Extreme makeover for the Nucleus Model 500 Geiger Counter by Mark McLaughlin

Transcription

1 Extreme makeover for the Nucleus Model 500 Geiger Counter by Mark McLaughlin Figure 1: The Nucleus Model 500 Geiger Counter with its makeover. The Geiger counters used in the introductory Physics labs at CSULB are old, but they are durable. The problem with most of these counters is that the switches are failing, and replacement switches can no longer be ordered. (However, some of the counters have more serious problems.) I have devised a way to restore these counters at a cost of about $70 per counter. The old switches have been replaced with simple push buttons. With any luck, the geiger counters should last another 30 years! The schematic for the original geiger counters is shown in figure 4. I replaced many of the logic circuits in this schematic with an AT89C bit microcontroller. The updated geiger counter consists of 5 blocks: Geiger tube and circuitry to generate pulses. This has remained unchanged. Circuits to count these pulses (when given permission to count). I call this block, Pulse Counter. LEDs to display the how many pulses have been counted. Two D-Flip Flops which control when pulses may be counted. I call this block, Traffic Cop. An AT89C2051 microcontroller. I call this block, Timer. When Traffic Cop gives permission for Pulse Counter to count, Timer keeps track of how long 1

2 Figure 2: The original Nucleus Model 500. There is a huge nest of wires underneath. Figure 3: The new buttons are much easier to maintain. 2

3 Pulse Counter has been counting. The duration of how long pulse counting is allowed is specified by a 1 pole, 6 position switch. Once this time is up, Timer informs Traffic Cop, and Pulse Counter is not allowed to count anymore. These blocks are shown in figure 6. Figure 7 explains how the Traffic Cop works. And figures 9 and 10 explain the strategy behind the assembly code that I wrote for the microcontroller. The actual assembly code is in figures The schematics for the updated geiger counters are in figures The printed circuit board layout is in figure 18. Also, sometimes the old geiger counters need replacement 5 Volt supplies. A circuit for this is shown in figure 20. Finally, the list of materials is in figure 21. 3

4 Figure 4: The original schematic for the Nucleus Model 500 Geiger Counter. 4

5 Figure 5: Outline of the blocks that make the updated Geiger Counter. 5

6 Figure 6: Circuitry to generate pulses, and the 5 Volt power supply. For the Geiger counters that need to have the insides totally replaced, I used a different 5 Volt power supply as shown in figure 20. Also, the NAND gates can be moved to the new printed circuit board for the counters that need the insides totally replaced. 6

7 Figure 7: The two D Flip Flops act like a traffic cop, coordinating when to enable or disable the counters. 7

8 Figure 8: An explanation of how a D Flip Flop operates. This is copied from the CMOS cookbook. 8

9 Figure 9: The inputs to the microcontroller. 9

10 Figure 10: Some of the strategies behind the assembly code for the microcontroller. 10

11 Figure 11: Assembly Code page 1 of 4 11

12 Figure 12: Assembly Code page 2 of 4 12

13 Figure 13: Assembly Code page 3 of 4 13

14 Figure 14: Assembly Code page 4 of 4 14

15 Figure 15: The block that I call, Pulse Counter, along with LEDs. 15

16 Figure 16: Traffic Cop, the two D-Flip Flops, and Timer, the microcontroller. 16

17 Figure 17: The NAND gates may be placed on the new printed circuit board if the old geiger counters need the insides completely replaced. 17

18 Figure 18: The pcb layout. Figure 19: The top layer of the pcb. 18

19 Figure 20: A 5 Volt power supply is sometimes needed on broken units. 19

20 C1 27pF Digikey 1015PHCT ND C2 27pF Digikey 1015PHCT ND C3 10uF Digikey ND C uF D1 White LED LED1 7 SEG LED CC Digikey ND LED2 7 SEG LED CC Digikey ND LED3 7 SEG LED CC Digikey ND LED4 7 SEG LED CC Digikey ND LED5 7 SEG LED CC Digikey ND R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 10K digikey 10KQBK ND R digikey 200QBK ND R38 10k Digikey 10KQBK ND R39 10k Digikey 10KQBK ND R40 8.2k Digikey 8.2KQBK ND R41 10k Digikey 10KQBK ND R42 10k Digikey 10KQBK ND R43 10k Digikey 10KQBK ND R44 10k Digikey 10KQBK ND R45 10k Digikey 10KQBK ND R46 10k Digikey 10KQBK ND R47 10k Digikey 10KQBK ND R48 10k Digikey 10KQBK ND R Digikey 510QBK ND R50 5.1k Digikey 5.1KQBK ND R51 10k Digikey 10KQBK ND R52 10k Digikey 10KQBK ND R53 10k Digikey 10KQBK ND R54 10k Digikey 10KQBK ND U1 CD4033 Digikey ND U2 CD4033 Digikey ND U3 CD4033 Digikey ND U4 CD4033 Digikey ND U5 CD4033 Digikey ND U6 AT89C2051 U7 CD4013 U8 CD4011 X1 24Mhz Digikey SE3440 ND Figure 21: The bill of materials. Also, the printed circuit board costs about $50. But this cost goes down as more boards are ordered. 20