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Project Notes

#256 Pease

A Bob Pease tribute, LM331 voltage-to-frequency kit from The Boldport Club (Project #1).

Here’s a quick demo video to show it works..



The Pease is a tribute to the legendary analogue designer Bob Pease.

‘My favorite programming language is… solder’ –Bob Pease

It is another beautiful PCB from the Bolport Club, and is a great excuse to find out more about the featured chip - the LM331 voltage to frequency converter.

I joined the Boldport Club rather late, and missed out on the original Pease kit (now out of stock). But I did manage to snaffle the PCB in sale of “Just Less the Perfect” boards. Sweet!


About the LM331

Although the LM331 is used here in its basic voltage-to-frequency converter mode, it is actually quite a bit more versatile, with applications in:

  • Voltage to Frequency Conversions
  • Frequency to Voltage Conversions
  • Remote-Sensor Monitoring
  • Tachometers

See the LM331 Datasheet for more.

Kit Modifications

Since I was kit-less, I decided to experiment a bit. Firstly, I’m using a 20kΩ LDR for input rather than a phototransistor. Secondly, after testing the original circuit on a breadboard, I decided to slow down the frequency.

  • the original runs at hundreds of kHz, so acts like a PWM LED dimmer
  • I pulled the operating frequency down to 4-10 Hz, so it visibly blinks

The end result is a light/voltage controlled “blinky”. Depending on the potentiometer trim, it will:

  • stay “off” in bright light
  • start to blink at ~10Hz with a very low duty cycle (~1%) as light levels drop
  • blink hard at <7Hz with high duty cycle (50% and over) in dark conditions

The specific component changes (with reference to the schematic below):

Ref Original Replacement Rationale
R6 330Ω 220Ω a brighter LED
R1 6.81kΩ 1MΩ massively reduce base frequency
C1 330pF 100nF massively reduce base frequency
C2 1nF 1µF achieve greater duty cycles at the reduced frequency
S1 Vishay BPW96C Phototransistor 20kΩ LDR ..because I didn’t have a phototransistor on-hand

The particular LDR used has a range of about 200Ω (bright light) to 20kΩ (dark).

The component selection means the board works happily from 5-9V. I repurposed an old mouse USB lead as a 5V USB power supply connector for the board.

The Build

No unboxing this time - a pity, as the original kit packaging looks really neat.

Testing the circuit on a breadboard to experiment with component values..


Final component selection..


Build complete, front:


Finished, around the back…

  • hmm, still trying to get perfectly round globules of solder.
  • Hot glue to the rescue for securing the USB power connector. Is that allowed on a Bolport Club kit???!



Obviously, ambient light conditions and the trim of the pot alter the circuit performance, but here are some scope traces for roughly “light” and “dark” situations.

Bright Light - Original R/C components

High frequency 175kHz; duty cycle ~40%


In Darkness - Original R/C components

Frequency reduced to ~ 30kHz, and duty cycle over 80%..


Bright Light - Replacement R/C components

Low frequency 9 Hz, and low duty cycle <5%


In Darkness - Replacement R/C components

Frequency reduced to ~4 Hz, and duty cycle over 50%..






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.