Category Archives: Electronics

Programming a Picaxe 08m chip

After I got my Arduino I felt the urge to brush up on my general electronics knowledge. The last time I’d really played with any circuits was back in about 1994 when I was studying my Technology GCSE – which now is quite a long time ago. So I picked up a copy of Make: Electronics and started reading through it. Sadly I was a bit lazy and just read through the book, rather than actually building many of the circuits suggested. It did come in handy for tips on soldering and I do intend to go back and make some of the circuits – I just got a bit distracted by a new arrival. However the last chapter covered microcontrollers and in particular the Picaxe 08m chip. This really piqued my interest!

The Picaxe 08m is a PIC chip with a pre-installed boot-loader that allows programming the chip via a serial port. This is similar to the way that the Arduino has an AVR chip with a pre-installed boot-loader. There’s also some software for the Picaxe for handling the programming, a bit like the Arduino IDE.

The 08m is massively less powerful than an Arduino. It has 512 *bytes* of program storage and only 14 *bytes* of variable RAM. However it has 5 outputs/4 inputs and can run a single servo. In fact I could have used it for the logic part of my Arduino doorbell. It’s not quite as friendly to get started with as an Arduino though. You need to wire up a simple circuit before you can get going. In fact I actually had to solder together a prototyping board, as I didn’t have much luck trying to used a breadboard. The circuit needed isn’t that complex (assuming you use 3AA batteries for power). The cost of the 08m chip is less that £2. The proto-board is £2, so altogether you can have a simple usable microprocessor for less than £4. It’s also small and doesn’t use much power. So for projects where an Arduino is overkill it’s perfect.

Soldered picaxe 08m prototype board

To make testing out circuits easier I also added a female header to the board, so I could plug in breadboard jumper cables (in the same way as one does with an Arduino Duemilanove):

Picaxe 08m prototype board with header attached

To verify everything worked I wrote the “hello world” program of microcontroller world, a program that simply blinks an LED (by turning pin 1 on and off):

main:
	high 1
	pause 1000
	low 1
	pause 1000
	goto main

That’s Picaxe BASIC by the way. Maybe not as pretty as C, but for a processor with only 14 byte sized variables it’s more than enough.

Then I wired up an LED into the prototype board, along with a small current limiting resistor:

Picaxe 08m prototype board + header blinking LED

Using the Mac version of AXEPad and the Picaxe USB serial cable (which has a stereo socket at one end) I uploaded the program to the 08m chip and saw the LED blinking once every second. I’ve recorded a video of the programming, which should show how straightforward it is:

I’ve got a few projects in mind that are actually space/weight constrained and one in particular that might put the parts involved into extreme peril, so having the option of using a very small and cheap microcontroller is very appealing. I’ll reserve the Arduino now for projects that need it’s extra power and amazing flexibility.

“Ultimate” Arduino Doorbell – part 1 (Hardware)

After a first couple of small Arduino projects I felt the need to make something a bit more useful and permanent.

I had seen Roo Reynolds talking about hacking his doorbell to get it onto Twitter and as our doorbell is a bit rubbish I thought this seemed like a good project. His hack only updated Twitter, so there was no direct physical sign that the doorbell had been rung. Given that our current doorbell is pretty rubbish anyway (it should ring, but just makes a rattling sound), I decided to start off with the physical/audible side of things and then later move on to pushing notifications out to laptops on the local network or beyond…

I’ve been documenting my progress on Flickr now for a while.

Take apart wireless doorbell

Following Roo’s approach, I picked up a cheap wireless doorbell from Tesco for about £9 and took it apart:

I was able to verify that I could turn an LED on/off with the doorbell at this stage, so I knew I could use that as a signal in the Arduino. Using a multimeter also let me figure out some more details about the voltage and current used to power the speaker:

Testing wireless doorbell with multi-meter

This was about 3.1 V and 75mA output. That’s a few milliamps more than the Arduino can tolerate, so I knew I had to make sure I put a resistor (22KOhm) in between the doorbell and the Arduino. The doorbell would also need to draw power from the Arduino, but luckily the Arduino has a separate 3V output, so that wasn’t an issue.

Testing connecting Arduino to doorbell

From there I verified that when the doorbell was activated I could read a drop in voltage from the wire on the doorbell labelled “SPI”. At this point I had the input for my Arduino.

Servos and prototypes

The next step was to provide some sort of audible output. Originally I had thought about just adding a buzzer, but fancied something a bit more old-fashioned. I figured it’d be good fun if the nice 21st century tech of the Arduino used some rather more ancient technology to create sound – a bell.

