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DESKTOP DIGITAL GEIGER COUNTER

By John Iovine

A Geiger counter is a radiation detection instrument. Since the nuclear disaster in Fukushima Japan in 2011, radiation hazards and radiation detection instruments like Geiger counters were brought to the public consciousness. The news media coverage of the disaster and its subsequent safety hazards created an increased demand for Geiger counters that is still felt today.

This article will show you how to build a laboratory-quality Desktop Digital Geiger Counter.

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John Iovine is a scientist and electronics tinkerer and author who owns and operates Images SI Inc. (imagesco.com), a small science company.

LOOKING FOR THE TRAIL

Radiation is emitted by the decay of unstable atoms. Geiger-Müller (GM) tubes detect 3 forms of this energy:

α ALPHA RADIATION Free-traveling helium nuclei, can penetrate a few sheets of paper.

β BETA RADIATION Free-traveling electrons or positrons, can penetrate 3mm of aluminum.

γ GAMMA RADIATION High-energy photons, can penetrate several centimeters of lead.

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Rob Nance

Image The wall of the GM tube is a cylinder of thin metal or conductorcoated glass. With metal cylinders, one end is mica, which (unlike metal) lets some alpha radiation through.

Image The tube is sealed and filled with halogen and noble gases.

Image An electrode runs through the middle of the tube.

Image The power supply takes power from a 9V battery or 12V DC power plug.

Image A high-voltage power supply subcircuit uses transistorized feedback with a step-up transformer to create a “ringing choke converter” that converts a 6V, 14.1kHz oscillation induced in the primary coil of the transformer into 325V across the secondary winding. A voltage multiplier boosts this voltage in steps up to about 600V DC. Three Zener diodes let you configure the output voltage to between 300V and 600V DC, to match the GM tube used.

Image The high voltage is applied across the tube’s wall and electrode, turning them into a cathode and anode, respectively. In the tube’s normal resting state, the resistance between its anode and cathode is very high.

When a radioactive particle passes through the tube, it ionizes gas molecules in its path, which momentarily creates a conductive trail through the gas (like a vapor trail in a cloud chamber). This lowers the tube’s resistance, in a momentary pulse.

Image In the detector subcircuit, the tube’s cathode directs the momentary pulse to the input of a comparator, alongside a reference voltage tunable by a potentiometer. The comparator cleans up each pulse by outputting a digital 1 while the tube output exceeds the reference voltage and a 0 when it’s lower. This first comparator downstreams its output in parallel to the 3 other comparators on the same quad chip, which simply act as buffers.

Image One downstream comparator feeds to the base of a transistor, which powers the light and sound output — an LED flash and speaker click — when each particle is detected.

Image The other 2 downstream comparators connect to an output connector for output to other devices.

After each detection, the high- voltage subcircuit takes some time to recharge the tube. During this “dead time” (which varies with different tubes; 90μs for an LND712), the instrument cannot make detections.

SET UP

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This Desktop Digital Geiger Counter contains three separate PCBs. Each board has its own schematic. Care must be taken when building the individual boards, as each board may share common component names, such as R7. So be sure, when you choose a component, which PC circuit board you are building.

The main circuit is the Geiger counter shown on the left in the “Looking for the Trail” image on the previous page. The HV section of the circuit is built around a single transistorized (TIP3055) ringing choke converter that has been around for years. This particular derivative of the circuit was designed by Sam Evans to minimize current draw. The important element in this circuit is the transformer.

In this configuration, the primary winding of the transformer and the feedback winding of the transformer are arranged so that the circuit begins a sustaining oscillation when power is applied. If you checked the output of the oscillator you would find the waveform’s duty cycle is symmetrical.

The high voltage output from this stage is regulated by Zener diodes. In the latest revision of the PCB, there is a jumper J1 that shorts out the D8 Zener. You would only use this jumper to power a GM tube that requires 400 VDC.

