My build of the K7TMG HF Morse Code Thermometer was fun, and it inspired me to use the same circuit to create a new HiFER beacon to honor my neighbor’s cat Boris. With some of my indoor cat-owning neighbors in the past, I have acted as caretaker when their parents are out of town or at work. I don’t have that kind of relationship with Boris though, so she and I stare longingly at each other through the window when she is out. (Edit – I’ve looked after Boris a few times since first writing this post, so kitty-human relations have definitely progressed!) We are two beings sharing a mutual admiration, but separated by a sheet of glass –
When there’s a kitty who you want to hang out with but can’t, the obvious thing, of course, is to build a little HF beacon to transmit their name in Morse code. It’s the thing to do and so, I found myself building another K7TMG HF thermometer, but without the temperature-sensing circuitry. I also added a 2-section LPF to attenuate harmonics. I used the capacitance and inductance values that Chris Smolinski uses in his HiFER beacon kit, but recalculated the number of turns on the toroids so that I could use T37-6 cores instead of the larger T50-2 ones he uses. I think that the tuned tank circuit in the collector of the oscillator transistor must also help reduce the harmonic output of this stage as the level of the 2nd harmonic at the antenna is further down on the fundamental than I would normally expect from a 2-stage low pass filter like this –
For the firmware, I found a very versatile and useful piece of AVR beacon freeware written by Nick SV1DJG. If you use the circuit above, in which pin 2 of the ATtiny85 keys the oscillator, you’ll need to change the line
#define ledPort 1
in Nick’s code, to
#define ledPort 3
If you leave the output port as port 1, you’ll need to make pin 6 of the ATtiny85 the keying line instead of pin 2. If you want to make pin 3 the keying line, just specify “ledPort 4″Also, in the code, you can specify the message (or callsign/ID) you want to send, the keying speed, and the length of pause before it repeats. My beacon sends the letters BRS at a speed of ~5wpm, with a pause of 2 seconds. If you want to send QRSS with this program, there is also an option to specify the dot length in milliseconds. It is currently set at 1200. The dash length is derived from that, being specified as 3 times the dot length. Inter-character and inter-word spaces are also defined in terms of the dot length, so when you change the dot length, everything else follows.
The build went smoothly, and there’s not too much to say about it. As always, Rex’s MeSQUARES and MePADS did a sterling job of making the process of building Manhattan-style a lot easier. I cut the board to fit into a specific enclosure, and it worked straight off the bat. The trimcap had very little effect on the amplitude of the output signal, and the oscillator started perfectly at all settings. A fixed capacitor of around 47pF probably would have worked as well. There is also room for experimentation with the values of the 2 feedback capacitors, which are 470pF and 330pF in my circuit. Lowering those values will shift the oscillator up in frequency. Two 100pF capacitors should work. You may even be able to go lower in the value of these capacitors. My oscillator came up on a nominal 13556.9KHz. It was a good frequency for me, and didn’t seem to conflict with any of the HiFER beacons listed over at the LWCA website page of MedFER, BeFER and HiFER beacons. Great – no need to change any components!
Unfortunately, a different enclosure was sent from the one I ordered. The one I wanted had 2 external lugs for fixing it to a wall, post etc. The lack of these meant that I had to drill holes and fix it with screws protruding from the inside of the enclosure. It wasn’t ideal, as it meant more holes needed to be sealed to prevent moisture ingress. At this point though, it was the enclosure that I had, so it was the one that I used. It’s a nice weatherproof enclosure, available from China for as little as $3.41 inc shipping, or just a couple dollars more if you want it quicker from a supplier within the US. There are versions with external mounting lugs and clear tops too, if you like that sort of thing. An eBay search for “85x58x33mm waterproof plastic box”, or similar, should show plenty of options –
I wanted to mount this little beacon outdoors and power it exclusively from a small solar panel with no battery. This meant that it would only operate during daylight hours, of course, but I’m thinking that some grey-line action should still be possible, as the beacon will still be operating when locations just to the east are entering their grey-line phase. Living in a rented multi-unit building means that I need to be cognizant of the wishes and sensibilities of others, and I didn’t want to take the chance of a battery exploding inside a very hot enclosure in the summer heat. It’s probably unlikely, but with little previous experience in this area, I didn’t want to take the chance. Besides, the idea of a little circuit that is entirely dependent on the sun in a very direct fashion appeals to me. The panel I used was an old one that I bought cheaply as a lot of two, from a fellow on eBay who decided he didn’t want it, immediately after purchasing it. When drilling and filing the hole in the enclosure for the cable from the solar panel, I was careful to keep it as small as possible, so that sealing it against the elements would be straightforward –
Boris seemed to like it –
The plan was to install it on top of a fence on the property line of the building I live in. Sitting on top of the fence, the solar panel would receive light until fairly close to sundown with little obstruction from nearby buildings. My tube of silicone marine grade sealant had dried up, so I decided to try using a product called Plastidip, a can of which I had on hand. It’s a black rubbery solution that comes in an aerosol can. You spray it on, and it forms a weatherproof seal. I’ve used it successfully in the past for sealing the ends of coax at dipole center-feed insulators, so figured it should be usable in this application too, though perhaps not quite as easy to keep to a small area as squeezing silicone sealant out of a tube. Here’s a close-up of the beacon just below the top of the fence –
