The Lazy Man's Germanium Tester

[Kevin Mitchell]

Afterthought (more info on previous page) - I may have to double up some of the circuitry to avoid sending a negative polarity to the ADC due to user error. Or just be lazy and hope people read red labeled instructions 

[Kevin Mitchell]

Had a better idea. I've added another SPDT IC that is toggled by a comparator. So if the voltage from the buffer goes below 0v it trips the comparator which actuates the switch that determines the ADC's probe point - being pre-inverter or post-inverter. This means the measurement voltage will never go below 0v - depending on any offset characteristics of the chosen opamp. Even so, if the ADC does see below 0v it wouldn't be enough to damage the microcontroller.

https://tinyurl.com/yaa63t39


From here I can drop the PNP/NPN polarity switch altogether and just use another device-under-test socket to accommodate the different devices - keeping this as simple as I can without risking any damaging user errors which would be a massive design flaw.

[Ben N]

This means the measurement voltage will never go below 0v - depending on any offset characteristics of the chosen opamp. Even so, if the ADC does see below 0v it wouldn't be enough to damage the microcontroller.

Why not just stick a Schottky in there for protection?

[Kevin Mitchell]

Why not just stick a Schottky in there for protection?

While that would add extra protection I fear it'll effect the measurement voltage. Besides, I believe there's protection diodes already in these microcontrollers - albeit, not as beefy.

Do you not like the automated routine? 

[Ben N]

That's not it, I just focused on what I could ponder with half a brain in the middle of my workday. 

[Kevin Mitchell]

Messed with it a little more for this neat circuit. Have at it.
https://tinyurl.com/y7kd26vn

I'm going to purchase some dual LED displays that will change color depending on what polarity device is being tested. Failure to insert a device properly should result in no display action (all LEDS off). I will be keeping the option to use normal less expensive displays with the same pinout - minus one anode pin. I still need to breadboard to verify - just waiting on the displays to arrive.

Power supply will be 9VAC, rectified to ~12v and converted down via adjustable regulators so we have a stable +/-9VDC supply.

So far with where the design is I literally have everything in my parts drawers minus the displays. All common parts though a long way from the simplicity of the OG circuit.
BOM so far

I also want to explore changing some of the Attiny85's pin configuration so I can free up the AREF pin. 5V resolution is not needed so if I can lower it while keeping it within the proper range I could improve the ADC resolution to monitor smaller voltage changes.

[pinkjimiphoton]

I got lazy. And so can you!

I had breadboarded the germanium leakage and true gain tester from Geofex as I finally decided to order boxes of those recently popular new-old-stock Russian germanium transistors.
Because I absolutely hate math I had put together a spread sheet with the formulas ready to go! All I had to do was punch in the two measurements - open base voltage & closed base voltage.
Well apparently that was too much work for me and I got sick of it real quick. Numbers kept changing and results were slightly different each time I retyped the measurements

So the thought occurred to me early this afternoon, can I make an arduino do it? Turns out I can! And it's surprisingly accurate in comparison to the manual method.
Devices tested in this photo are MP39B - with noted gains between 20 & 60 according to our archives around the forum.


Before I draw up a schematic I will say that the parts in the photo are certainly overkill as the majority of both ICs & Arduino are unused. I had also kept some uneeded values in the code just in case I wanted to display them to manually to check the results against a DMM & my pre-formulated spreadsheet.

Here's my code for anyone interested. I'll try to put up a schematic tomorrow, but it's rather straight forward. The number .00488 is to convert the resolution of the 10-bit ADC @ 5 volts to the actual voltage (5/1024 =  ~0.00488).

Code Select Expand

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x3F, 16, 2);

int pulsePin = 55;
int adcPin = A0;

float voltageA;
float voltageB;
float voltageLeak;
float voltageGain;

void setup() {
  lcd.begin();
  lcd.backlight();
  pinMode(pulsePin, OUTPUT);
}

void loop() {
  //read open base
  digitalWrite(pulsePin, LOW);
  delay(50);
  voltageA = analogRead(adcPin)*.00488;
  //read closed base
  digitalWrite(pulsePin, HIGH);
  delay(50);
  voltageB = analogRead(adcPin)*.00488;
 
  //print result
  voltageGain = (voltageB-voltageA)*100;
  lcd.clear();
  lcd.print("TGain = ");
  lcd.print(voltageGain, 1);
  lcd.print(" hfe");

  voltageLeak = voltageA/2472*1000000;
  lcd.setCursor(0, 1);
  lcd.print("Leakage = ");
  lcd.print(voltageLeak, 0);
  lcd.print(" uA");
}

That's it for tonight 

dude.... DEWD!!! i think i love you!!! long time!!! outstanding!

[Kevin Mitchell]

Thanks for your kind words, Pinkjim! However a few months late to the party 

The OP is a great resource for folks new programming with arduino. I was a bit rusty when I wrote it up but since then I've gotten better at writing efficient code. The final stand-alone version will be much more gratifying