We are using a 500 VDC output because our wand contains a LND712 GM tube. The cathode of the tube is connected to a 100K resistor. Each time a particle is detected, a voltage pulse is generated across this resistance. The reason I didn’t use the speaker on this board is I didn’t like the volume of the sound.

MATERIALS

All the PC boards, wands, PDF files, and other miscellaneous components used to build the Desktop Digital Geiger Counter are available on the Images SI, Inc., website. Part numbers are included where appropriate. For more information, visit imagesco.com/geiger/desktop-geiger-counter-kit.html.

Geiger Counter Circuit (#GCK-02)

» PCB printed circuit board (1): (#PCB-52)

» C5 C6
C7 C8

0.01μF, 1KV capacitors (4)

» C1

0.1μF, 100V capacitor (1)

» C2

330μF, 16V capacitor (1)

» C3 C4

10μF, 16V capacitors (2)

» D1–D6

1N4007 diodes, 1000V (6)

» D7

5.1V Zener diode (1): (#1N751)

» D8

100V Zener diode (1): (#1N5271B)

» D9 D10

200V Zener diodes (2): (#1N5281B)

» Q1

TIP 3055 regulator (1): (#ALT.H1061)

» R2

33Ω resistor (1)

» R3 R7

1 MΩ resistors (2)

» R4

470KΩ resistor (1)

» R6

1 MΩ, multi-turn potentiometer (1)

» R8 R9 R10
R11 R14

10KΩ resistors (5)

» R12

100Ω resistor (1)

» R13

100KΩ resistor (1)

» T1

Mini step-up transformer (1): (#HVT-06)

» U1

7805 voltage regulator (1)

» U2

LE33 (1)

» U3

Bridge rectifier (1): (#W01M)

» U4

LM339N (1)

»

14-pin IC socket (1)

» J1

2-pin header (1)

Digital Meter Adapter (#DMAD-04)

»

Printed circuit board (1): (#PCB-74)

» C1

0.1μF, 50V capacitor (1)

» C2 C3

10μF, 16V capacitors (2)

» D1

SMH-16 (1)

» D2

1N4007 diode, 1000V (1)

» P1 P2

SMH-02 (1)

» R1

33Ω ¼W resistor (1)

» R4 R5 R6

10K ¼W resistor (3)

»

LCD display (1): (#LCD-01-16×2)

» U1

Preprogrammed PIC16F88 (1)

»

18-pin IC socket (1)

» U2

Voltage regulator (1): (#LDO-5V)

Soundboard

» Prototyping board

» 10K ¼W resistor (1)

» 220K ¼W resistor (1)

» 330Ω ¼W resistor (1)

» 1K ¼W resistor (1)

» 220Ω ¼W resistor (1)

» 0.1μF, 50V capacitors (2)

» 2N3904 transistors (2)

» Piezoelectric speaker (1): (#SPK-05)

» LM555 timer (1)

» 8-pin IC socket (1)

Other Components

» GCW-01 Geiger counter wand (1)

» SPST 2-position toggle switches (4): (#SW-10)

» DPDT 3-position, center off toggle switch (1): (#SW-18)

» Red panel mount 5mm T1 LED (1)

» Green panel mount 5mm T1 LED (1)

» 20K panel mount potentiometer and knob (1)

» Panel mount TTL-stereo jack (1)

» Panel mount mono jack (1)

» Panel mount power jack (1): (#Jack-18)

» Panel mount 3-pin mini DIN socket (1)

» 22 gauge wire, various colors

Hardware

» Bud IP-6130 enclosure with stand (1)

» 4-40 x 1" nylon screws (8) (for mounting PCBs)

» 4-40 nylon hex nuts (16)

» 4 x 9/16" nylon spacers (8)

» 2-56 x 1" nylon screws (4) (for mounting LCD)

» 2-56 nylon hex nuts (4)

» 2-56 × ½" black nylon screws (2) (for mounting mini DIN connector)

» 2-56 black nylon hex nuts (2)

» 9VDC wall transformer (1)

Tools

» Soldering equipment

» Multimeter

It was too faint. So I created a soundboard. The difference is that the soundboard stretches the pulse, creating a better click. The soundboard follows the construction of the GCK-02 board.

