## SimCity: Reminiscence Edition

If you read this for electronics stuff, prepare for disappointment. Because this is a sleep-deprivation induced rant about a video game.

# SimCity: What the hell happened to you?

Every now and then I feel like going back and playing SimCity. When I was the right age to really appreciate the game, SimCity 3000 was the latest one at the time. It had pleasant, pleasing graphics and soundtrack, lots of scope and didn’t take itself too seriously. The cities you built could grow to enormous size, enough to push the 900MHz Athlon I had at the time to its limits when drawing the whole thing.

Crucially, it wasn’t SimCity 4. I got it on Steam a while back. Argh, that was a mistake.

When I play SimCity, I want to build and run a city. I don’t want to drive around it (that’s what GTA and Driver are for). I don’t want to care about individual Sims in it (more than any others).

What I really want is a more realistic “local government simulator”. For me the best bit about SimCity, apart from the long-term building, was choosing all the progressive, green, lefty policies and having them work.

## What I want from SimCity

Features I would love to see in a better SimCity would be:

### More realistic building timescales

When you zone land for buildings in the game, it takes time for buildings to appear. If the economic conditions aren’t right, they may not appear at ask. I want the same thing for other building. You want three miles of road? 8 months, and the cost is an estimate. You want a new water pipe under that industrial zone? That’s lots of digging. One year minimum, and the economic output of that area will fall. Oh, you need that pipeline now? Too bad, should have planned earlier. You’ve changed your mind about the construction of that new motorway? It’s going to cost time and money to remove what we’ve started.

### Realistic limits to what you can build

I was always a bit dubious about building my own power plants. Isn’t that a national government/utilities thing? Since when does a town with a population of 5000 just decide to build a coal fired power station?

Towns aren’t (typically, at least)  owners and operators of their own utilities. However, the local government gets to decide who gets to build what, and where. I want the Sim equivalent of EDF asking to build a new nuclear plant, and the likely impacts presented. I want to be able to refuse, and then I want the chance that the national government forces me to have it anyway and my citizens and I have to cope with that.

### The feeling that I’m part of a bigger country

On the subject of national government, I want much more interaction with them. A huge part of local government is balancing the needs of your citizens with the wider national (and indeed global) interest. For instance, the government is going to put a high speed railway through your countryside. Do you support it or not? What do your citizens think? Will it stop in your city, and if it does, will it be a boon or a drain?

On the subject of the countryside, I want something akin to greenbelt laws. Why do my Sims not kick up a fuss when I fill in that lake or raise a forest to the ground? Maybe I should only be allowed to zone a new commercial block if I agree to build a load of new parks – with penalties if I don’t. Maybe the government will decide to stop me zoning altogether in some places. Who knows, but I do know that local authorities can’t just decide that a bit of land will be for houses now – as the regular e-mails from my MP will attest.

### The feeling that I’m part of a wider economy

You already get a fair few economic choices in the game, but it’s all fairly local. I want to see the impact of the wider economy. You got to trade water, power, waste with the surrounding towns and cities, but there was never a sense of a fully developed economy. I want the town up the road to get a new factory, pulling activity away from my town and causing massive traffic problems. I want my local economy to be affected by the national one. Employment and investment as part of a national and global whole.

### In short…

I want the plans I make to be forced to take time and the wider world into account, not just – as was most often the case in SimCity – money.

That sounds simple enough. Over to you Maxis.

## Cheap-ass Logic Probe in 15 minutes

I had a sudden need for a logic probe. Here’s how to build one in 15 minutes.

Element 14 pen

Get an Element 14 pen. IT MUST BE FROM ELEMENT 14. No other normal biro will work.

Cut off the clicker end with a junior hacksaw and discard the useless innards.

Drill a 2mm hole in the side. Stick a black wire through and terminate with a crocodile clip.

Cut a single header off a header strip. Solder on a red wire.

Pick a soldering iron you don’t particularly care for. Melt the plastic of the header until it fits in the end of the pen.

Solder an LED and appropriately valued resistor onto the red and black wires. Use heatshrink to cover up your soldering crimes.

Use polymorph to secure the LED into the end of the pen.

Hey presto! Cheap-and-dirty logic probe.

Hacking Ugandan Toilets http://nicnaknoe.wordpress.com/2014/03/09/hacking-for-ugandan-toilets/

## PCB Software Choices

I have an imminent decision looming, and I’m hoping that spelling it out in a blog post will help frame my thoughts. <lie>It’s a tough one, with serious short- and long-term consequences either way.</lie>

I know, it’s up there with the biggest of ethical dilemmas, like that one about killing people on train tracks (I forget the details). But have no fear, let’s first state the reason why I need to think about it.