I sprayed the the screws that fixed the enclosure to the fence with Plastidip, and at this point began to wish that I had either held out for the enclosure with the external lugs, or at least used silicone sealant. I had forgotten how very liquid Plastidip is before it sets. Much of it dribbled down to the bottom of the enclosure and pooled. You can see it oozing out from the bottom of the board in the next shot. I’m not sure whether it conducts when in the liquid state, but I didn’t much like this. Plus, it just looked messy –
All this time, I had been monitoring the beacon signal with my K2 on a battery, to make sure that I didn’t break any connections during installation. Strangely, at this point, the beacon had stopped, and was just emitting the occasional dit or dah. I guesssed (incorrectly, it later turned out) that perhaps the liquid Plastidip was conductive, and was the cause –
I pulled the board out, cleaned up the oozing mess with Q-Tips, then reinstalled it in the enclosure. Poking around the micro-controller with my fingers, the beacon sprang back to life. I wasn’t able to determine exactly what had caused the problems, which concerned me. Unfortunately, I was pushed for time, as I was trying to complete the installation before one of my neighbors returned, a woman with whom, sadly, relations have completely broken down. It’s a long and uninteresting tale but at this point, nothing I can do or say will help things. It seems that I have been identified as a mortal enemy. The fact that she doesn’t like cats doesn’t help either 😀 At this point, I decided to press on with the installation as swiftly as possible. I stapled the dipole antenna just underneath the top of the fence in both directions, and mounted the solar panel on top with two short screws –
This is the type of install I was aiming for – unobtrusive. My neighbor on the other side might see the solar panel, but I was hoping that they wouldn’t mind. You never really know with folk what will bother them and what won’t. It’s at times like this that I can see the advantage of owning my own place with a big plot of land in a lesser populated area. The dipole is horizontal and only about 8 feet above ground level, so it’s probably a bit of a cloud-warmer. Definitely a compromise installation –
I went back inside and, monitoring from indoors, was happy to hear a good signal coming from the beacon. The letters “BRS” were being sent at 10wpm (my initial setting) with a 2 second pause before repeating, and absolutely no chirp on the signal. Monitoring the signal on and off throughout the rest of the day, I was happy to note that it stayed on the air for about 2 1/2 hours longer than it had when located indoors with the solar panel mounted in the window. It continued to transmit until about 48 minutes before local sunset. Exposure to direct sunlight makes a huge difference to solar panels. If I had been able to angle the panel toward the sinking sun, no doubt I could have eked out a bit more time on the air. All was good. I was happy, and fell asleep that night with the K2 on, expecting to wake up to the next morning to the sweet sound of the letters “BRS” singing from my K2.
Instead, I awoke to the sound of a minute or two of dits, with the occasional pause, a few meaningless dits and dahs, then another minute or so of dits. Perhaps as the sun rose higher in the sky, the situation would correct itself, I thought. It didn’t however, and at 10:30, with the sun fairly high in the sky, and more than enough light to power the beacon, it was still sending out long series’ of dits, punctuated by occasional pauses, a few dits and dahs, and then the next long series of dits. Not a BRS to be heard anywhere. This was disappointing, and not what I expected. I decided to dismantle the installation, so that I could take my time trouble-shooting inside, as opposed to at the top of a step ladder. Bringing the beacon inside, I put the solar panel in the window and – lo and behold, the beacon started up perfectly, sending out it’s BRS callsign exactly the way it was supposed to.
So – why wasn’t the micro-controller starting up properly in the morning? At this point, I did what any modern 21st century renaissance man would do, and Googled it. A few others had experienced this exact same issue, of an ATtiny not starting correctly when powered just by a solar panel with no battery. One explanation offered in a forum seemed very likely – that when the solar panel is beginning to receive light, as the voltage gets to the point that the micro-controller wakes up, a small panel still isn’t capable of supplying much current. Anything else in the circuit that draws current, such as the crystal oscillator, will cause the voltage to drop below the point at which the micro-controller can operate properly. At this point, I was using a 78L05 regulator, which was drawing ~4mA of quiescent current. It’s not a lot, but when light is low, and the panel is only supplying a few volts, that extra current draw was most likely enough to cause the voltage to sag when the oscillator kicked in. Listening to the beacon, it seemed very likely that this is what was happening. The ATtiny, in the low light, had enough current to operate, so it turned the keying pin high, at which point, the oscillator began drawing current. However, that extra current draw caused the voltage to sag below the point at which the ATtiny could operate. As a result, the ATtiny turned off, the keying pin went low, the oscillator turned off, the voltage went back up, and the whole process started over. This gave way to the transmission of a constant series of dits, instead of the beacon callsign. Unfortunately, as the sun rose higher, and the light level also rose, the ATtiny was not recovering. What was needed was to set the BOD (Brown-Out Detection) to a voltage level such that when the voltage from the panel equals or exceeds this voltage, it is also capable of supplying enough current to the entire circuit without the voltage dropping below the BOD voltage. I reset the BOD to either 2.7V or 4.3V (I forget), from it’s previous level of 1.8V and this seemed to solve the problem. However, with the beacon in it’s new (temporary) position indoors, with the solar panel in the window, the higher BOD meant that the beacon often didn’t come on until late in the morning, due to the fact that a) it was a small panel and b) panels in windows tend to generate much less power than panels outdoors.
