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Project Notes

#385 widlar

Hacking the BoldportClub Widlar (Project #21) to get the classic μA723 to do something other than voltage regulation (LED blinky).


Here’s a quick demo..



BoldportClub Widlar project is really all about two things:

  • Bod Widlar’s classic μA723 voltage regulator, in SMD packaging
  • a beautiful circuit board that breaks out the chip to allow building any of the standard circuits supported by the μA723 “voltage regulator toolkit”

The μA723 comprises a number of functional sub-systems:

  • internal voltage reference, 7.15V (typical)
  • voltage reference amplifier
  • error amplifier
  • current limiter
  • output control transistor
  • output zener diode

With a few external components, these allow the μA723 to be tailored for specific voltage regulation goals. I’ve experimented with some of the basic 723 circuits previously:

Although not recommended or particularly useful for modern designs, it is widely discussed and documented, so makes an excellent learning tool.

As it came out in the same year I was born (1967), I also find it a personally interesting benchmark from which to judge how much things have changed, … and how they haven’t;-)

The Kit

Another wonderfully packaged kit from Boldport..

widlar_kit widlar_pcb_front widlar_pcb_rear


Qty Item
x1 μA723, TI UA723CDR
x1 NPN BJT TO-220, Fairchild BD239BTU
x5 470Ω resistor 250mW, Multicomp MCF 0.25W 470
x5 4.3KΩ resistor 500mW, RS PRO739-8809
x5 1.5KΩ resistor 500mW, RS PRO739-8767
x3 10KΩ resistor 250mW, Multicomp MCF 0.25W 10K
x1 10Ω resistor 600mW, Multicomp MCMF006FF100JA50
x1 3Ω resistor 600mW, Vishay MBB02070C3008FCT00
x1 10KΩ trimmer, Bourns 3306W-1-103
x2 100pF ceramic capacitor, Suntan TS15001H101KSBSA0R
x2 470pF ceramic capacitor, Suntan TS15001H471KSBPA0R
x1 PCB

Going Off Road..

This kit came packed with components for building a wide range of possible μA723 configurations.

But since I’ve already looked at the basic μA723 circuits, and I don’t really need another regulator, the attraction of course it to try and do something different (even though the 723 is generally though to “not have applications beyond the relatively narrow field of voltage regulation”).

But I spy an error amplifier, so that means it should be possible to get the 723 to behave like some form of oscillator.

Using the 723 voltage reference as a virtual ground, I configured an opamp oscillator circuit around the error amplifier, disabled the current limiter, and got a nice effect for “blinking” and LED at about 2Hz.

NB: Since I’ve left a DC offset on the oscillation, the LED blink is more a “glow”, as it never goes fully off.

I used a breadboard build to experiment and confirm component values:


Without load, I’m getting a square-ish wave with low duty cycle, and Vref providing a DC offset:


With the LED load attached, the waveform is more complex:







Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

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

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - 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
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.