Electronics is fun

even making it.
Really!
Beleive me!
Even when it does look only tedious, error-prone and boring. Its fun.

(if I keep repeating that to myself, I will start to beleive it again, I guess).

Actually, I have a lot of fun making something electronic, every time.

Constructing a wedding present

We have

• a rotary dial
  
• a 3-position switch
  
• an LC Display

all these are pretty much no-brainers. The LCD is a well-known thing and libraries exist to drive it.
The LCD just needs 7 port pins to connect to the µC (Microcontroller). Solutions exist to use less pins by using port expanders but on a simple project like this one, we should have 7 pins available, right?

The rotary dial is basically two switches with one common pin.
When you connect the common pin to ground and the two others to port pins on the µC, you are all set to decode the numbers dialled.

The 3-position switch that came with the lamp is connected in the same way as the rotary: the middle pin to ground, the two others to port pins, this way the µC can tell the position lfet-middle-right of the switch.

We also have

• a lamp
  
• a motor

that still need control.

The 12V/10W bulbs in the lamp will run on DC as well as AC (glow bulbs are so wonderfully simple. As simple as they are inefficient for making light :-). And when the DC is pulsed, we can easily control the brightness.
So the lamps will be controlled by a PWM signal. The AVR Mega8 processor I am using on every project like this one will generate up to 3 different PWM signals in hardware, so that is easy.

The lamps use considerably more power than a µC pin can provide, so we need some amplification.
If you understand electronics, you may want to stop reading, becaus my solution is somewhat underengineered¹.
But I like it: it is simple, cheap and works quite well :-)

I simply drive the gate of an IRLIZ44N MOSFET from a processor pin. With the processor running from 5V this is enough to switch the transistor on and off and the processor pin can source and sink enough durrent to charge/discharge the gate fast enough. At least at the low switching frequencies we use here.

Everything but the motor is covered now. All with solutions I had done or used before.
A motor I had never driven from a µC myself. That is one of the reasons I wanted to use some electronics here, to learn about that.

Googling and researching a bit I found there is one standard solution to driving a motor that everybody and his dog uses.
The L293D chip includes the power transistors to drive motors up to 600 mA current consumption.
My motor uses at most 300mA so we are on the safe side here.

The chip is pretty simple to use: 3 port pins drive the enable and the two direction inputs. When both direction inputs have different logic value, the motor turns, when they have the same value the motor is stopped (and acts as an electric brake).
The enable can be driven by a PWM signal again to control rotational speed of the motor – perfect.

I am actually using the PWM to soft-start the motor (ramp up the voltage slowly). This makes the sound and motion appear very smooth, not jerky.
I also use the PWM control to use less voltage when closing the box; this way opening (lifting) and closing (lowering) the box works at pretty much the same speed. Applying maximum voltage when closing makes the closing process visibly faster than opening – and that gives an underpowered or chinese-engineered look :-)

Lets heat up the soldering iron

Why is that thing still called a soldering iron? There is no iron in there nowadays, right? Whatever.

The type of wiring I use here is known as „Fädeln“ in german. The small copper-coloured wires are 0,2mm strong copper wires with an enamel insulation that can withstand about 60V. Therefore the wires may cross each other without forming a connection.

For a small circuit like this one this method is much faster than making a layout and etching a PCB. It also avoids messing with chemicals lile FE-III-Cloride.
Debugging that stuff can be quite annoying, though.

The enamel insulation is supposed to melt and burn at 300°C, so the 370°C hot tip of the soldering iron should burn it right off, exposing the raw copper below. That copper the tin then bonds with forming a solder joint.
But the stupid stuff does not know that and sometimes does not burn off – so the joint looks like it is soldered but the electrical connection is not there. Grummel.

End-Switches: Electronic or mechanic?

We will have limited-length linear motion on this project.
We must limit the motion of the drawer, the motor must turn off at tow points.

Reliably.
Or the whole thing will self-destruct.

Reliable sensors for the end positions are...

