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Project Notes

#447 Capacimeter

Measuring capacitance with old-school 555 and 4017 digital logic - a Boldport Club remix of a PEAK, Project #31, December 2018.


Here’s a quick demo..



Capaci-meter is a Boldport remix of a classic capacitance measurement circuit designed in high school by Jez Siddons of Peak Electronic Design.

The device takes a very interesting approach - rather than trying to measure the actual capacitance or charge time, it uses a derivitive function (the capacitor’s effect on the frequency of a 555 timer) to infer capacitance.

How it Works

There’s a full write-up in the User Guide.

It is quite ingenious; here’s my paraphrasing…

Basic operation boils down to four points:

  • the capacitor under test alters the frequency a “Cx-dependent” 555 astable multi-vibrator
  • this gates the output of a second, Cx-independant 555 astable multi-vibrator
  • CD4017 decade counters are used to count the number of pulses let through the gate
  • the 4017s drive the LED display, with 10 LEDs arrange clock-face style. Two 4017s make for two-digit resolution

Finer details to note:

  • the “range selector” is changing the R2 value (thus frequency) of the Cx-independant 555
  • the Cx-dependent 555 counter is in “measurement” when output high:
    • LEDs are disabled (by pulling LED catchodes high)
    • enables the Cx-independant 555 counter
    • triggers 4017 reset at the start of the pulse
  • the Cx-dependent 555 counter is in “display” when output low:
    • LEDs are enabled (by pulling LED catchodes low)
    • disables the Cx-independant 555 counter

The schematic is from the Boldport design sources on Github:




Reference Qty Description
R1 1 1.5M 1%
R2 1 150k 1%
R3, R7, R10 3 13k 1%
R3’, R5, R6, R8, R9, R12 6 1k 1%
R4 1 560R 1%
R11 1 130k 1%
R1’, R2’, R4’ 3 Not fitted (optional to adjust accuracy)
C1, C2 2 10uF, 10V or higher, 20%, electrolytic
C3, C4, C5 3 100nF, 16V or higher, 20%, ceramic, Y5V/X7R
C6 1 4.7nF, 1% or 5%, ceramic, COG/NPO
C7 1 47pF, 20%, ceramic, COG/NPO
C8 1 10nF, 20%, ceramic, X7R
D1, D2, D3, D4 4 1N4148 Signal Diode
U1, U2 2 CD4017BE Decade Counter (Use socket!)
U3 1 NE556 Dual Timer (Use socket!)
U1’, U2’ 2 16 pin DIL socket (7.62mm)
U3’ 1 14 pin DIL socket (7.62mm)
U4 1 78L05 5V 100mA Regulator
Q1 1 2N7000 N-Ch MOSFET
LED1-L, LED1-R 2 Low current or high brightness green LED, 5mm.
LED2-L to LED10-L, LED2-R to LED10-R 18 Low current or high brightness red LED, 5mm.
LK1, LK2, LK3, LK4 4 Header pins (2 pins per header) for range selection jumper
JP1 1 Jumper needed for header pins (only 1 jumper needed)

kit_parts kit_pcb_front kit_pcb_rear


Luckily the PCB was designed with standard 3.54mm spacing between jumpers, meaning a breadboard can be used to align pins for soldering:


Soldering complete, and initial test under power:



Custom Power switching and Base

I added a centre-positive barrel jack with as battery bypass and a small switch so that:

  • there’s power on/off control
  • and the alternative of providing external power instead of a battery

I also did a little wire-craft to make a custom base for the kit. It’s stuck to a bit of packing foam for stability and isolation.

capacimeter_build_360_a capacimeter_build_360_b capacimeter_build_360_c capacimeter_build_360_d


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.