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!
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.
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.
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.
High frequency 175kHz; duty cycle ~40%
Frequency reduced to ~ 30kHz, and duty cycle over 80%..
Low frequency 9 Hz, and low duty cycle <5%
Frequency reduced to ~4 Hz, and duty cycle over 50%..
- Pease - in the Boldport shop
- Pease - project page
- Pease - OSH files on GitHub
- Pease - club community site, packed with resources for the project
- LM331 Datasheet
- "Just Less the Perfect" - where you can still obtain the Pease board
- Bob Pease - wikipedia
- ..as mentioned on my blog