This project started in early 1998 after encountering an article on the internet complaining about the high cost of radiation detection equipment. Upon investigating for myself, I had to agree with his conclusion that the radiation detectors were being priced above what the average consumer could afford. Another problem was that most of these radiation detectors were designed like survey equipment, often requiring both hands to operate and demanding constant visual scanning of indicator lights or dials.
What I felt was needed was an early-warning device that could be carried on the belt or in a purse. Similar to the canaries that were carried into coal mines to signal miners about the presence of poisonous gasses, this canary would signal the presence of radiation. For example, an entire company of firemen could be wearing such a detector. Upon entering a building, if one of the fireman's alarms sounded, he would immediately alert his companions and exit the building. The fire captain could then dispatch a team equipped with normal radiation survey equipment to determine exactly what was tripping the alarm, and exactly how much radiation was present.
A lot of people have misconceptions about how a radiation alarm works. In its simplest form, a radiation alarm would consist of a normally-open push button, a relay, a buzzer, and two batteries (see illustration 1).
Upon being alerted of possible radiation (say for example I was visually monitoring a Kearney detector) I would push the button causing the high-voltage relay coil to energize. The energized coil would close the relays leaf-switch allowing low-voltage electricity to flow through the piezo buzzer (sounding the alarm).
To further refine the system, the electromechanical parts could be replaced with entirely electronic devices. The electromechanical relay we will replace with an NPN transistor, and the mechanical push button switch we will automate with a Geiger Müller switch (see illustration 2).
Most persons have had experience with NPN transistors and know how they work. Electricity flowing through the transistor from the Base to the Collector is functionally equivalent to the energized relay coil closing a leaf-switch; it allows electricity to flow from the Emitter to the Collector (the fact that the two batteries are no longer physically isolated on separate circuits is irrelevant for our purposes, because their electronic flowpaths are still functionally separated).
Although you may have never seen a Geiger Müller switch, it is a simple device consisting of a gas-filled metal container with an isolated wire in its center. The center wire is connected to the Positive side of the battery, and the metal container to the Negative side. Similar to the way a solar-cell becomes momentarily conductive when a photon particle passes through it; while a particle of gamma radiation is passing through the gas, the gas becomes momentarily conductive, allowing electricity to flow from the center wire to the metal case.
Now, whenever a particle of gamma radiation strikes the switch, electricity momentarily flows through the transistor, which allows electricity to then flow through the piezo buzzer, causing the buzzer to chirp.
One of the problems with any radiation detection device is that it can be triggered by background radiation coming from both outer space and the earth itself (you can actually see this radiation if you build yourself a Cloud Chamber). It would become pretty annoying if for example while sitting in your home the alarm would chirp, say, once a minute. One way to counter this problem would be to add a step-relay into the circuit (see illustration 3).
For illustration purposes we are using a four position continuously-cycling step-relay. Now let us assume that radiation was striking the alarm once every minute like before. Instead of electricity always flowing directly through the buzzer, it now also energizes a relay coil, causing the step-relays switch to cycle to the next contact point. Electricity will now flow through the buzzer only when the contact switch is in position four. Therefore the buzzer will only chirp at a more tolerable rate of, in our example, about once every four minutes.
Unfortunately, de-sensitizing the alarm to background radiation also desensitizes it to so-called dangerous radiation. However, the tradeoff is acceptable. This is because radiation classified as dangerous is just the same as background radiation; it's just that it is coming at you at a very rapid rate. Instead of, as in our above example, one particle per minute; the radiation may be coming at you at a rate of, say, one particle every quarter second (often much faster). Even using the step-relay to reduce the alarms sensitivity by one fourth, the alarm would still be chirping at you at the frightening example rate of once per second.
In fact, this very same great separation between the rate of background radiation and the rate of dangerous radiation can be used to make our final refinement. Let's replace the step-relay with one that has a button to reset the contact switch back to position one whenever it is pushed. Now let's add a timer that will push the reset button once every three minutes. Assuming the previous example rate of background radiation, the contact switch will never get to position four before the timer resets the relay. As such, the example background radiation will never make the alarm chirp. Only in the presence of dangerous radiation will the alarm sound, because this radiation will advance the relays contact switches many times between each of the timers slow resets of the relay.
Illustration 4 shows the final schematic diagram for the electronic radiation canary. To keep costs down, all but three parts (the 12vdc battery, 50x isolated DC to HV DC converter, and gamma detector) are standard (1998 catalog) Radio Shack components.
When built, all the electronic components will fit inside a small project box about the size of a VHS camcorder battery. Total price for the project is around 100 dollars (approximately $40 for the GM switch, $50 for the isolated DC to HV DC converter, and $10 for the other components [1998 US Dollar prices]).
