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Project Notes

#257 Emergency

Pimp the Boldport Club Emergency kit (Project #2) .. now a LED blinky with 3 passives and a transistor (Esaki Effect)

Here’s a quick video to show it working..

Build

Notes

I joined the Boldport Club rather late, but manage to snaffle up the Emergency kit PCB in sale of “Just Less the Perfect” boards.

The tiny ‘engineer superhero’ emergency kit, second edition was Boldport Club Project #2.

As originally intended, it is a kit with a few components and a nice travel case that will possibly save your engineering superhero status at some point!

With all the components soldered into the (of course beautiful) PCB, it works as a battery/continuity tester (the LED lights up if you apply voltage)

Emergency_board_front

Emergency_board_rear

Circuit Modifications

Since I’m working from scratch (PCB only - not the full kit), there’s an obvious challenge to do something a bit different.

@mehradzie asked on Boldport Slack if it is possible to make it blink. Challenge accepted!

I was actually thinking along similar lines, at first using a blinking “joule thief” circuit. But I just couldn’t figure how to get it to work within the constraints of the PCB.

So then I turned to an interesting idea: Negative Differential Resistance (NDR) effect or Esaki Effect. Basically causing an Emitter-Collector avalanche, a behaviour noted by Leo Esaki and exploited by many since to create interesting effects with BJTs.

After a bit of experimentation, I was able to get something to work with only a few liberties taken with the original PCB:

  • I had to accommodate larger capacitor. Anything over ~100µF is workable, but best effects achieved with 330µF electrolytic. I selected the smallest package I could find, but it is still a bit of a squeeze to fit it in where the capacitor should be.
  • an NPN BJT replaces the n-channel MOSFET. I’m using an S9013. Note: yes, this is in “backwards” (emitter to +ve, collector to -ve)
  • moved one resistor to make wiring easier
  • cut and rewired a few traces

Transistor selection is crucial, as the blinking effect is really exploiting a “defect” in the transistor. Of all the small-signal NPN BJTs I had on hand, only two worked at all, or at reasonable voltages (S9013, S9018). I went with the S9013 because I could excite the effect conveniently around 9V.

The revised circuit essentially works like this:

  • battery charges the capacitor
  • over a certain voltage, the emitter-collector junction experiences an avalanche and inrush of current from the capacitor
  • the LED turns on
  • as the capacitor discharges, the avalanche breaks down and the transistor junction closes
  • repeat…

Here is a scope capture from a breadboard build/test:

  • CH1 (yellow; AC-coupled): capacitor voltage
  • CH2 (blue; AC-coupled): collector (LED anode) voltage

scope

Construction

Breadboard

Schematic

Testing on a breadboard:

Emergency_breadboard

Final layout and component selection. Build complete:

Emergency_layout

Power on, and it’s blinking! Why? Because!

Build

Credits and References

About LEAP#257 BoldportPCB DesignOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.