Well, there is a very good reason why the big CNC machines almost always use inductive end-switches.
Mechanical switches are very reliable. My mouse has two and they always work – unless I am targeting a boss mob in WoW.
Mechanical microswitches are very reliable – unless used in one of my projects.
They never work there.

Always remember the wise words my first boss told me when he found me using pen and paper to find the proper values for the feed-back resistors on an Opamp-based amplifier:

„The experiment is a well-accepted scientific method for gaining knowledge“

Or in other words: just try it. If smoke appears, try something else. :-)

The circuit above gives me an optical-end-switch with a pretty good sesitivity. It will trigger when the reflecting surface is about 5mm above the CNY70.
The hysteresis is about 3mm: the signal will switch again when the reflecting surface has moved away to 8mm above the CNY70.
Even if the drawer is vibrating when moving or the movement is a little jaggy, the vibration will not exceed 3mm for sure.
The processor should see a very stable „destination has been reached, turn off motor“ signal.

Wiring it all up

with the controller board and end-switch modules finished it is time to wire it all p for the first time and see if the motor actually runs.
And – surprise? - it does and creates exactly the right sound: audible but not too loud.

Firmware

Time to get started on the firmware.
The firmware takes input from the rotary dial, controls the motor (stop, direction, speed), test for end-switch conditions and control the light (brightness).

The firmware also reads some values from the EEPROM in the processor and displays a riddle to be solved before the box opens for the first time.
And while we are at the EEPROM anyway, it also remembers the last setting for the lamp brightness.

After solving the riddle the user can choose to enter a 6-digit pin. Once that is set, it can be used to open the box without entering the solution to the riddle.
I admit: the PIN-feature I put in just for me, so I could test the mechanics without solving the riddle all the time :-)

Features

All my programs for µC end up to be some state machine. There must be a reason to that and it may be very interesting for some psychiatrist – but for me that is just the way I program :-)
Basically all µC programs are endless loops and my main loop typically has some code checking stuff (i.e. Flags from ISRs) and then goes into a switch(state) {}.

On powerup the firmware checks the 3-position-switch.

In middle position, we are in idle state: stop motor, display „Ready“, load the last-remembered brightness value into the timer compare register and that is it.

Switch in left position activates the dimmer function.
In this state the display shows „Dimmer“ and the rotary dial becomes a 10-level dimmer. Level 0 switches off the lamp alltogether, level 9 is 100% power, the other are somewhere in between.
The value is stored in EEPROM every time it is changed.

The switch in the right position brings the firmware into motor control mode.
This mode can be left only by entering something or poweroff. That is a feature, not a bug!

In motor control mode I first check if the riddle was solved before and a PIN has been stored. In that case the display shows „PIN“ and the controller reads 6 digits. The box opens with the correct pin.

If the riddle was never solved, the Display shows „Zauberwort“ and a blinking cursor in the second line.
Now the user must enter the solution (which is „SESAM“, the german equivalent of „SESAME“ as in „Sesam öffne Dich“ or „open sesame“).

This is done by entering letters with the rotary dial. A mobile phone came in handy for the bride and groom when they tried to figure it out.

I had a really good time watching them fiddling with my contraption :-)

The digits 2 to 9 produce the usual letters we all know from entering SMS messages without the T9 dictionary. The digits 1 and 0 move the cursor one position forward or backward respectively.
Once the user has figured out to dial „77771331777712161“ the diplay shows „richtig!“ and one second later the motor starts.

The motor is started with approximately 50% duty cycle on the PWM, the cycle is increased a bit every 2 microseconds. That creates a nice soft start and the movement ramping up to full speed.
The motor is run until the end-switch for „open“ triggers or a timeout is reached. The timeout is really important: I had the end-swtich disconnected on one test and without the timeout, the motor would have done something very destructive.

Once the box is open, the user is kindly asked to place the 3-position switch in the middle position again. That triggers the closing.
Again the motor is run starting at 50% duty cycle and ramping up to about 86%, so the motor gets less power on closing. That makes it run about the same speed as in the opening cycle.
Again this runs until the end-switch fires or a timeout is reached.