#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..

Notes

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!

Pease_board_front

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..

build_breadboard

Final component selection..

build_components

Build complete, front:

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???!

build_complete_back

Performance

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.