Geiger counter with TI LaunchPad MSP-EXP430G2 (MSP430G2553)

Hi all,

I present you my Geiger counter project, now that it is done thanks to your help. (I use this lib and I wonder why it's not included in Energia)

I could reach over 350000 CPM (Counts Per Minute) with a rather hot Americium-241 source and the very alpha sensitive SBT-11A pancake probe (I checked, it's not light sensitive) (the displayed µSv value for that is not correct because not calibrated for Am-241), and in a theoretical test over 1.5mio CPM, so it is good to know the chip is fast enough, but I'm not sure what limits it (don't have oscilloscope).

There's a limit how many CPM can come out from a tube, called "dead time". For this pancake probe, which has a dead time of 25µs, it's 1s/0.0000025s = 40000 impulses per second = 2400000CPM, which is much higher than for a SBM-20 probe: 1s/0,000190µs*60 = 315789CPM. 2.4mio sounds a lot however and I'm not sure if it still could be reached, other limit factors aside.

Formula: (xCPM/60)/((xCPM/60)/(1-((xCPM/60)*0.000190))) = factor on how many impulses are registered.

Let's say we have 150000CPM:

for SBM-20: (150000/60)/((150000/60)/(1-((150000/60)*0,000190))) = 0,525 -> only 52,5% are registered.

for SBT-11A: (150000/60)/((150000/60)/(1-((150000/60)*0,000025))) = 0,9375 -> 93,75% are registered, so this is much better.

The last step is to put the chip on a breadboard and in a case.

#include "LCD_5110.h"

LCD_5110 myScreen(P2_4,    // Chip Select *  #3 on sparkfun breakout SCE
                  P2_3,    // Serial Clock * #7 on sparkfun breakout SCLK
                  P2_2,    // Serial Data *  #6 on sparkfun breakout DN(MOSI)
                  P2_1,    // Data/Command * #5 on sparkfun breakout D/C
                  P2_0,    // Reset *        #4 on sparkfun breakout RST
                  P2_5,    // Backlight      #8 on sparkfun breakout LED
                  PUSH2);  // Push Button 2  

#define LOG_PERIOD 10000
#define MAX_PERIOD 60000

long unsigned counts, cpm, cumulative;
unsigned int multiplier;
unsigned long previousMillis;

char string[10];

void count(){ counts++; cumulative++; }

long unsigned maxCounts=0;
float usv=0.0;
void setup() {
  //=== LCD ===
  myScreen.begin();
  myScreen.setFont(1);
  myScreen.text(0,0,"DeepBlueSky");
  myScreen.text(0,2,"v1.0");
  //=== geiger counter ===
  counts = 0;
  cpm = 0;
  multiplier = MAX_PERIOD / LOG_PERIOD;
  attachInterrupt(P1_2, count, FALLING);
}
char uSv[12];
int unsigned times; //this doesn't need to be long unsigned (in theoretical test it goes up to ~ 9000 [µSv/h] (= 9mSv/h))
void loop() {
  unsigned long currentMillis = millis();
  if(currentMillis - previousMillis > LOG_PERIOD){
    previousMillis = currentMillis;
    cpm = counts * multiplier;

    if(cpm > maxCounts)
      maxCounts = cpm;
 
    //CPM
    sprintf(string,"%lu",cpm);
    myScreen.text(0,0,string);
 
    //CPM to µSv/h
    unsigned long long count57 = 57 * cpm; // factor 0.0057 für SBM-20 tube
    sprintf(string,"%u", (unsigned) (count57 % 10000));
    times=cpm/176; //µSv number before . //this mysterious 176 comes out if you do 1/0.0057, and so when you divide cpm/176 you get the number before the dot, e.g. 1000CPM/176=5 (no rest)
    sprintf(uSv,"%d", times); //start concatenating the number -- add number before . (dot)
    strcat(uSv,"."); //add .
    strcat(uSv,string); //add number after .
    myScreen.text(0,2,uSv);
    memset(uSv,0,sizeof(uSv)); //clear µSv array
    
    //maximum CPM
    sprintf(string,"%lu",maxCounts);
    myScreen.text(0,4,string);
    
    //cumulative -- dosismeter //with setFont(1) we can only fit three rows on the LCD, use setFont(0) if you want
    //sprintf(string,"%lu", cumulative);
    //myScreen.text(0,4,string);
    
    counts = 0;
  }
  delay(LOG_PERIOD);
  myScreen.clear();
}