Let's begin at the 12vdc battery [B1] on the far right side. Originally I wanted the unit to operate 24/7, and I initially chose a common #23 alkaline battery due to its size (this battery is often used in wireless doorbells buttons, garage door remotes, and older car alarm remotes). Unfortunately, like a flashlight left on for a long period, the active circuit drained this small battery below operating voltage in less than four days. This somewhat negated its usefulness as an always-on belt device that I wanted to change the battery in only once a month (I was however being somewhat overly optimistic, as even hearing aid batteries which operate 24/7 only last about a week). However, the canary will easily operate for months when attached to a large capacity (and unfortunately physically larger) battery such as the sealed lead acid batteries used in uninterruptable power supplies (or virtually indefinitely when run from an AC power line to 12 volt DC adapter).
The circuit also requires a 600vdc battery to power the Geiger Müller (GM) switch. As batteries of that voltage are quite costly, I took a different approach and used a 50x isolated DC to HV DC converter [V1]. This electronic module acts like a DC transformer, converting 12vdc into 600vdc. The unit I used was an EMCO GP06. The positive side of the battery [B1] is connected to pin 1, and the negative side to pin 2.
The positive high-voltage output (pin 3) is routed through a 4.4meg ohm load resistor (the schematic shows two 2.2meg ohm resistors [R1 & R2] connected in series [Radio Shack 271-8051], because Radio Shack does not normally stock single 4.4meg ohm resistors). Note that resistors R1 & R2 are used only to ensure proper functioning of GM1 (and for flow-tracing can be considered as just a wire).
Electricity then flows into a GM switch [GM1]. The part I chose was a LND 714. This particular detectors advantage is that it will activate with mild radiation, yet it will not saturate (stick in the on position) in strong radiation environments (saturation issues are why special "Civil Defense" meters, rather than normal "survey" meters, should be used around nuclear fallout).
Whenever gamma radiation strikes the GM switch, electricity will flow from the switch into the Base of an NPN transistor [Q1]. The transistor I used was a silicon 2N4401 [Radio Shack 276-2058]. From the Collector of the transistor, the electricity flows back (via R3) to the negative high-voltage return (pin 4) of the isolated DC to HV DC converter [V1], completing the circuit (note that common pins 4 and 2 are tied together). Note that resistor R3 is used only to ensure proper functioning of IC1 (described later); so for flow-tracing of the circuit described thus far, R3 can be considered as just a wire.
All the other circuitry in the schematic uses a low 12vdc. The negative side of the battery [B1] goes to a common bus. The positive side of the battery, besides going to the isolated DC to HV DC converter [V1] as already mentioned, is used to power all the integrated circuit chips and to provide the electricity that powers the piezo buzzer [P1] (see illustration 2).
12vdc flows from the positive side of the battery [B1] to the Emitter of NPN transistor Q1. Whenever the GM switch [GM1] turns on, electricity will flow through the transistor from the Emitter to the Collector. From the Collector, the electricity flows into a counter/divider [IC1].
For IC1 I used a 4017 decade counter/divider [Radio Shack 276-2417]. This chip acts like a resetable 10 position continuously-cycling step-relay (see illustration 3). As IC chips require power to operate, pin 16 is connected to 12vdc positive, and pin 8 to 12vdc negative.
The electricity from the Collector of Q1 flows into pin 14, flows out pin 8, and completes the circuit back to the battery [B1]. The electricity flowing from pin 14 to pin 8 is similar to the function of the coil in a step-relay. Every time the circuit is energized, it advances the switch to the next contact. Note that there is also a pulldown 10K ohm resistor [R3] connected between pins 14 and 8 [Radio Shack 271-1335]. Resistor R3 is used only to ensure proper functioning of IC1 (and for flow-tracing it can be considered as just a wire).
A piezo buzzer [P1] [Radio Shack 273-059] is connected between pin 9 and the negative side of the battery [B1]. The ten pins 1-7 & 9-11 are equivalent to the contacts of a ten contact step-relay switch. Each time pin 14 is pulsed, 12vdc positive is cycled to the next output pin. When the IC chip is powered up, the contact will always be reset to the first output position. Pin 9 is coincidently the ninth output position. Therefore, GM1 will have to pulse eight times before electricity will flow through P1.
As long as no power is going into pin 13, the chip will cycle the outputs continuously every time pin 14 is pulsed (outputs will cycle from zero to nine, then start back on zero again). If pin 13 is connected to 12vdc positive, then the chip will ignore any pulses on pin 14, and prevent the output from cycling away from its current position. I have connected pin 9 to pin 13. With this configuration, once the chip cycles to pin 9, it will cause the piezo buzzer to sound, and the buzzer will continue to sound as long as the circuit is powered.
