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B efore we start, we should perhaps emphasise that there is no way that you could rely on a simple breath tester such as this to determine if you’re fit to drive (or operate machinery, etc). While it will give you an indication that you are at or over “oh-five”, it could just as easily be reading slightly under when the boys in blue say you’re slightly over. So be warned – this is not a legal instrument and makes no pretence to be one. Then again, what do you expect for less than $30? (The “real” units cost many hundreds, even thousands of dollars!) Also, you may not realise that when you’re asked to provide a By Ross Tester roadside breath test, even those (no relation to Breath Tester) testers are only supplying a relative reading. If this reading is between the alcohol in your blood“over the limit” you will be destream and the alcohol in your breath tained for testing on one of the larger, but there are reasons why it could be very accurately calibrated machines. different. Only at this stage do the Police obtain (A quick tip: it’s often higher in the a blood alcohol level reading which bloodstream than on the breath, so if will stand up in court. you’re determined to demand your Even then, you usually have the “rights” you could end up in even right to demand a blood test to deterdeeper doggie doo-dah!). mine the actual blood alcohol content By the way, just in case you’re (or BAC) because that’s what you are thinking “gee, I wonder if all this is actually charged with. from experience” the answer (touch There is a very close correlation 28  Silicon Chip wood) is no. But I am the proud owner of a “responsible service of alcohol” course certificate. So there! Our breath tester Now we’ve covered the legalities, let’s get on with the project. It’s built into a disposals case and has an alcohol sensor at one end, into which you blow. This detects the amount of alcohol on your breath and registers it on a series of LEDs, the more LEDs alight, the more alcohol you’ve consumed. It is powered by 12V DC, for example, from a car cigarette lighter (a suitable lead & plug is supplied in the kit). As such, it could be left in the car – but once again, please don’t rely on it!!! The circuit is by no means new – it’s been around for perhaps twenty years. In that time there have been significant advances in the alcohol sensor, which forms the heart of the unit. The display side is based on an old friend, the LM3914 LED driver. The sensor A high quality, thick film semiconductor gas sensor is used to detect the presence of alcohol on the breath. There are many different types of gas sensors – some detect the presence of the various “fuel” hydrocarbons such as propane, octane, etc and are used extensively in bilge alarms for boats. The sensor used in this project, It might not quite stand up in court but this simple little breath tester can give a quite reasonable indication of your soberity sobrity sob how much you’ve had to drunk. It’s easy to build – and fun to calibrate! though, is made to detect alcohol – specifically ethyl alcohol. In the presence of gaseous alcohol, its resistance decreases dramatically, in direct proportion to the amount of alcohol detected. It is this feature which makes it ideal for our project. Incidentally, while each sensor is made to detect a certain type of gas there is almost always some sensitivity to other hydrocarbons, so the reading may be quite erroneous if there are other gases about. As you might imagine, the sensor is extremely sensitive. It must detect the presence of alcohol at better than one part in 5000 (.05%). Not only that, the sensor needs to be free of any contamination – even lying around not even turned on it can be affected by other gases. For this reason, the sensor has an inbuilt heating element which “purges” the sensor unit itself so the readings are of the gases detected at that time and not before. The relatively heavy current drawn by the heater (125mA) means that battery operation of this unit is not really a proposition. Yes, it could be done but you’d be always changing or charging batteries. So we have a sensor which has an element (actually two elements) October 2000  29 Fig.1: the circuit could hardly be much simpler! A semiconductor sensor changes resistance in the presence of alcohol, varying the voltage at pin 5 of IC3. This in turn lights the appropriate LED(s) to indicate the amount of alcohol. which change resistance in the presence of alcohol. Now all we need to do is detect that change and display it. Detector/display As we mentioned before, the display uses the LM3914 LED driver, a device we have often used in such things as audio level meters, fuel mixture meters and similar projects which require a LED “bargraph” display. This IC is designed to have a LED connected to each of its ten output terminals and will progressively light the LEDs according to the voltage at its input (pin 5). If the voltage is low none or few light but as the voltage increases, more and more LEDs will light up. (This of course assumes we are using the LM3914 in its “bargraph” mode (ie, it lights all LEDs in a column from the minimum up to the LED indicating the reading). It can also be used in “dot” mode by leaving out the link – only the LED indicating the reading is lit in this mode. We use a selection of green, orange and red LEDs to simulate OK, borderline and not OK alcohol levels. Ideally, the orange LED, or perhaps the transition between the last green and the orange should occur at the 0.05% alcohol level. 