Take me to the Steps

Whether it’s the “Fukushima plume” and giant monsters on your mind, or you’re pioneering thorium reactors in your backyard, the classic Civil Defense CDV-715 Geiger counter has a perfect retro-style case to house a modern, sensitive digital Geiger counter.

fig3In the 1960s, at the height of the Cold War, over half a million CDV-715 ion chamber radiological instruments were manufactured. In its day the CDV-715 could be found in government fallout shelters across the United States. When FEMA decommissioned the CDV-715, the instruments made their way onto the surplus market, where they’re available cheap. Unfortunately, this survey instrument only detects high levels of gamma radiation that would be encountered post-nuclear attack or incident, so it’s not sensitive enough for most radioactive detection work.

fig2You can easily fix that by swapping the counter’s old electronics for a new alpha-beta-gamma Geiger counter circuit and LCD Analog-Digital Meter. The top line displays Counts Per Second (CPS), alternating with Approximate Radiation Level in either Imperial measurements (mR/hr) or metric (mSv/hr). The bottom line is a power meter that provides a quick visual indication of the current CPS reading.

In Make: Volume 29, I showed how to build a Geiger counter circuit that easily fits this application. (Build it at makezine.com/projects/geiger-counter, then come back to this project for the retrofitting details.)

fig4

BewareofGodzillaI like this circuit because it lends itself to a number of GM tubes available on the market and can power any GM tube that requires either 400 or 500VDC. It’s also compatible with the Radiation Network (radiationnetwork.com), so you can share your readings with others worldwide — in the event of global radioactive lizard outbreaks.

 

 

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Project Steps

Purchase a surplus CDV-715.

It doesn’t matter if it’s in working order, since you’ll be gutting all the old electronics out. It doesn’t even need a meter, since that will be replaced with a new faceplate that holds the new Analog Digital Meter.

Drill mounting holes.

Find the printed circuit board from the kit and drill 5 mounting holes as shown in photo 2-1 (marked in red).

Place the PCB at the bottom of the CDV-715 case where you want to secure it. Then use the mounting holes in the PCB to mark the locations where you’ll drill mounting holes for the 3/4″ hex spacers.

Also mark and drill holes for the 9V battery holder and the GM tube holder. The GM tube holder I used was a 3/8″ plastic cable clip. If you’re using the LND712 GM tube, try a 1/2″ clip instead.

Build the new Geiger counter.

Construct the Geiger counter PCB per the instructions in the original article, except for a few minor changes:

  • The speaker switch is not needed; it should be jumped in the On position. (Alternatively, you could mount an external audio switch.)
  • The S1 switch is replaced with an external rotary switch (see Step TK). You can solder two 9″ wires in the S1 position for connecting the external switch.
  • The terminal blocks are not needed; the Pulse In wire from the Analog Digital Meter is soldered directly into one of the “D” (digital output) pads used for the terminal blocks.

The Analog-Digital Radiation Meter.

When I first thought of doing this project, I tried mounting my standard 16×2 character LCD, but it was too large to fit into the space vacated by the original meter. So I designed a new PCB that uses a smaller 8×2 character LCD to create an Analog Digital Meter. The 8×2 LCD fits nicely into the allocated meter space.

The smaller LCD presented new challenges regarding the display of information from the Geiger counter. When using the 16×2 LCD display I had plenty of space to place the Count Per Second (CPS) on the top line and the equivalent radiation level in mR/hr (or mSv/hr) on the second line. In addition to this I wanted something that would replace an analog meter. I wasn’t happy with the available analog meters or the electronics to implement them, so I decide to create my own.

The LCD’s 8-character line doesn’t permit me to write mR/hr, so I shorten this to mR. This left 5 characters to display the radiation level. I took a similar approach to displaying CPS. This left 4 character to display a CPS numerical value up to 9,999, more than enough.

The second line of the 8×2 LCD is a dedicated analog power meter. To create the LCD analog meter I programmed 8 custom characters into the LCD, each representing a binary number: 1, 2, 4, 8, 16, 32, 64, and 128. The chart below (Figure H) provides an indication of how to read the power meter. Using the information we can obtain a reasonable approximation of CPS from 1 to 2,040 from the second line of the Analog-Digital Meter. The chart below illustrates the CPS range capabilities of the ADM meter.