MAKE IT.

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BUILD YOUR GEIGER COUNTER

Time: A Weekend

Complexity: Moderate

1. Assemble the PCB

You may hardwire this circuit to a breadboard or use the available PCB. Although you do not need the PCB, it will make construction easier (see Figures 1 and 2).

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When using the PCB, it’s merely a matter of mounting and soldering the components in their proper position. All the parts are outlined on the top silk screen (Figure 2). For our application, however, we can leave quite a few components off the board.

Begin by mounting and soldering all the resistors:

» R2 — 33 ohm (color code — orange, orange, black, gold)

» R3 and R7 — 1 mega-ohm (color code — brown, black, green, gold)

» R4 — 470 K-ohm (color code — yellow, purple, yellow, gold)

» R5 — Jump with a small length of wire (in this unit a 10 Mega ohm resistor is housed inside the wand)

» R8, R9, R10, R11 and R14 — 10K-ohm (color code — brown, black, orange, gold)

» R12 — 100 ohms (color code — brown, black, brown, gold)

» R13 — 100 K-ohm (color code — brown, black, yellow, gold)

Next, mount and solder diodes D1 through to D10. Make sure to orient the line on the diodes with the line on the silkscreen diode drawing on the printed circuit board.

» D1, D2, D3, D4, D5, and D6 — 1N4007

» D7 — 5.1V Zener

» D8 — 100V Zener

» D9 and D10 — 200V Zener

Install and solder the regulator 7805 and the 3055 (or H1061) transistor. Mount and solder the capacitors:

» C1 — 0.1μF-50V

» C2 — 330μF-16V

» C3 and C4 — 10μF-16V

» C5, C6, C7 and C8 — 0.01μF — 1KV

Solder a small piece of wire connecting the top two holes of S1 as shown in Figure 2. Mount and solder the mini-step-up transformer T1. Then mount and solder the 14-pin IC socket for the LM339, lining up the notch on the IC socket with the silkscreen outline on the PCB.

Mount and solder U2, making sure the profile matches the silk-screened outline of the part before soldering. Next, mount and solder the bridge rectifier U3. Align the positive terminal of the bridge rectifier with the + pad on the PCB. Mount and solder the 1-Mega-ohm potentiometer in the R6 position on the PCB.

Finish the board by mounting and soldering the two-pin header in the J1 position. J1 is used to vary the voltage to GM tube. By shorting J1, diode D8 is shorted out and the voltage to the GM tube drops from 500VDC to 400VDC.

2. Connect the Boards

Next, we must add wires to connect this PCB to the other boards used in the assembly, as well as to panel-mounted components. To simplify the construction, we have color-coordinated the majority of the wires. Where power is involved, a red wire is used for + and black for ground, in most instances. As a standard, eight-inch pieces of wire are used and may be trimmed down as needed.

Solder a red wire to P1, and black wire to P3 for power. Also, solder a red wire on the lower right-hand side of the PCB, in the pad marked +5V, and a black wire in the pad marked GND. These wires will connect power to the soundboard.

Solder a purple wire to the third hole down on the right side of the PCB marked with the letter D. This will connect to the digital output that will go to the input of the soundboard. Solder another wire to the other “D” output that will connect to the Pulse Input of the DMAD.

Next solder three wires to the 3-pin mini DIN socket: in the photograph a dark green wire to the — output, a red wire to the + pad, and a white wire to the — pad on the top part of the PCB. Connect these wires to the appropriate connections of the panel-mount 3-pin mini DIN socket as shown in the schematic.

Solder a wire into the right-most hole of Q2 (this is the base of Q2, not shown in picture). This is a digital output that will connect to the 1/8” jack for the digital pulse output on the front panel.

