Branch: master
Find file History
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.

#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..


▶️ return to the LEAP Catalog


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