Radioactivity And The Geiger Counter – Part 3

Doc Mike Finnley, Going Postal
The radiation spectrum

Before I get to making a Geiger counter I want to pay a quick visit to the opposite end of the radiation spectrum, all the the way from gamma rays to the shortwave. It’s interesting to note that alpha and beta radiation do not appear on the above diagram. Why? Well, they are particles and not rays, but their energy does place them between X-rays and gamma rays.

The reason for this detour is simple: I want my counter to be portable.

The Spectrum 48K And Three LEDs

A Geiger tube needs around 400 volts for it to work. Now, 267 AAA batteries are not portable! I want my counter to work with just three.

Another type of spectrum, of the home computer variety, also had power supply design problems. Mr Sinclair’s power supply for the spectrum 48K was an ugly black plastic ‘brick’ that plugged into the mains. It converted the 240V into 9V DC, the same as a PP3 battery.

Sir Clive had a problem, though. The memory chips in the spectrum needed some awkward voltages for them to work: +12v and -5v. A total voltage of 17v.

How do you magically turn 9 volts into 17 volts?

The answer lies in something called an inverter. It turns a direct current into an alternating current (DC to AC). You then feed that signal into a transformer.

In the case of the spectrum this DC to AC conversion was done by TR4, or transistor number 4. An electronic switch. The problem was that this switch was underrated for the current that it had to supply to the transformer. So, it occasionally failed and needed to be replaced.

* * *

The inverter has to be one of my favorite circuits. They’re great fun and seemingly magic! Let’s build one. All you need are three electronic components, a cardboard tube and some insulated wire:

Doc Mike Finnley, Going Postal
Left to right: A 4.7K resistor, 0.47uF capacitor, TIP41C transistor (TR4!). The coil is made from a 3/4 inch diameter cardboard tube. The green wire is the feedback and consists of 10 turns. The red wire is the drive (or primary) and consists of about 20 turns

What we are making is, essentially, a Tesla coil. The beauty of this circuit is that not only does it self oscillate, but it also finds its own optimum frequency. Its natural resonance.

On its own it’s pretty useless. We need another circuit to couple with the primary coil. Here I’ve wound five turns of thick copper wire and attached three LEDs in series (the wire does not need to be that thick, it’s just for show):

Doc Mike Finnley, Going Postal
The ‘secondary’ coil and three LEDs
Doc Mike Finnley, Going Postal
Left: Components wired-up. Right: The circuit diagram

‘Jolly good, Doc, but big deal. So you’re going to light a few LEDs up.’

Yes, but each of those LEDs needs at least 1.8v to light-up; and, in case you’ve missed it, I’ve only got one 1.5v battery to light three of them. That means I need at least 5.5v to make them work – from 1.5 volts!

Doc Mike Finnley, Going Postal
Magic! Each turn in the secondary coil is receiving over one volt. There are five turns of wire, so we get enough voltage to light the LEDs

Short video of it in action:

I mentioned at the beginning that we were looking at the shortwave end of the spectrum. What you can’t see in the above photo is the speed or frequency that the TIP41C transistor is switching at. If we connect the LEDs to an oscilloscope all is revealed. The circuit is, in fact, switching on and off at an incredibly high speed.

Doc Mike Finnley, Going Postal
Magic! Each turn in the secondary coil is receiving over one volt. There are five turns of wire, so we get enough voltage to light the LEDs

In the above oscillogram you can see that there are about 1.9 squares between each peak in the trace. The oscilloscope’s time base was set to 0.5 microseconds per division. This means that there are 1.9 times 0.5 microseconds per cycle – or just over one megahertz. This frequency is at the low end of the shortwave/high medium wave spectrum.

The vertical deflection is set to 5 volts per division. This gives a peak voltage of about 9 volts.

It’s this high-speed switching that makes modern power supply units so compact. Just look at how small your mobile phone charger is!

I don’t want to give the impression that you are getting something-for-nothing. The LEDs are drawing just a few milli-amps. The battery, on the other hand, is supplying nearly 160 milli-amps.

In the final part I’ll use this technology to make a battery powered Geiger counter and test it with the uranium glazed tile pieces from part 2.

© Doc Mike Finnley 2019

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