Switching from Imperial to Metric Measurements

If you place a jumper on the 2-pin header on the back of the Analog Digital Meter, the display will change from Imperial measurements (mR/hr) to metric (mSv/hr), shortened to “mS” to fit on the 8-character line. The CPS reading stays the same regardless whether meter is set for Imperial or metric.

Build the Analog-Digital Meter (ADM).

Because the LCD meter is connected to and powered by the main Geiger counter board, there are a few components you can eliminate from the LCD’s PCB to simplify its construction. The power and backlight switches S1 and S2 are eliminated and replaced with a jumper. Since you’re pulling 5V power from the Geiger counter board, you can also eliminate the voltage regulator U1 and input capacitor C2 shown on the PCB. What’s left is a spoonful of components to mount to get the meter working.

Build the Analog-Digital Meter (cont'd).

Begin by soldering a jumper wire on S1 and S2. Mount and solder the 18-pin socket, aligning the indentation on the socket as shown on the silk-screen outline on the PCB. Solder the 10K resistor in the position shown, then the 2-pin header, potentiometer R2, diode D1, and the the 0.1µF capacitor C1.

Solder 22-gauge stranded wires 9″ long to the ground pad, +5V pad, and the Pulse In pad. If you have colored wire, use black, red, and white wires respectively. At this point your ADM PCB should look like photo 5-1.

All that’s left is to solder the headers for mounting the LCD and inserting the pre-programmed chip into the IC socket. Flip the PCB over to mount and solder the two 8-pin headers

Next mount and solder the LCD to the 2×8 header pins.

Prepare the CDV-715 case and power switch.

When you open the CDV-715 case it will resemble photo 6-1. Remove the circuit board, ionization chamber, and panel meter. Keep the screws for the panel meter.

The CDV-715 has a rotary power switch that’s integrated with the old PCB, so it’s not useable for our circuit. I wanted to keep a rotary switch for the retrofit to keep the overall look the same. Remove the old switch fixture and ream the hole to 3/8″ to accept the new switch. The thickness of the case prevented using the faceplate nut to secure the switch. Rather than create a bracket for the rotary switch, I glued switch into position.

Final assembly.

With the switch mounted, it’s time to assemble the unit. Replace the old panel meter with the new faceplate, which has holes for mounting the Analog-Digital Meter. The 3 holes along the circumference align with the screw holes in the case. The ADM is attached to the inside of the faceplate using four 0-80 machine screws and nuts.

Final assembly (cont'd).

Finally, attach the PCB to the ¾” hex standoffs using 4-40 nuts, and secure the battery clip and the GM tube to the inside of the CDV-715 case.

TUNING THE GEIGER COUNTER: Turn on the Geiger counter and if you have a radiation source, bring it as close to the GM tube as possible. Now simply adjust potentiometer R6 until you hear clicking and see the LED blink. Each click and flash represents the detection of one alpha, beta, or gamma particle passing through the tube (but not all such events are detected; it depends on the tube‚s sensitivity). Continue to adjust R6 to achieve the highest CPS count on the ADM Meter. Stop turning the pot after the peak is reached; stay at that point and do not go past.

Even without a radiation source, background radiation from natural sources on Earth and cosmic rays will cause the Geiger counter to click. Where I live, background radiation triggers about 16–32 counts per minute (CPM) with an LND-712 tube. If your Geiger counter continues to count well above 32 counts per minute, for a few consecutive minutes, you have to back down R6 slightly to bring the CPM count down. That’s it — you’ve made and calibrated your analog-digital Geiger counter.

Close it up and you’re ready to flip the switch and detect some particle flux!

Going further.

Here are a few things you can do to improve on this retrofit Geiger counter project. Let us know how it goes!

Cut holes or louvers near the GM tube to improve the sensitivity of the GM tube. The metal can of the enclosure blocks alpha radiation and attenuates beta radiation and gamma radiation. If you’re using an alpha-sensitive tube like the LND-712, drill a 1/2″ hole in the case to allow the mica window to see outside. Cover the hole with a screen to help protect the sensitive mica window while still allowing it to see alpha radiation.

The GCK-02 Geiger counter PCB has an LED that flashes with each detected radioactive particle. This LED can be made external to the PCB and mounted in the top cover of the case.

In the same manner, the speaker can be extended and made external.

Repaint your Retro Digital Geiger Counter for an updated, post-Cold-War look.