Don McRae recently stopped by our shop last week to show off his homebrew CNC project, the “Mug Marker” — a wonderful little cardboard robot that can draw on mugs.

Much like the Mug Plotter on Instructables, it uses the same EBB controller board and stepper motors as the Eggbot, but with linear motion for the pen instead of rotation. However, unlike that version, Don has incorporated the same winch-drive mechanism that we use on the WaterColorBot to provide motion for the linear axis– meaning that it can go fast or slow, with very good accuracy.

Don laser cut the large white winch on the back of the machine from acrylic. It controls a string that pulls the pen carriage back and forth as it rides on a pair of rods:

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The pen holder itself slides into some additional bearings, and has a small protrusion on the back that rests on the servo horn, allowing it to be lifted up or lowered down.  Like the Eggbot’s pen-lift mechanism, this mechanism only (actively) lifts the pen, which means that it can ride over uneven surfaces, or plot on mugs with variable diameter.

 

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Underneath the pen carriage, the opening of the mug fits onto a three-jaw coupler on the motor shaft, and the base of the mug is held against a rubber faced spring loaded plunger. Small copper tubes are used as bushings to allow the coupler pieces to rotate inward or outward to allow mugs of differing diameters to fit on. A little silicone on the surface of each of the three parts provides a gripping surface, and the upcurved lip keeps the mug from sliding too far.

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Looking down through the pen carriage, you can see the mug below held in the coupler.

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The chassis of the machine is made from cardboard, either hand or laser cut to slot together, and held together very cleverly with pins. The whole machine is put together from a combination of off the shelf parts and found materials, many of which are laser cut for the correct shape. For software, Don uses the Eggbot Inkscape extensions with very reproducible results.

Thanks to Don for bringing it by and letting us take pictures!

Oddities at the Electronics Flea Market

eFlea July 2013

Today was the monthly Electronics Flea Market in Cupertino, and we came across some gems this month.

Above, an AN-OIL-IZER. The seller said her geologist father used it for testing oil purity.

It’s described in patent number 3182255, a device for capacitively testing lubricating oil (e.g., engine oil) for contaminants, by looking for changes in its dielectric constant. To use it, you place a drop of the oil in the holder, and the ball bearing into that drop of oil.  The bearing is held down by a leaf spring, keeping it indexed against the holder.  This forms an oil-filled capacitor between the ball bearing and a lower curved plate that is insulated from the bearing. The capacitance will vary as the dielectric constant of the oil changes due to contamination.  It comes with two ball bearings, as well as oil samples for calibration.

eFlea July 2013

The E-Z-Code Jr. is a tool for learning morse code: when you draw the “electric pencil” through the slots, it crosses contacts in the correct spacing to make the characters. It also has a hinged telegraph key which can be tucked away below the device.

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The seller of the E-Z-Code Jr. told me that the thing I really should be photographing was this magnetron. It is a beautiful old piece of hardware, with its wave guide and high-power tube.

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We found a book on Magnetic-Bubble Memory Technology. We also saw a book on tube delay memory.  We’re not sure if these are a step up from the single-bit flip-flop memory in our Digi-Comp II.

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I’d love to see the circuit diagram for the Cosmic Energy System by Psy Herabel [sic] Health Town, Inc.!  (Sadly, their domain no longer seems to be active.)

A round up of our “Basics” articles

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Soft Circuit Merit Badge01  g22023

Over the course of the past few years, we’ve been writing occasional “Basics” articles, about introductory topics in electronics and microcontrollers.  In the spirit of making things easy to find, we’ve now tagged them so that you can find them with this link, and we’re collecting them together in this index that will be updated from time to time.

Our “Basics” articles about electronics in general:

Additional “Basics” articles about working with AVR microcontrollers:

Field Trips: Atmel Headquarters

Sylvia

Super Awesome Sylvia and I were invited to attend Bring Your Kids to Work Day at Atmel recently. (Atmel, of course, is the company that makes the microcontrollers found inside Arduino products and in many of our own projects and kits.) We were there to help provide tangible, interesting, and playful examples of how Atmel chips can be used. And of course, we weren’t going to miss an opportunity to visit Atmel headquarters!

Photo courtesy of Atmel

The biggest hit with the kids were the Octolively interactive LED modules (sporting the Atmel ATmega164P). When the kids waved their hands over them, the LEDs would light up and ripple. Some of the kids would start out by poking and grabbing at the LEDs until they lit up, but as soon as I told them it would work “even without touching it” their eyes would get big, and they’d wave their hands over the top, enthralled.

Some of the other things we brought were our handheld game, the Meggy Jr RGB (with the ATmega328P); a Bulbdial Clock (Atmega328P again), which points rings of LEDs at different heights down at a central point to create shadow hands of different lengths; our giant Alpha Clock Five (ATMega644A); and the Larson Scanner (ATtiny2313A), which lights up nine red LEDs to make a scanning robot eye.

Photo courtesy of Atmel

Another project that captured the kids’ attention was a Keepon by BeatBots. Other demonstrations included a quadcopter and a hacked hexabot.

Photo courtesy of Atmel

We got to have lunch in the bright sun in the courtyard with Avary Kent, who was demonstrating the PuzzleBox, a brain-controlled helicopter.

Photo courtesy of Atmel

Sylvia got to give the PuzzleBox a try, triggering it to fly as soon as she concentrated hard enough.

Workbench

After lunch, we got to tour of a couple of labs. This workbench was well stocked with a Metcal soldering iron (our favorite) and lots of tools and supplies.

Workbench

Apparently the poor Pleo on this bench needed some repair.

Chip testing machine

This machine is for inspecting and testing chips after they have been removed from their housing.

We got to go into the RF anechoic chamber, and watch as our cell phones stopped receiving any signals.

Horsing around

We also had some time to hang out and horse around with friends new and old. Our friend Paul Rako seemed to be having as much fun as the kids.

Photo courtesy of Atmel

Thanks to Paul and Atmel for inviting us to visit!

How Printed Circuit Boards are Designed (1960 Edition)

We found this gem in A Manual of Engineering Drawing for Students and Draftsmen, 9th Ed., by French & Vierck,1960, p. 487.

Printed Circuits allow miniaturization and the elimination of circuit errors—advantages that cannot be obtained by other methods. Once a pattern or suitable design is established, preparation of a black and white drawing can start. Scales for reduction, for example, 4 to 1, 3 to 1, or 2 to 1, are used. To insure sufficient bonding area of the metal laminate during soldering operations, lines should not be less than 1/32 inch in width when reduced. Line separation should never be closer than 1/32 inch on the final circuit. Figure 19.24 illustrates the drawing of printed circuits.