3. Make the soundboard

Figure 3 shows the schematic, while Figure 4 shows the soundboard constructed using point-to-point wiring on a prototyping breadboard as outlined in the schematic.

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The Pulse In connection is fed from the purple wire of the Geiger counter circuit to the soundboard. The +5V power and ground connection for the soundboard come from the wired connections on the lower right-hand corner of the Geiger counter circuit. Figure 5 shows the GCK and soundboard connected.

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Brown wires to the soundboard are for the Audio on/off switch, S1. Connect these wires to an SPST toggle switch (not shown in the photograph).

4. Make the Digital Display Board

The schematic for the Digital Meter Adapter (DMAD) is shown in Figure 6.

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Begin by mounting and soldering the 16-pin header on the bottom of the board (the side with no silk screen). Now, on the top silk screen side, mount and solder the resistors:

» R1 — 33 ohm (color code — orange, orange, black)

» R4, R5, and R6 — 10K ohm (color code — brown, black, orange)

Solder a small piece of wire to connect the center and right holes of S1. Repeat for S2. This jumps these switches in the on position.

Next, mount the 1N4007 diode, D2, making sure to align the stripe on the diode with the line on the silk screen. U2 is the regulator. When mounting the regulator, be sure the flat side is oriented with the flat side of the silk screen.

Mount and solder a 2-pin header at P1 and P2. A wired cable will attach here to provide power to the entire unit; P1 is +V and P2 is ground. Mount the 3-pin header marked P8.

Now, mount C1, the 0.1uf capacitor, and the two 10uf 16V capacitors marked C2 and C3. When mounting these capacitors be sure the longer lead is oriented to the hole marked positive.

Next, mount and solder 18-pin socket U1 to the PCB making sure that it is oriented according to the outline on the silk screen. Insert the preprogrammed 16F88 microcontroller matching the notch on the chip to the notch on the socket.

Connect the following wires to the PCB before attaching the LCD module to the PCB. Connect 3 wires to the holes at R2, shown as 3 yellow wires in the photograph. These will connect to a panel-mounted potentiometer to allow you to adjust the contrast of the LCD screen.

Connect three wires, shown as dark green wires in the photograph (Figure 7) to the largest of the pads at J2. The center pad is ground. These wires connect to the stereo Serial Digital TTL output socket on the front panel.

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Connect a wire, shown as a gray wire, in Figure 7 to the right-most hole at J1. (You may choose to connect this wire at P4 instead.) This wire will connect to the “D” digital output of the main Geiger counter circuit. Connect a black power ground wire to the center hole at P3, and a red wire for the +9VDC to the right-most hole. These will connect to the power supply.

Connect eight wires, shown as white wires to each of the holes at S5. These will connect to the three switches labeled B4, B6, and A7, B5 switches in the schematic and provide several different functions of the unit.

Finally, mount and solder the LCD module to the 16-pin header.

5. Attach Switches to the DMAD Board

Begin by wiring switches to the white wires connected to S5 of the digital display board. (Refer to Figure 7.) All of the bottom wires are connected to ground.

Connect the top and bottom wires of B6 to an SPST toggle switch. This switch is used to select between mR/hr and mSv/hr.

Connect the top and bottom wires of B4 to an SPST toggle switch. This switch is used to change between one-second and sixty-second time selections (CPS and CPM).

The wires from A7 and B5 are connected to a DPDT toggle switch with center off for Random Number Selection and range. Connect the top wire at A7 to the left-side top and bottom prongs of the DPDT switch. The top wire of B5 is connected to the top right prong of the DPDT switch. Both bottom wires for A7 and B5 are connected to the appropriate center prong of the DPDT switch, see Figure 7 for switch details.

Next, connect the yellow wires attached to R2 of the digital display to the appropriate connections of a 10K panel-mount potentiometer.

