Figure 3 CDV-715 Ion Chamber Radiological Instrument
When FEMA decommissioned the CDV-715, the instruments made their way onto the surplus market. Since so many instruments were manufactured, and because of their lack of sensitivity the CDV-715 can be found on Ebay for low cost. If you would like to retrofit a sensitive Geiger Counter inside a Cold War style case, read on. First step purchase a surplus CDV-715 from Ebay or elsewhere. It doesn't matter if the CDV-715 is in working order or not. We will be gutting all the old electronics out. It doesn't require to have a meter either since that will be replaced with a new faceplate that holds the new Analog Digital Meter. In Make magazine 29, I wrote a construction article for a Geiger counter circuit that easily fits this application. While I will not repeat the construction details for the Geiger counter circuit, I will point out the retro-fitting details. I like this circuit because it lends itself to a number of GM tubes available on the market, see figure 4.