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Building a Geiger Counter

This tutorial covers how to build a geiger counter based on on the nuts and volts article by John Lovine which can be found here.

You can find the bill of materials used here. geiger_counter.xlsx

below is the circuit diagram featured in the Nuts and Volts article.

Construction in completed in 3 major parts, the construction of the square wave generator, the high voltage circuit/power supply and the timer circuit. To illustrate this visually I have hilighted the circuit diagram accordingly.

Construction of the square wave generator

Construction started with the square wave generator. For this step add the components hilighted in purple in the annotated circuit diagram. Some of the capacitors and resistors were found to be uncommon and unavailable at local electronic supply shops. To avoid delivery times on these component several resistors or capacitors were soldered together to get the desired resistance or capacitance. R1 (4.3k ohm) was created from 2, 2.2k ohm resistors and c1 was created by soldering a .0027 uf and two .001 uf capacitors in parallel. The completed circuit can be seen in the image below.

High voltage and power supply

Now the components hilighted in green on the circuit diagram were installed. In my case the correct transformer did not arrive on time so it will not be installed until later. After the components are in place disconnect and test the vcc out of the lower left part of the circuit (power supply) and make sure it reads 5 volts. If if 5 volts is sow reconnect the VCC it and switch it on again. Place the positive lead of a voltmeter between C5 and D2 and the negative lead on ground. The voltmeter should read 500-800 volts. You can see an image of the high voltage section and the power supply below.

Timer circuit

Finally install the timer circuit and complete the high voltage section by inserting the components hilighted in blue. You will note the 3 zener diodes in the high voltage section, they are used to regulate the voltage to 500v. The completed timer circuit can be seen below. Note that multiple capacitors were used to create capacitor C11 in the circuit diagram.

Finishing assembly

After the timer is completed the Geiger Meuler tube can be installed between R4 and R5. BE ADVISED: the GM tube is polarized, make sure the positive end attaches to R4. In the picture below you can see the completed circuit and the locations to connect the GM tube leads denoted with red arrows.

Testing and modifying the circuit

When the proper transformer arrived it was installed, the timer circuit was removed and the circuit was powered on. A voltmeter was placed at the junction of D2 and C5 and no voltage was observed. Oscilloscope leads were then placed at the base (pin 1) of Transistor Q1. The square wave generator was not functioning. A further study of the hex inverting buffer and its used as a square wave generator revealed that the circuit requires that the 4049 chip have vcc connected to pin 1 and pin 8 connected to ground. For more reading on the square wave generator and how it works the following article is recommended IC 4049 Square wave oscillator generator. In addition the data sheet for the 4049 proved helpful TI 4049 hex inverting buffer datasheet.

The 555 timer circuit was found to be missing connections on the circuit diagram as well. Pin1 on the 555 timer needed to be connected to ground. The monostable section of 555 timer wikipedia article proved useful for troubleshooting the circuit

Upon completing the high voltage section the 555 trigger circuit was reconnected and the geiger counter was switched on again. The led D7 lit and stayed on so an oscilloscope was placed at both ends of the led. The led was not consistently on as it appeared but flashing rapidly so the oscilloscope probe was placed at the base of transistor Q2 to see why the led was be constantly triggered. The base of Q2 was found to have an extremely noisy signal so the Geiger Mueller tube was removed yielding no change. Eventually the noise was found to be due to leakage from the high voltage section of the circuit. The high voltage section was then isolated by placing it on a different breadboard and the circuit was re-tested. The nose was significantly reduced and Geiger counter functioned properly.

Updated circuit diagram

Modified connections pictured in red

Summary of modifications

• connect 4049 pin 1 to vcc</li>
• connect 4049 pin 8 to ground</li>
• 555 timer pin 1 to ground</li>
• high voltage section on separate breadboard.</li>

Results

Below is a video of the geiger counter in operation using fertilizer with high phosphate content as a radioactive source.

The scratch built Geiger counter was used to measure background radiation. An Arduino Uno was used to count pulses over ten second period. The results were then compared with the kit built geiger counter.

• Scratch built Geiger counter background radiation measurement: 13 +/- 8 CPM
• Kit built Geiger counter background radiation measurement: 14 +/- 9 CPM

The background radiation measurements easily agreed within uncertainty.

Both geiger counters were then used to measure the radiation levels from a cesium source at 30 cm distance.

• Scratch built Geiger counter cesium source radiation measurement: 538 +/- 92 CPM
• Kit built Geiger counter cesium source radiation measurement: 255 +/- 69 CPM

The scratch built geiger counter was found to be significantly more sensitive when exposed to the cesium source. The scratch built design has a higher voltage across the Geiger Mueller tube 500v compared to 275v. Higher voltage allows lower energy particles to be detected. It is believed that this higher voltage is responsible for the higher count rate in the scratch built counter.