Image The author and publisher do not make any warranties (express or implied) about the radiation information provided here for your use. All information provided should be considered experimental. Safety and health issues and concerns involving radioactive contamination should be addressed, confirmed, and verified with local and national government organizations or recognized experts in this field.

NOTE: You have two sets of power leads into the DMAD board. The two leads you just soldered at P3, and the header you soldered on P1 and P2. You can feed the main power into either one of these power leads. The power leads you do not use are then used to feed power to the GCK-02 Geiger counter board.

6. Calibrate Your Device

After you have everything wired and before mounting anything into the case, power up the unit and make sure everything is functional. See Figure 8 on the previous page.

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Now is the proper time to calibrate your Geiger counter!

To calibrate your Geiger counter, you need a radioactive source. See “Radioactive Sources” on page 84.

Set the digital meter to CPS. You will need to adjust potentiometer R6 on the Geiger counter board. Turn on the Geiger counter. If you have a radiation source, bring it very close to the front of the wand — but BE CAREFUL! The front of the GM tube inside the wand has a thin mica window that is easily damaged. Make sure you do not touch the mica window. Adjust R6 to obtain the highest CPS reading on the digital display. If the clicking sound is annoying, use the audio switch to mute the sound.

You can use a simple procedure to get a ballpark calibration for the Geiger counter. The difficulty in calibrating the digital meter has much to do with the variables in play. The tube’s response can vary +/- 20 %. The strength of the radioactive source can also vary in addition to variances in the electronic components. All these factors affect accuracy. With this being said, you can still proceed to get that ballpark approximation for the digital display meter.

This calibration procedure uses a 10 uCi Cs-137 source. The source is held at specific distances from the front of the GM tube in the wand without any shield.

Distance from front of GM tube:

Approximately 1 mR/hr = 6”

Approximately 10 mR/hr = 2.5”

Approximately 100 mR/hr = 0.5”

7. Mount the Unit Inside the Case

Once the circuit has been tested, you can begin mounting the unit inside the case. Begin by lining up the boards inside your case the way you want them orientated, and then mark and drill the mounting holes. The holes I used for attaching the PCBs to the bottom of the case are 1/8.”

Insert (8) 4-40 x 1” Phillips-head nylon screws into the mounting holes on the bottom of the case and secure with nylon hex nuts. Four 9/10” nylon spacers are used to elevate the Geiger circuit and soundboard inside the case.

Secure the PCBs to the case with hex nuts. Figure 9 shows the end result.

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The front panel hole guide, Figure 10, is provided as a drilling template. This assumes you are using the BUD enclosure listed in the parts list.

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The drill sizes for the holes are detailed in the diagram. The decal used on the front Panel is shown in the Figure 11 on the following page.

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Full-size PDF files for both the holes and the decal are available for free download at the Images SI Inc. website (imagesco.com).

When assembling this unit into the case, some of the components will need to be mounted on the face panel before being wired; others, like the switches, can be wired and then mounted to the face panel.

Mount the TTL serial and digital output sockets into the appropriate holes in the front panel. Connect the three wires, shown as dark green wires in Figure 7 from the digital display to the appropriate connections on the TTL serial socket. The center wire is ground.

Connect the wire from the base of Q2 on the Geiger counter circuit to the + lead of the digital output socket. Use a small piece of wire to connect the ground of the two sockets.

Insert the green power LED and red pulse LED into the appropriate holes on the front panel. Connect the red and green LEDs, using wires, to the appropriate connections as diagrammed in the soundboard schematic.

Power is fed to the completed unit from a 9VDC wall adapter. It is fed through the panel- mounted SPST toggle power switch. After the switch, the +9V is fed to the DMAD power input leads. The secondary power leads connect to and power the GCK-02 Geiger Counter board. The power supply is in parallel powering both the Geiger counter circuit and digital display. The soundboard receives +5V from the Geiger counter circuit.