30  Silicon Chip The range of voltages over which the LM3914 responds is set by the ratio of resistors connected between pins 7 and 8, and 8 and ground. These are set up to match the output levels of the sensor. Back to that input, pin 5. As you can see, it is connected directly to one of the elements of the sensor, pins 4 and 6, with a resistor and pot connected to earth. The other element, pins 1 and 3, Parts List 1 Breath Tester PC Board* 1 Plastic Case* 1 Cigarette-lighter plug & lead* Semiconductors 1 alcohol sensor (IC1) 1 LM7805 5V regulator (IC2) 1 LM3914 LED Driver IC (IC3) 5 3mm green LEDs (LED 1-5) 4 3mm red LEDs (LED 7-10) 1 3mm orange LED (LED 6) Capacitors 1 100µF 25VW PC electrolytic 1 10µF 16VW PC electrolytic Resistors (0.5W, 5%) 1 1.8kΩ (brown-grey-red-gold) 1 2.7kΩ (red-violet-red-gold) 1 3.9kΩ (orange-white-red-gold) 1 10kΩ PC-mounting trimpot * see text is connected to the +5V supply from the 7805 5V regulator. Remember that the resistance between the elements is variable. This then forms a “voltage divider” across the 5V supply. Normally (ie, no alcohol detected) the sensor resistance is high, so the voltage at pin 5 will be low. But when it detects alcohol the sensor resistance drops, so the voltage at pin 5 will be high. The actual voltage is in direct inverse proportion to the sensor element’s resistance, which itself varies in direct inverse proportion to the amount of alcohol detected. The 1.8kΩ resistor and 20kΩ pot are used to calibrate the circuit – we set the LED display so that the orange LED lights when 0.05% alcohol is detected. We’ll look at calibration shortly. The 100µF and 10µF capacitors on the input and output of the 7805 regulator help to smooth out any variations in the supply and also to remove any noise on the supply line, both of which could affect the accuracy and/or readings. Building it All components, including the gas sensor, mount on a single PC board. There are only a few components to solder in place but as usual, it is wise to start with the lowest profile Fig.2 (above) is the component overlay. At right, reproduced same size, is the completed PC board connected to a section of the PC board from the disposals case. The only reason for using this board is to take advantage of its 2.5mm DC socket. Note there is an error in the PC board overlay – the green LEDs go to the left and the red to the right. ones (ie, the resistors and trimpot) first. Next move on to the capacitors, the regulator and the IC socket. All of these must be soldered in the right way around or your circuit will not work. Also, the regulator pins must be bent down 90° so that it lies parallel with the PC board. A hole is provided for securing the regulator to the PC board but this is not really necessary. You need to decide whether you want a bargraph display or a dot display. If you want a bargraph, use one of the component lead off-cuts to solder a link onto the PC board where shown. If you want a dot display, don’t solder the link in. When you come to solder the sensor in, you will note that there is nothing to tell you which way around it should go – the six pins fit in the holes two different ways. While we said before that this was a semiconductor device (and therefore you might expect it to be polarised) this particular semiconductor can go either w­ay around. The sensor is meant to mount high off the PC board – poke only enough legs through as you need to for reliable solder joints. The last components to be soldered in are the LEDs and we have left these until last because these need special treatment. They need to be lined up in both the vertical and horizontal planes. Before poking them through their holes, though, a warning: some PC boards may have the screened printing (on the component side) wrong as it applies to the LED positions. Holding the board so its label (K080B Breath Tester) is at the bottom, the RED LEDs go on the left and the GREEN LEDs go on the right. The orange, of course, is between them. The symbols for the LEDs show a flattened side (the anode) all facing the same way. Make sure you put the LEDs in this way around or they won’t work! Now, how do you line them all up? Simple: holding the LEDs so they won’t fall out, turn