Remote control (part 2)

~ 12 Sep 2014, 01:09

In the previous post I promised to tell you more about the remote control of household loads I made. But first, some background...

Last January I had to do some small house renovation, and I contacted this guy, recommended by friends, who demanded some outlandish amounts for the work. Waay above the market average. It was my mistake, of course - I had told him what I did for living. In Bulgaria, it seems, a lot of people think that programmers dig boatloads of money, so no price is too high if you client is one of those. For that reason, the guys I actually hired later had it that I work as an Electrical Engineer. The cables, breadboards and soldering iron scattered around helped with that ploy ;)

Well, it was a bit unexpected when it came to laying and connecting power wires. The workers left that to me - after all, I should be very good at this kind of stuff. In my case, I ended up with a bunch of spotlights that I had no way of connecting: the only lightswitch in the kitchen was a simple one: a live wire comes from below, and a single wire continues toward the ceiling lamp. Even if I managed to bring neutral from somewhere, I had to dig channels in the walls to reach the spotlights.

For that reason I wired the lights to regular hidden wall sockets, with lazy switches to command them. But this was all meant to be a temporary solution; it was inconvenient to operate them, and I definitely wanted a centralized lighting control, from a single switch.

In the same time, I had bought some RF modules long ago (these ones). They use a 434 MHz band and are intended for hobby work, with pins to be directly inserted into a breadboard. They are capable of transmitting "1 pin" of information - whatever voltage you send to the input pin of the transmitter gets generated on the output pin of the receiver. So the idea I got was to create a circuit that stands in the way between the hidden sockets and the spotlights, where the receiver controls a few relays. And the sender would be used into a miniature circuit, that would fit into the wallswitch socket (without any mains connection there).

As mentioned, the 434 MHz modules are hobby-oriented, and even though the interface to them is very straightforward, the abstraction they provide turns out to be a really flaky one. First, the state transmission is discretized in time - 4800 bps - so it's quite slow. I didn't need speed, so this wasn't an issue, and I initially envisioned a purely "analogue" circuit without any logic - one side has a switch, directly controlling the transmitter, and the other and has the receiver hooked up to some transistors, which command the relays. Well, probably with a RC circuit in between, to filter out any noise. But I was told by somebody about the flakyness of these modules so I went on to check the feasibility of the idea.

Indeed. When the transmitter is turned off, the receiver gets some random noise. Even when the transmitter is powered (and it was expected to be only intermittently), there were bursts of wrong bits on the receiver side - so the necessity of digital logic, employing some kind of error-correcting code, was apparent, least not if I wanted a random disco in my kitchen :)

I prototyped the two parts of this thing on a breadboard first. The sender/receiver were communicating well, and as a guard against the random noises, the transmission starts with a fixed "magic byte", which is checked for correctness at the receive end. I turned off the transmitter and must have had forgotten the receiver running for half an hour, when, to my dismay, I saw that the test LEDs powered up. The noise from the ether had managed to create the magic byte, and the transmit side interpreted the noise afterwards as a command ...

I solved that by increasing the magic bytes to four, and I send the command byte twice, for consistency checking. That got it running, but now it was not reaching every time, especially if there was a wall between the devices (these are the cases with YOU SHALL NOT PASS). I.e., the random noise apparently changes some of the bits of the magic header and the receiver drops the message. To combat that: the message gets sent 10 times, hoping that at least one should gets through.

So the prototype was working, and I went on to solder a miniaturized version of the sender::


After trimming:

The idea is that the whole thing must fit into a wall hole for a light switch - which is very limited volume indeed. Luckily for me, the designers of the transmitter module obviously had applications like mine in mind and had made it quite compact:


The perfboard is just for convenience. I could have point-to-point soldered everything:


The thing is finally connected like this:


An interesting problem that took me a while was how to power the thing. The transmitter requires around 8 mA at ≥3V (it gladly accepts more). But at this usage, any normal battery would drain in a week, so I turn off the transmitter when not in use, ditto for the microcontroller, and they're woken up only when needed. The standby consumption is around 0.1 µA, but the active one is a bit high for coin cells like CR2032: under load it would drop below 3V, so they got out of the question. I thought about a 3-pack of silver-oxide SR44s, which would have worked, but their capacity was too small, the device would need replacements every year or two. A similar solution would have been to use an A23 battery, but it had the same issue, and it also required a regulator. A small lithium-ion cell would have been okay on voltage and capacity, but I doubt that a rechargeable one would hold the charge for more than a year. In the end, I found out what they use in the industry for exactly these kind of things: lithium thyonil-chloride. These beasts last for decades, losing only 1% of charge per year in standby, and pack a huge amount of energy per volume. The nominal voltage is 3.6 volts, which is also welcome. The downside of this chemistry is its intolerance to short circuits: it can explode, and its contents are quite toxic. I even got an order rejected from my electronics supplier for this reason - these batteries are considered hazardous materials, so are not readily shipped. Luckily, another supplier had them in stock, even if the smallest size he had was unnecessarily big for my application.

After the transmitter was ready and tested, I got to make the real receivers as well (I needed two, for the two sides of the kitchen).