Using a micro servo from Oomlout I created a couple of prototypes.

I made a pretty simple circuit to hook up the servo, doorbell and arduino:

The first prototype used some lego and a bell from a Christmas ornament:

This worked, but the servo was probably louder than the bell!

Next I got hold of a small brass bell from a music shop. Luckily the top unscrewed and would let me easily attached it to a piece of lego. So another prototype was created using the small brass bell:

Prototype doorbell with brass bell

This worked much better!

The bell is about 100g in weight, which isn’t massive, but when it’s flung around the lego prototype would tend to move too. Not so good when this will live on the kitchen window sill. So I’d need something sturdier for the finished version.

A little soldering

As well as trying out a better bell I also made the circuit for the bell and servo more permanent, by soldering everything up on a piece of stripboard. I also used a few headers, so most of the circuit would plug into the Arduino – like a rudimentary shield. The wire to the pin controlling the servo, was left to be plugged in separately. I hadn’t soldered anything since school, so I ended up swearing quite a bit trying to do this. For me it was probably the trickiest part of the whole exercise, but it did work in the end. Just glad I had a multi-meter with a continuity mode to find the short-circuits – a knife worked well to clear up some of those.

Stripboard soldering

Full soldered doorbell + arduino

Woodwork

I next turned to my (t)rusty woodwork skills to create a sturdier version. First off some pieces of wood to mount the Arduino, doorbell circuit and servo:

Arduino and doorbell mounted on wood block

Servo mounted in wooden block

Next came the base and first part of the bell moving arm:

Then I added the top piece of the arm as well as the picture wire to make the bell swing properly:

Closeup of bell innards

At this point everything worked as expected:

Tidying up

After the sawing and sanding was finished there was still some general tidying up to do.

Once I’d taped down the servo wire and antena wire with electrical tape I needed to deal with the movement of the whole piece when the arm swings out. First I got hold of some Sugru and made some small rubberised feet to prevent the bell sliding around:

Sugru rubber feet on bell

The next task was to add a counter weight so the bell wouldn’t tip over. For this I made two piles of fifteen pennies, which I wrapped in electrical tape and taped to the ledge on the reverse side from the Arduino:

Counter weights on back/front of doorbell

Eventually these will be covered up and will probably actually make up part of the internal structure of the outer decoration. This should also mean they’ll end up being attached better. For now though electrical tape has proved sufficient.

One final tweak involved inserting some small nylon washers either side of the arm:

Washer and tidier wire

This was done to help make the movement of the arm more consistent and slightly smoother. Previously I was finding that the arm would shift position slightly, which sometimes caused problems for the servo.

Deployment

With the hardware finished I then set about actually getting it all up and running as our real doorbell. So imagine my horror when upon placing the doorbell on our kitchen window sill and connecting the power everything went haywire and the arm just constantly moved! After some tweaking of the code and kitchen-side debugging I realised that I really should make use of the Arduino’s internal pullup resistors for the doorbell pin. Up until that point I had left it “floating”. When powered by the USB this rarely drifted to zero, so I thought it was working fine. However on mains power it regularly dropped quite low, making the Arduino think the button had been pressed, triggering the servo. Configured the pullup resistor and tweaking the activation threshold (as it was now higher than zero) sorted out this problem perfectly:

Conclusion

So we now have a slightly Heath Robinson doorbell. It works better that our old one. There’s also plenty of scope for making it do more too. In fact phase two will be mostly about connecting the doorbell to my Chumby to get it communicating with the local network.

Check out my doorbell git repository on github for the code, as well as a fritzing circuit file.

Arduino Tone Generator/”Theremin”

So here’s another quick Arduino project. It is a “theremin” of sorts, but whereas a real theremin uses antennas to control pitch and volume this on uses a potentiometer for pitch and a push button for volume. The tone is output via a piezo buzzer (as used for generating morse tones in the previous project).


Arduino "Theremin"

The voltage from the potentiometer is read via analogRead, then is mapped to a freqency to output on the buzzer. To make things sound nicer the frequencies are limited to the twelve “well tempered” notes. As the potentiometer is turned the note played changes. There’s a little bit of smoothing (legato) in between note transitions too (so it sounds a bit more theremin-like).

All code for this Arduino “theremin” is available on github, as part of my Arduino Sketches project.