When a particle of gamma radiation strikes GM1, electricity flows from V1 through the transistor Q1 and back to V1. This stimulates electricity to flow from B1, through the other side of transistor Q1, through IC1 (causing the chip to cycle one output), and back to B1. After the eighth particle strike on GM1, the IC chip will have cycled to a position where electricity can flow from B1 (via IC1), through P1, and back to B1, causing the piezo buzzer to sound continuously thereafter (see illustration 3 for a similar schematic).
The remainder of the circuit is dedicated to the reset timer. Tests were made to determine the approximate rate of background radiation striking the detector. This included cosmic rays and natural earth rock radiation. Based on these tests I determined the maximum amount of radiation particles per minute expected to be detected by the GM switch from background radiation, added a slight buffer, and set the timer rate accordingly. Tests in the proximity of a dentists X-ray verified that the chosen timer rate was not too fast as to squelch legitimate radiation detection.
Tip: Increasing the pulse rate of the reset timer will proportionately Decrease the canaries sensitivity (and visa-versa).
When 12vdc positive is applied to pin 15 of IC1, the chip will always reset the output back to the first output position (regardless of whether pins 13 or 14 are being triggered). I connected pin 15 to the output from the timer circuit. Every time background radiation causes GM1 to pulse, IC1 will advance the output. Before the eighth background induced particle strike however, the timer circuit will reset IC1. Therefore, the radiation canary should never squawk unless in the presence of significant radiation levels.
If the canaries alarm does sound, it will only do so for a short time, before the timer resets IC1 and turns off P1. If you have moved the detector (and hopefully yourself) far enough away from the triggering radiation source, the alarm should not sound again. If however the detector is still within close proximity to a radiation source, the alarm will again sound (the GM switch will have pulsed enough times to sound the buzzer before the timer can again reset the chip), and the process will repeat all over again.
IC3 is the timer. I chose a TLC555 timer [Radio Shack 276-1718]. To power the chip, a 12vdc positive is applied to pin 8 and a 12vdc negative is applied to pin 1. The remaining pins are wired as shown in the schematic. The values of R5, R6, and C1 determine the timers pulse rate. For my purposes, I chose a 10K ohm resister for R5, a 220K ohm resistor for R6 [Radio Shack 271-1350], and a 47uf condenser for C1 [Radio Shack 272-1015].
Please note that the output from this timer is a signal pulse that is mostly on (sending a 12vdc positive signal) and briefly off. This is the exact opposite of the kind of pulse desired to be injected onto pin 15 of IC1 (as long as 12vdc is being injected onto pin 15, the signal on pin 14 is disabled). In order to achieve the opposite effect (a signal mostly off), I added a NAND logic gate [IC2] into the circuit to reverse the signal. Although a single NAND gate was all I needed, the smallest chip available from Radio Shack is a 4011 Quad NAND gate (four gates on one chip) [Radio Shack 276-2411].
To power IC2, a 12vdc positive is applied to pin 14, and a 12vdc negative is applied to pin 7. I chose to use the gate located on pins 11-13. Normally a 12vdc positive will flow out of pin 11. If however a 12vdc positive is applied to both pins 12 and 13, the electricity flow will stop.
The output from IC3 (pin3) is tied to both pins 12 & 13 of IC2. The output of IC2 (pin 11) is tied to the reset pin 15 of IC1. Because there is normally a 12vdc positive being emitted from IC3, the output from IC2 will be turned off. However in the brief period when IC3 is turned off, IC2 will be allowed to output a 12vdc positive signal which will trigger IC1.
Note that there is also a pulldown 10K ohm resistor [R4] connected between pins 11 and 7. This is used only to ensure proper functioning of IC2 (and for flow-tracing can therefore be ignored). The pins 1-2 & 5-9 belong to the other three unused NAND gates, and are tied to pin 7 simply to prevent spurious signals from entering the IC chip (so for flow-tracing these connections can be completely ignored).
Like its namesake, I believe the Electronic Radiation Canary to be an effective early warning device for persons concerned about the possibility of encountering radioactive environments. Outside a laboratory it is difficult to find sources of radiation to test with. However, the device proved effective at detection when placed in the close proximity of a radioactive mineral sample, and when placed in the presence of an operating X-ray machine (it would be of great help if someone would develop a "sensitivity" table based on different reset timer pulse rates).
Lowest cost was the primary criteria for the choice of components (Caution: To reduce premature failures, make sure you use components rated for High Voltages!). However, if one is willing to spend slightly more money, EMCO makes a very tiny DC conversion module to substitute for their GP06 that would allow construction of a Canary no larger than the size of a pager. A further developed Canary circuit makes a good Science Fair project.
Late News: Radio Shack has discontinued the majority of its parts line. Therefore some of the part numbers listed here are now invalid. Feel free to substitute equivalent parts from other vendors.
FYI: here is a link for persons interested in preparing for Radiological Incidents (also has info on Exposure Doses and such).