Here the perfboard would have been ugly and dangerous - having line voltages on random tracks and sticking wire ends is a no-no. So I designed, and LUBO_1 printed me, PCBs for the purpose:


Added: relays, a rectifier, and a fuse holder:


The rest of the electronics:

In the beginning I fitted these fuses:

0.5 amp ones, which should have been enough for my 80 W spotlights. However, loads like this - lamps, especially incandescent ones - have a huge initial surge current, and the fuses blew almost right away. Lesson learned, I bought and installed slow-acting ones.

The receiver is ready:


I made two copies:


Note that the PCBs are identical - they both have tracks routed for four channels. The first board has a relay and transistor only for channel 1, the second has for channels 2 & 3.

The PCB solves the problem with the line voltages dangling around. Nothing can electrocute you on the upper side. For the back side, I had 4 adhesive rubber pads (which I had randomly picked from Lidl, one of these purchases where you regretted you didn't buy more), so the solder joints are also not touching anything potentially conductuve below. I realized afterwards that the fuse holder and the fuse itself are still at line voltages and went on to search for a solution... and indeed, they have standardized plastic covers for these holders:

plastic cover for fuse holder - standardization ftw

The two receivers, ready for mounting:


Channels 2 & 3:

Channel 1:

The digital control logic has its obvious advantages. I programmed a self-timer to turn off the lights if I forgot them for more than 1½ hours.

So this is the main system. For the fun of it, I decided to make a computer-controlled transmitter, as it was shown in the previous post. There are a lot of ways to connect a PC with a microcontroller, but one of the simplest is to use a parallel port. Sadly, these wonderful things are phased away, but I still have a somewhat old home server laying around... The interesting part here is how I managed to power the sending device from the parallel port directly. As you know, unlike USB, the parallel port interface has no power lines, but it's still possible to "steal" a small amount of current though the data pins. The limit is really small, like 2mA in the strictest specs, but usually you can get around 8-10 mA per pin. Just in case, I wired up four of the eight pins just for power (after a schottky diode each) - so I have 8-40 mA at my disposal for powering the transmitter. When you programmatically send a byte through the parallel port and you set the 4 least significant bits, my device there detects that it's fully powered and turns on the transmitter. The higher four bits/pins are used for communications. Commanding the parallel port is made easy by libieee1284, interfaced though a small C program on Linux. And, of course, I did a simple webinterface, that runs on an apache server. So now, I can control my spotlights (or any other load if I wish) via a wall switch, using a smartphone or a tablet, through ssh, or even periodically via cron...

The hardware of this thing is till on breadboard:


Here's the connection to the server:


I was slightly worried whether the RF would work, since the server and the receivers are in different rooms and the signal must go through two walls. Luckily, it seems that the send-ten-times thing works even here.


In the future I plan to utilize more high-level communication modules, e.g. ZigBee, since coding and debugging noise-proof protocols is not as much fun as it seems. For example, while testing the computer interface, I had a very weird issue. There was no way to turn on the first channel, only to switch it off, if you sent a command for switching off the THIRD one. And also, commands for channel 1 affected channel 2, and commands for 2 affected 3. In the beginning it seemed like a simple bug, but after reviewing each part of the stack that turned not to be the issue. It was just... the tranmitter sending out the bits slightly too fast (which I discovered through trial and error). This is a common theme actually - when you have only one pin for communication, you need to make your protocol self-clocking, i.e., you need to ensure that the two endpoints don't go out of sync. UART does this by employing start and stop bits, but I was sending bits verbatim, since, gee, what could possibly go wrong for 48 bits? In the end, slightly lenghtening one delay solved it. So indeed, are you still thinking that printf-debugging, or through a terminal with gdb is painful, ugly, and obsolete? Meh.


#1 by Иван, posted on 26 Sep 2014, 13:43

Оле, само видях Robotev и прекратих всичкото четене за да стигна до долу и напиша това: НЕ!

Цените там са с пъти (някои - ДЕСЕТКИ пъти) завишени цени. Мога да ти дам конкретни примерни, ако искаш. Електроника си купувам от Bangood (ще се въздържа от УРЛ да не ми реджектне поста), Comet/Quazar и разни китайци от Ебей.

Е, ако много бързаш и модула ти трябва за вчера, тогава си плащаш като поп при Роботев.


Така, очевидно трябва да изчакам, защото блога си мисли че го флудя заради другия коментар от преди 10 минути на съседния пост :)

Захранването на трансмитера мисля че можеше да стига до 12V?

#2 by anrieff, posted on 26 Sep 2014, 14:19

Е, аз съм писал - тези модули съм ги купувал преди години, и хич не ме интересуваха тогава цените. Пък особено за единични бройки...

Иначе да, трансмитера е от 3 до 12 волта. Лошото е, при по-високо напрежение, PIC-а му се увеличава осезаемо консумацията в спящ режим, най-оптимално е на 3 волта, ама там не измислих как да го захраня. CR2032 щеше да кляка под товар.

ПП. намалих изискването за интервал между постовете, вече е 60 секунди.

#3 by Николай, posted on 8 Dec 2016, 01:06

Страхотна статия, може ли да се свържа с теб да питам 2-3 неща?

#4 by anrieff, posted on 8 Dec 2016, 01:23

Разбира се, пиши ми на [името на сайта, без ".net" накрая] в "".

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