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:
|R3, R7, R10||3||13k 1%|
|R3’, R5, R6, R8, R9, R12||6||1k 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)|
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.