Category Archives: EMSL Projects

XL741: Principles of Operation


Our two “dis-integrated circuit” kits are the Three Fives Discrete 555 Timer, and the XL741 Discrete Op-Amp. These two kits are functional, transistor-level replicas of the original NE555 and μA741 (respectively), which are two of the most popular integrated circuits of all time.

Last year, we wrote up a detailed educational supplement for the Three Fives kit, that works through its circuit diagram and discusses its principles of operation down to the transistor level. Today, we are doing the same for the XL741 kit, and releasing an educational supplement that explains how a ‘741 op-amp IC works internally, down to its bare transistors and resistors:

XL741 Documentation (PDF)

This ability to peek inside the circuit makes the XL741 a unique educational tool. In what follows, we’ll work through the circuit diagram, discuss the theory of operation of the ‘741 op-amp, and present some opportunities for experiments and further exploration.

You can download the supplement here: XL741 Principles of Operation (1.1 MB PDF)

Additional Resources:




Lemon Plum Jam


The plums on our tree ripened all at once this year! Making this sweet and tart Lemon Plum Jam took care of some of the excess fruit in a tasty way.


  • 4 cups plum pieces (pits removed, skin still on)
  • 2 whole lemons—large meyers if you can get them—cut into pieces (seeds removed, peel still on)
  • Juice of 2 more lemons
  • 1/2 cup water
  • 4 cups sugar

Heat the plum and lemon pieces, lemon juice and water in a pot on medium, stirring occasionally. After about 15-20 minutes, the fruit should be softening. Macerate the fruit in the pot—a potato masher works well for this. Add the sugar. Stir regularly and cook to the desired consistency. To test consistency, put a spoonful on a plate in the fridge. If it’s too runny after cooling for a few minutes, keep simmering and test again after a few minutes.

Makes about 2-3 pints.


If you want to can it for longer storage, Ball has a nice introduction to canning (pdf), and additional resources on their website.

Other fruit preserves from the Play with your food archives:

WaterColorBot and BeetleBlocks

The Tinkering Studio posted on twitter:

BeetleBlocks is a system for enabling people to explore algorithmic 3D geometry by snapping together Scratch-like programming blocks.

BeetleBlocks block programming example

They posted a picture of the finished painting, which looks awesome.

Painted output in the WaterColorBot

Politicians To Poop: A Chrome extension

Politicians To Poop

Politicians To Poop is a new extension for the Chrome web browser that replaces the names of presidential candidates (US, 2016) with the “pile-of-poo” emoji. Options allow you to “poopify” the names of Democrats, Republicans, or both.

Politicians To Poop is available now, for free, at the Chrome web store.


Politicians To Poop

Politicians To Poop

Politicians To Poop

Possible reasons that you might want to use this extension include:

  • You are from outside the US, and don’t need to hear these names every day.
  • You are temporarily overloaded by the amount of poop that the candidates sling at one another.
  • Because it is funny.

No judgement upon any of the named individuals, nor their platforms, parties, or beliefs is either implied or intended. This is intended to be an equal-opportunity text replacement tool, for the good of all humanity.

More information:
This project was inspired by Millennials to Snake People. Much more information including source code, the list of names, and additional attribution is available at our GitHub repository.

Retro-tech Style in the New York Times

Component Wine Charms

I’m excited to be included in an article on retro tech style in the New York Times today, titled Nintendo 64s and Vintage PlayStations as Home Décor.

Not surprisingly, these techie hobbyists share their passion in online communities. One of the more popular forums is a D.I.Y. tech blog run by Evil Mad Scientist Laboratories, a family company in Sunnyvale, Calif., that produces open-source hardware. The site features tutorials on making earrings out of linear regulator chips, wine charms from capacitors and a wooden footstool in the shape of a classic 555 integrated circuit chip from the ’70s.

A Diamagnetic Demonstration


Our friends stopped by with a simple apparatus to demonstrate the diamagnetic properties of bismuth metal. Diamagnetism is a extremely weak magnetic effect — generally orders of magnitude weaker than everyday permanent magnets, which exhibit ferromagnetism. However it is also an extremely interesting effect because diamagnetic materials are repelled by magnetic fields. This is different than the case with ferromagnets, where one pole of a magnet repels another — rather, the entire material is (weakly) repelled by any magnetic pole.

Now, how might one observe such a weak effect? One way is to build a magnetic levitation rig, but the field configurations there are a little less obvious. With a simple but sensitive balance, we can see the repulsion directly. The balance above has a long wooden beam, a central pivot on two blocks of plastic, and a couple of coins on the far end for counterbalance.


At the business end of the scale, there is a cylinder of bismuth metal about 1 cm in diameter, held in place by a rubber band. We also have a larger rectangular block, which is our test magnet, made of grade N50 NdFeB and painted black. And finally, the Lego Astronaut Twins are here helping out as a scale and position reference.

Moving the block magnet beneath the bismuth, we can see what happens in an animated GIF:

After the balance settles, the resting position of the end with the bismuth is considerably higher. With some calibration in terms of weights and/or positions, one could even measure the exerted force with some precision.

A slight improvement to this apparatus would be to reverse the roles of the bismuth and the block magnet. That is, to affix the magnet to the arm of the balance, and to slide the bismuth beneath it instead. You could then use a nearby block of aluminum to damp the motion of the beam through magnetic (eddy current) damping. Many commercial balance-beam type scales already use magnetic damping so that they settle down to their final values faster.


Hard Drive Earrings

Wearing my new earrings

The newest in my collection of geeky jewelry: glass hard drive platter earrings.

tiny platters

We picked up a tray of tiny glass platters at a local surplus shop some time ago, marked “Tear Down Qty: 25 pcs.”

CF card for comparison

These one inch platters were used in Microdrives, which were designed to fit into CompactFlash card slots. (Shown with CompactFlash card above for scale.) They were used in (among other things) the iPod mini. After 2006, CF cards outpaced the capacity of the fragile Microdrives, and they’re no longer manufactured.


The platters are attached to the earring hooks with magnet wire. They’re incredibly reflective, and everywhere I wear them, spots of light are dancing around me.

Related: Hard Drive Wind Chimes