Nuts & Volts - September 2005

Tech Forum

Amazing Devices

mount device) microcontroller would be capable of the calculations and LED driving logic. A cheap electret condenser microphone capsule (less than $1) and likely a single transistor "mic preamp" could provide impact information to the microcontroller.

Richard Crowley Hillsboro OR

place of the normal Geiger-Muller tube. If radiation strikes a neutral atom of gas, a charged ion may be formed, which is accelerated toward the oppositely-charged electrode. The accelerated ion may strike other atoms forming more ions, continuing to form a multiplying cascade of ions until the neon lamp briefly conducts a current of thousands of ions. The ion arc is extinguished when the voltage across the lamp drops below the sustaining voltage. The voltage across the neon lamp is adjusted to just below the point where it would normally light. That way, it will light in response to radiation, even visible light. So, keep this circuit in the dark.

The point of the above, is that considerable amplification is required to light the lamp and click the speaker. This amplification took place in the neon lamp. This circuit does not give you the analog response which you require. It also illustrates that we need to apply a voltage across the neon lamp to have any hope of current flow due to radiation. There is a major difference in your application from the circuit in Figure 1. We do not want the neon lamp to ever go into avalanche mode and conduct. Therefore, we will never apply any voltage high enough to the neon lamp to allow that. All we want to do is measure an extremely small leakage current. The ideal circuit is shown in Figure 2.

This is an imaginary circuit because the op-amp (operational amplifier) does not exist as a discrete part. It converts current into the op-amp to a voltage at the output. Charles Wenzel ( www.techlib.com/ science/ ion.html) has built it up from discrete transistors, resistors,

etc. If you need

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[#05056 - May 2005]

I would like circuit suggestions for measurement of the dark (Townsend) current in a neon bulb. I would like to use a Keithley 181 nanovoltmeter to take measurements up to six figures. I know I am using a voltmeter to measure current, but I want to get the benefits of the sensitivity and accuracy of this particular instrument. The ultimate reason for doing this is to detect radiation from biosources.

Gary Grinnell via Internet

Measuring radiation to the degree of accuracy implied by the use of a six digit meter may not be possible with simple circuits. Though, there is a more complex circuit with some hard-to-find parts that may come close to your requirements. Should you not require high accuracy, there are some simple circuits that we can look at on the way to the complex one.

First, a circuit (Figure 1) published by Am Sonnenrain 4-71543 Wuestenrot, Germany; (e-mail: info@peterlay.de) in Electronic Design, March 18, 2002. He has also written a book about experiments with radiation sources (in German), Experimente mit Strahlenquellen im Haushalt ( www.peterlay.de).