I wanted to make this little setup as efficient as possible before putting it back outside, so swapped out the voltage regulator for the one shown in the schematic – a 5V LP2950. This series of regulators has a much lower dropout voltage than the 78L series – between about 0.04V and 0.38V, depending on current draw. They also have much lower quiescent current, at less than 0.1mA, compared to around 4mA for the 78L series. My final version of the Boris Beacon had an LP2950, and the BOD on the ATtiny85 set to 1.8V. You can do this with the 10PU version. By contrast, he lowest BOD on the 20PU version is 2.7V. It worked like a charm! I’ve had the beacon in the shack, powered just by the small 1.8W solar panel sitting in an east-facing window. It starts running early in the morning, even on overcast days, and stays on until fairly late in the afternoon. It would run for even longer hours if the panel was mounted outside. This was a very encouraging result.
You’ll notice that the capacitor on the input side of the voltage regulator is shown as a 100uF part. Normally, I’d use something in the range of 1 – 10uF, and I did start with a 1uF cap in that position. A larger value capacitor in that place helped to smooth out the voltage swings when the light level was marginal. When the ATTiny was beginning to send a semi-random series of dits, due to the sagging voltage issue previously described, a larger value capacitor helped to mitigate that somewhat. A 330uF, 470uF or larger part could help a little more but ultimately, when the light level falls, it falls. At most, I doubt that a large cap here would buy you more than an extra few minutes of operation at the very beginning and end of the day. Another idea for experimentation would be to try a different transistor. I’m wondering if, say, a 2N2222A would provide a little more power?
At this point, I feel that the experiment is complete, and am not feeling the need to mount it outside again. It would be interesting to see if the mighty 1mW RF output could earn me any spots, but that was really not the main purpose of doing this. My primary motivation was an interest in the circuit – building it, and optimizing it. I did order another enclosure, with lugs, which would be perfect for outside mounting. Alternatively, this case could, as well as housing the circuit board, effectively act as the center part of a dipole, with the lugs acting as strain reliefs. The wire carrying the power could hang down from the center of the enclosure –
The above case was bought on eBay, from a US seller, for $4.83 including shipping. I saw what looked like the same case from a Chinese seller, for the lower price of $1.56 + $2 shipping, so bought that as well, to compare the two. Interestingly, the cheaper one directly from China looked like exactly the same case, but of inferior quality. It looked like it had been cast from essentially the same mold, but wasn’t as nicely finished. One imperfection almost made for a bad seal with the lid. I intend to purchase a few more of this case, for future use, but will make sure to get them from the US seller. For sealing the holes where the wires enter, a silicone sealant should be more practical than the rubbery Plastidip that I had used earlier. This stuff should do the job nicely –
Another idea for an outdoor beacon enclosure would be an electrical junction box. I found this in my local hardware store. It’s 4″ x 4″ by a little over 2″ high. It has a rubbery seal around the lid, and is certified for burial, so should certainly withstand the weather above ground. It also has 4 lugs on the outside for fixing the enclosure to a wall, fence, or post. For securing the circuit board, the adhesive standoffs pictured should work well, so that you don’t have to drill holes in the enclosure. They are available for a 5/16″ hole, and in several different lengths. The ones pictured hold the board 3/8″ away from the box, and were part # 91443A130 from McMaster Carr. Someone in one of my discussion groups, when talking about outdoor enclosures for transmitters, suggested that if you have a completely sealed box, with no ventilation, it might be an idea to add a silica gel packet or two, to prevent condensation from forming inside the enclosure. I haven’t had any issues with the completely sealed non-metallic enclosure that houses my outdoor Part 15 AM transmitter, but it’s a good idea, and definitely worthy of consideration –
This little beacon has been greeting me in the mornings for the last few days, with the letters “BRS” sent at 5wpm. I rather like the fact that it comes on every day with the light, and goes to sleep at night – the way that we all did before gas and, especially, electric lighting came on the scene.
UPDATE – The Boris Beacon is now on the air. Details here!