Hardware » Cablemeter

h1. Eval

h1. Overall design

* 4 x red+green LEDs for condition

* Restart button

* Iterate over the ADC channels and get Voltage level

* Self-Calibrating via MOSFET (shortcuit cable interface)

* draw 100mA. Why? Sonst habe ich keinen Spannungsteiler. Je mehr Strom desto hoeher der Spannungsabfall; optional via mosfet.; 3/0,1= 30 Ohm

* ADC loesst theoretisch auf "3 / 4096 = approx. 0,0007" Volt auf.

* Die Kabel haben AWG28, 212.9 Ohm/km, 1.4 A nach erde,	0.23 A versorgung

* 0,2129 Ohm/m; 1,0645 Ohm/5m; 2,129 Ohm/10m;

* https://de.wikipedia.org/wiki/E-Reihe#/media/Datei:E12_values_graph.svg

* Buzzer would be nice for indication; durchklingeln. Da werde ich lieber Multimeter nehmen. Ist aber kein Aufwand.

* PWM Output

* Pinsocket connected to wires to measure resistance/Voltage directly

* https://www.aeq-web.com/spannungsteiler-microcontroller-berechnen-und-dimensionieren/

* Spannungen einzeln schaltbar damit auch kurzschluesse detektiert werden koennen

h1. Schema

       3V

        |

       | |  Cable

        |-------------->ADC

       | | 30Ohm

        |

       ===

h1. Theoretische Spannungen bei AWG28

theoretically an device that draws 0,1A, 5V and 5m AWG28 cable:

U2=((5*50)/ (50+1,0645) ) = 4,8957691; that should work

theoretically an device that draws 0,5A, 5V and 10m AWG28 cable:

U2 = ( (5*10) / (10+2,129) ) = 4,1223514 V

theoretically an device that draws 0,1A, 3V and 5m AWG28 cable:

U2=((3*30)/ (30+1,0645) ) = 2,8971978; that should work

theoretically an device that draws 0,1A, 3V and 10m AWG28 cable:

U2=((3*30)/ (30+2,129) ) = 2,8012076

theoretically an device that draws 0,25A, 3V and 10m AWG28 cable:

U2= (3 * 12) / (12 + 2,129) = approx. 2,547951

theoretically an device that draws 0,5A, 3V and 10m AWG28 cable:

U2= (3 * 6) / (6 + 2,129) = approx. 2,2142945

U2= (3 * 6,8) / (6,8 + 2,129) = approx. 2,2846903

h1. Calculations I

U2=((U*R2)/R_GES)

U

R*I

5V/0,1A= 50 Ohm

5V*0,1A= 0,5 W

5V/0,5A= 10 Ohm

RGes = R1+R2 = 50

3V=(5*R2)/50

*R2 = (3*50)/5 = 30 -> 27

R1 = 22*

RGES = 49

U2 = 5*27/49 = 2,755102

h1. Calculations II

* Ein Kabel >= 10m soll durchfallen. Koennen trotzdem groessere Wiederstaende verwendet werden?

Ohne Widerstand:

U2 = 2,755102

Obige werte 1fach, 10m Kabel:

R2 = 27

R1 = 22

mit 10m 

U2 = (5*27)/(49+2,129) = 2,6403802

2,755102-2,6403802=0,1147218

0,1147218/0,00073242188 = 156,6335

Obige werte 10fach, 10m Kabel:

R2 = 270

R1 = 220

mit 10m 

U2 = (5*270)/(490+2,129) = 2,7431832

Bei 12Bit ADC: 3 / 4096 = approx. 0,00073242188 V pro ADC-Wert.

2,755102-2,7431832=0,0119188

0,0119188 / 0,00073242188 = approx. 16,273135. Thats not super much

h1. Calculations III

h1. Keyfeatures

* Cable checker

* Buzzer Durchgangspruefer

* PWM Output

h1. MCU

* "stm32f051c4":https://www.mouser.de/datasheet/2/389/stm32f051c4-1851079.pdf

** I have 17; 

** 16 ADC channels

* STM32G030C8T6

** nearly same specs but smaller footprint; LQFP 48

** 2,68€ inc. Mwst.

** ADC faster

** Mentions an 16Bit ADC value via oversampling, but thats complicated: adding white noise in order to calculate further 2 bits by software. Every MCU can do that.