Here’s a demo of the tone generator in action:



My First Arduino Project (Morse Code)

So I finally have an Arduino. I’ve been hankering after one for a while now, after seeing what Build Brighton and others have been able to do with them. So I was very happy when my better half paid attention to the hints I’d been dropping and got me an Arduino starter kit for my birthday. That was only a few days ago and thanks to how wonderfully easy the Arduino IDE is, it hasn’t taken long for me to get things working. The simplicity of writing the code, hitting upload and seeing the effect on the Arduino within seconds makes it really quick to try out different ideas. The only bit that’s tricky (for me at least) is the electronics side of things. Due to a slight fear of accidentally burning out the 8bit AVR chip at the core of the Arduino I’m being super cautious. Though this is due to my at best high school knowledge of electronics. That and the fact that I’m pretty much a software boy.

So what to make first? Well morse code seemed like a good starting point. I’d written an app to output morse on the capslock LED before and the starter kit had LEDs, so it seeded like a good starting point. On closer inspection there was also a piezoelectric buzzer – so even better it was possible to generate a tone too!

Getting the LED to turn on and off was pretty easy and it forms the standard “hello world” program for the Arduino. Getting the buzzer to generate a tone was marginally trickier, but not that hard and basically just consisted of turning it on and off quick enough to cause a vibration and therefore make a sound. With the help of this guide to making the Arduino play the Can-Can I also had details of how to wire the circuit correctly (in particular adding in a suitable resistor to avoid overloading the AVR chip).

Using the starter kits breadboard, jumper wires, a 330 Ohm resistor, an LED and the piezoelectric buzzer I put together this:


Arduino Morse Code Circuit

The buzzer had it’s positive pin wired in to pin 4 on the Arduino and the LED was in turn wired in to pin 13. In the picture above, I arbitrarily used yellow jumper leads for “ground” and green for positive. So one green lead goes to each pin (4 and 13) and only one yellow lead goes to the “ground” on the Arduino.

Initially I hard-coded the message the Arduino was going to output, but having to re-compile each time to change the message seemed a little tedious. So I decided to get the Arduino to read from the serial port, store the characters it read in, then output the relevant morse. To achieve this the program has a 512 byte buffer that it fills up with data read from the serial port. If the buffer is full it signals this by writing back a -1 down the serial port – otherwise it echos back what it has read in. As it plays the morse it moves along the buffer looking for the next character. Once all characters are output it resets the buffer. The program is actually able to read from the serial port whilst it is turning the LED on/off and generating tones. It’s only limited by the size of the buffer it is using and the fact that it can read faster from the serial port than it can output morse code.

Most of the work in the code revolved around trying to both read from the serial port and generate tones. If the morse was known ahead of time, it would be pretty easy to right several for-loops to run through each character generating tones until we were finished. However the program would not start reading from the serial port until it had finished outputting the morse. So apart from using some basic state-machine style antics to keep track of which character we were on and whether we were currently during a dot, dash or silent period the major a-ha moment for me was realizing how to easily generate a consistent tones without using delays in the code. In previous code I’d seen for generating tones, the code looked like:


for (long i=0; i < numCycles; i++){
    digitalWrite(targetPin,HIGH); // write the buzzer pin high to push out the diaphram
    delayMicroseconds(delayValue); // wait for the calculated delay value
    digitalWrite(targetPin,LOW); // write the buzzer pin low to pull back the diaphram
    delayMicroseconds(delayValue); // wait againf or the calculated delay value
}

This is nice and easy to follow, but all those calls to delayMicroseconds mean you are hanging around waiting for the air to move to make a sound.

As I wasn't too concerned with the particular fidelity of the sound from the buzzer I opted for the following instead:


void morse_pulse_on() {
  digitalWrite(LED_PIN, HIGH);
  unsigned long period = micros() % buzzer_delay;
  digitalWrite(BUZZER_PIN, period < (buzzer_delay>>1)? HIGH : LOW);
}

This was called every time through the loop() function if a sound needed to be played. As there is no delay in calling it, one could then immediately perform other tasks, such as checking for new serial data or turning the LED on and off. It's quite simple real - the modulus operator (%) is used to get the sub-divide the current time in microseconds into a number between 0 and buzzer_delay (a value pre-calculated to let us generate the right frequency) representing the current "period". If we are halfway through this period then we set the buzzer to HIGH, otherwise we set it to LOW. By repeatedly calling morse_pulse_on we end up with (in this case) a glorious 700Hz tone.

All code for this Arduino morse code program is available on github, as part of my Arduino Sketches project. Hopefully I'll add more code to that project as I get a chance to delve deeper into what the Arduino can do.

Here's a demo of the whole thing in action:



UPDATE: not long after finishing this, I discovered the tone and noTone functions. These produce a better tone than the technique outlined above. Though they do involve use of one of the interrupts on the arduino, that's normally used for PWM on pins 3 and 11 - not that it's a problem for this project.