The power jack must be attached to the back panel before wiring. Attach a red wire to the positive lead of the power jack. Connect the opposite end of the wire to the center prong of an SPST toggle switch. The other pole of the switch is then connected to P1 of the DMAD. The positive lead of P3 on the DMAD connects to the positive power input on the Geiger counter circuit labeled P1. A black wire is connected to the ground of the power jack is connected to the P2 ground on the DMAD. The ground connection of P3 on the DMAD board is connected to the ground of Geiger counter circuit board (P3). (See diagram.)

How Much Radiation Is Safe?

In the United States the U.S. Nuclear Regulatory Commission (NRC) determines what radiation exposure level is considered safe. Occupational exposure for workers is limited to 5000 mrem per year. For the general population, the exposure is 500 mrem above background radiation in any one year. For long term, multi-year exposure, 100 mrem above background radiation is the limit set per year.

Let’s extrapolate the 100 mrem number to an hourly radiation exposure rate: 365 days/yr x 24 hr/day equals 8760 hours. Divide 100 mrem by 8760 hours, and it equals .0114 mrem/hr or 11.4/hr microrem. This is an extremely low radiation level. The background radiation in my lab hovers around 32 uR/hr. Am I in trouble? No. Typically background radiation in the United States averages 300 mrem/yr, or 34 microrem/hr. The NRC specification is for radiation above this 34 urem/hr background radiation.

Notice that my lab readings are in microrads (uR/hr) and the exposure limit is given in microrems (urem/hr). I do not know what type of radiation (a , b or y) the Geiger counter is reading in my lab at any particular instant, so I do not know the Q factor of the radiation and therefore can not calculate the mrem. For general purposes, I consider them the one and the same. The digital Geiger counters I use are calibrated using a Cs-137 radioactive source. Therefore, the highest accuracy in reading radiation levels will be from Cs-137 sources. See makezine.com/go/bom2 for more information.

Exposure

Source Dose (conventional)

Dose (SI)

Flight from LA to NY

1.5 mrem

.015 mSv

Dental X-ray

9 mrem

.09 mSv

Chest X-ray

10 mrem

0.1 mSv

Mammogram

70 mrem

0.7 mSv

Background radiation

620 mrem/year

6.2 mSv/year

Secure the LCD display to the front panel of the desktop unit with (4) 2-56 x 1” nylon screws and hex nuts. Attach the mini DIN connector to the front panel with (2) 2-56 x 1/2” nylon screws and hex nuts. For the face of the unit, black nylon screws are preferred but not required.

Now insert your switches into the appropriate holes and check that they are orientated correctly before securing in place.

The front and back panels of the unit will slide into the groves on the case. Slide on the top of the case, and insert screws into holes in the bottom of the case to secure.

You can now plug in your Geiger counter wand and connect the Desktop Digital Geiger Counter to a power source.

FINISH X

NOW GO USE IT »

Radioactive Sources

Uranium ore is available from a number of sources including eBay. You can find a source list on the imagesco.com website. Small amounts of radioactive materials are available for sale encased in 1-inch diameter by ¼” thick plastic disks. The disks are available to the general public, license-exempt. This material outputs radiation in the micro-curie range and has been deemed by the U.S. Federal government to be license-free and safe. The cesium-137 is a good gamma ray source and has a half-life of 30 years.

USE IT. » THE HIDDEN LANDSCAPE OF RADIOACTIVITY

Test for Radioactive Contamination

If you’re testing for contamination, your GM tube should be sensitive to alpha radiation as well as beta and gamma. Geiger counters can only test for gross levels of contamination that show up clearly above background radiation; they are not the proper instruments for detecting low-level contamination. That said, here is how to test for radioactive levels above background:

1. Establish the background radiation level by measuring CPM for at least 20 minutes. Longer is better. Note the lowest and highest levels and then average them all to establish the baseline minimum, maximum, and average.

2. Position the GM tube very close to the top surface of the material you’re testing, and run the counter, recording the CPM output. The longer the run, the more accurate the results.

3. Compare the radiation output of your sample against your baseline.

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