the PC board over and support it long its edges on two books or blocks about 25mm thick. The LEDs will all drop down to the bench below and be exactly the same height. Solder just one of each LED’s legs in while the board is supported like this (it doesn’t matter whether you solder anode or cathode). Then turn the board over and check the horizontal alignment – if necessary, push the LEDs until A close-up view of the PC board end-on, showing how the alcohol sensor mounts up in the air (as do the LEDs behind). The reason for this is so both poke through the front of the case when assembled. October 2000  31 300mA and up would be fine) go right ahead. The regulator will cure any of the ills (hum, noise, etc) from the plugpack. Just make sure you get + to + and – to – or you will probably do some damage! The case You’ll need to drill 11 holes in the case – a 17mm diameter hole for the alcohol sensor (top) and 10 3mm holes for the row of LEDs, seen here just above the label. they are all lined up perfectly and equally spaced. Then turn the board over again and solder in the other leg and clip all the excess legs off. Apart from the power lead, your Breath Tester PC board is finished. Power In the kit there is a car cigarette-lighter lead. Its plug has a LED to show you power is applied and inside the plug is a 3A fuse (accessed by unscrewing the plug tip). On the other end is a moulded 2.5mm DC line plug. You can attach power in one of two ways – sacrifice the plug and solder the lead directly to the PC board; or connect it via the PC board which comes with the surplus case. This board has a 2.5mm DC socket so it’s simply a matter of plugging in. We’ll look at the second option in a moment. If you want to solder the lead direct to the board, cut the cable as close as you can to the plug (it’s only a 600mm cable so you haven’t got a lot to play with). Strip back 10mm of outer insulation to reveal the twin cables inside. Now here’s a trap for young players: the white wire is connected to the centre pin of the cigarette lighter plug which, of course, makes it the positive. That means the red wire is the negative! Yes, red is negative. Don’t say you weren’t warned! Strip 5mm of insulation off each of these and solder each to their appropriate points on the PC board. Of course, there’s nothing in the good book which says you MUST run this project from a cigarette lighter. If you want to run it from a DC plugpack (anything from 7-12V or so at about 32  Silicon Chip The Breath Tester is assembled in a surplus case which used to hold a mobile phone in a car. It fits quite nicely and requires only a little surgery, mainly to expose the sensor unit itself. First, though, we’ll look at power. We mentioned another PC board a moment ago. With a little ingenuity and thought (or is that thought and ingenuity?) you can cut this board so that it still fits in the bottom of the case, with its 3.5mm DC socket intact. That way you can simply plug the cigarette lighter lead in without cutting the plug off. The main Breath Tester PC board can then occupy the rest of the case, as our photographs show. Of course, you will have to solder a couple of wires (insulated hookup wire) from suitable tracks on the cut-off PC board to the Breath Tester PC board but this shouldn’t prove difficult. You don’t need to connect power –simply plug the DC plug (on the cigarette lighter lead) into the DC socket and with your multimeter on an “Ohms” or “continuity” range, find the points on the board that show zero Here’s how it all fits together inside the disposals case (which was once a car adaptor for a mobile phone). No screws are used to hold the boards in place –they simply drop over the mounting posts and are held tight. ohms (or close to it) between the tip of the cigarette lighter plug (the “+” connection) and the springy metal bits on its side (the “–” connection). Connect these points via short lengths of suitably coloured insulated hookup wire to the + and – power connection points on the Breath Tester PC board. Now, about that surgery we mentioned. The alcohol sensor needs to have a good airflow around it to detect properly. For this reason, the end must poke through a 17mm diameter hole in the case. This hole is drilled where the speaker used to go in the disposals case – right in the middle of the holes which let the sound out. In fact, the extra holes around the sensor are useful in increasing air (gas) flow. The other holes required are of course for the 10 LEDs – not much point in having them flash merrily away inside the case, is there? The holes for the LEDs should be 3mm in diameter, spaced 5mm apart. Only the tips need to emerge through the holes. These holes are drilled in a straight line across the front of the case, exactly 45mm down from the middle of the sensor hole, as shown in the photo. In the kit, a printed label is supplied. This fits perfectly in the “well” in the disposals case and can be glued into position after being cut to shape and size. However, it will need to be protected with some clear self-ad- What we are aiming to do is set the Breath Tester up so the orange LED comes on at 0.05% BAC. Start by adjusting the calibration pot so that the lowest LED just comes on. Now, according to that Responsible Service of Alcohol course I was telling you about before, 0.05 corresponds to 3 standard drinks in one hour (or less). Unfortunately, there is no easy way (at least for the lay person) to calibrate to this standard so, with much regret, we’re going to have to ask you to drink 3 middies (or 15oz/385ml glasses) of full strength beer during the next hour. We can only apologise for putting you through this but we haven’t been able to think of any other way. Honestly, we wouldn’t ask you to do this if it wasn’t absolutely necessary in the interest of global peace and Fig.3: full-size artwork for the Breath harmony and the closer alignment of Tester PC board, viewed from the the galaxies. Just make sure the beers copper track side. Use this pattern to check your PC board for any defects (eg, are nice and cold! bridges). If beer is not to your liking, you can substitute a nip of spirits or a 200ml hesive contact film or other suitable glass of wine as a “standard drink”. material. We’ve got to warn you, though, Construction is now complete – all that wine varies all over the shop in that remains is calibration. its alcohol content so could be less First of all, though, you need to accurate. (Full strength beer is not so apply power for at least an hour, variable). preferably several hours, to allow the And just as importantly, this methheater to completely purge the sensor od does not take into account differof any gas residue it has picked up ences between males and females nor since being manufactured. metabolic rates, bulk, whether you’ve (A long purge is normally only eaten food, etc – all of which make required for a new unit. Later purges a significant difference to a person’s will be done much more quickly – just apparent sobriety. a few minutes is usually tons of time). How are those drinks going? The When you apply power, turn the first one didn’t even touch the sides calibration pot to both extremes of and you’re into the second already? its travel. One way you should see Hey, slow down a bit: you have to be all the LEDs come on (or the top LED of sound mind to calibrate this thing. in dot mode). If one or some of the OK, so we’re too late. Call in a mate LEDs don’t light, the odds are about and make sure he doesn’t touch a drop 100:1 on that you have it or them in till it’s done. back-to-front. Come to think of it, that’s a good Perhaps the easiest way to check move. Once calibrated he can have that it is working is to open a bottle three standard drinks and see what of metho and waft the fumes over the his reading is! sensor. Metho is almost pure alcohol Ready? Good. You’ll need to wait, so you should get an instant response! say, 15-20 minutes after your last drink to make sure it’s well and truly Calibration in your system. If you’re younger than 18 years, you Blow into the sensor and see if any must not read the following section. more of the LEDs light. You may need Avert your eyes immediately lest a fate to do this several times, adjusting the worse than death befall ye. calibration pot a little each time. Are they gone? Great. We couldn’t You should be able to get it to the get into the serious part of calibration point where all the greens and the oruntil they’d left. ange LEDs come on each time you blow The cigarette-lighter lead supplied with the kit has an integral fuse and LED plus a 2.5mm DC plug. You can cut this off and solder direct but be warned: red is negative, not positive! into it, then slowly die down again. If all you get is red LEDs, you’ve either got the calibration pot way too high – or you’ve sneaked a couple more beers without us looking. Once you get it to work, get your mate to repeat the process. Remember it takes a while for the alcohol to enter your bloodstream and then your breath so results do change significantly over time. And to finish… Finally, let us repeat the warning we started with. This device must NOT be relied on to give any real test of sobriety or otherwise, let alone the ability to drive. Only the Police Breath Analysis Unit or a hospital blood test can do that – by which time it is probably too late. SC Where do you get it? The Breath Tester project, including the PC board pattern, is copyright © 2000 Oatley Electronics Pty Ltd. They have available a complete kit of parts, including the disposals case and the cigarette-lighter lead, for $29.00 inc GST plus $7.00 pack and post. Contact Oatley Electronics at PO Box 89, Oatley NSW 2223, or phone (02) 9584 3653, fax (02) 9584 3651, website www.oatleyelectronics.com.au October 2000  33