Currently, I use the free version of Eagle, the popular PCB package from Cadsoft (and heavily promoted by Element14). This is fine for me. I don’t need more layers or a larger board size. BUT, I’m hoping to start selling some stuff on Tindie. And the license for the free version of Eagle doesn’t let you use it for commercial purposes. I think that clause is aimed at businesses making hundreds of boards, not hobbyists who will be lucky to sell one. But still, I feel that Cadsoft are doing good by providing a free version of their software, and it would be morally wrong to exploit that good for profit.

KiCAD is an open-source PCB package that’s also very popular. It appears to have come on leaps-and-bounds recently, and (excitingly) CERN have put someone on it full-time. That’s the kind of awesome stuff CERN does. Moving to KiCAD would eliminate the licensing issue.

Here’s all the pros and cons of this momentous decision:

Stick with Eagle Move to KiCAD No new learning curve Sparkfun/Adafruit etc. libraries available No need to port old PCBs to new software No cost for selling stuff Supporting open-source software ~£70 cost for the right license, which I might not ever make back Paid version still has limits on layers and board size Closed-source software MASSIVE learning curve If I want to sell an old board, need to port it across to KiCAD Seems to be fewer libraries/standard components than Eagle

Looking at this, all the cons of the “stick with Eagle” option after fairly minor compared to those for KiCAD. Closed-source isn’t really that big of an issue for me, I can afford the £70, and I don’t expect that the board limits will be an issue in the long term. My only concern is that the £70 will be wasted if I don’t sell much on Tindie.

After all this, it looks like I’m sticking with Eagle. £70 is less than ten hours at a UK living wage (reckoned at £7.65 per hour). I’d waste at least that much time learning KiCAD before I’d even start using it. While I don’t really think measuring my free time that way is completely fair or reliable, it’s a useful metric. And in this case, I think it’s a valid one. Either way I’m spending money or time, and I think my time is worth more here.

You have been hearing my brain think.

## Mains Frequency Display

I seem to be making a lot of displays lately. Here’s another one.

# Design Concept

I’ve had this planned for a while. The concept is to display the frequency of the UK mains electricity supply. This is meant to be 50Hz, but the frequency actually fluctuates around this value, as a result of changes to supply and demand.

If the frequency drops, it’s because demand has risen and/or supply has dropped, so generators have to work harder. This slows then down a bit, and the control systems take time to speed them back up.

If the frequency rises, it’s because demand has falled and/or extra supply has come on, so generators have to work less. This increases their speed a bit, and the control systems take time to slow them down.

So the frequency is constantly rising and falling. You can see a real-time graph of the last 60 minutes on the national grid website, and a nice real-time analog-style meter at the Dynamic Demand website.

I wanted to make a display that could show the mains frequency to 3 decimal places. I’d be using the same seven-segment display modules that I used in my UNIX clock, so all I had to do was design the part that would work out the frequency.

# How to work out the frequency

There are lots of techniques to do this. My choice was to count a fixed number of mains frequency periods and time how long that takes to occur.

Here’s a drawing of the concept: The sine waves represent the mains. A high-frequency signal is run in parallel and used as a counter. The count after a fixed number of cycles is inversely proportional to frequency.

The concept of measuring a varying low-frequency signal.

For example, if the frequency was exactly 50.000Hz, and I counted 200 periods, that would take exactly 4 seconds. If the frequency was actually 50.001Hz, 200 periods would only take 3.99992 seconds.

That’s not a lot of difference! In order to time that accurately, I need a high-accuracy, high-stability timing source. The RC oscillator in the microcontroller wouldn’t do. I would be using an ATTINY84, which has an accuracy of only +/- 10%, or +/- 1% after user calibration. Nor could I use an external crystal, without paying a LOT of money for a high-stability one.

Luckily, there is one class of cheap(ish) ICs that have high-accuracy, high-stability oscillators in them, and that’s real-time clocks. The DS3231 RTC chip, which was also used in the UNIX clock, has a +/- 2ppm 32.768kHz output. By counting pulses from this, I can determine the time very precisely.

# Maths!

After doing some maths, I figured that I could count for 100 mains cycles (2 seconds at 50Hz) and get my required accuracy.

The formula to translate count to mains frequency is

$f=\frac{1}{\frac{\frac{C}{32768}}{N}}$

where N is the number of mains cycles counted and C is the number of 32768 Hz cycles counted.

I re-arranged this, and added a 1000x multiplier in order to only use integer maths, so 50.000Hz would actually be stored as 50000. This is much nicer for implementing on a microcontroller.

$f = \frac{32768 * N * 1000}{C}$

Because we’re doing integer maths, we need to account for rounding, so the final formula is

$f = \frac{(32768 * N * 1000)+\frac{C}{2}}{C}$

For example, a count of 65724 would equate to a frequency of 49.857Hz, stored as 49857. A count of 65723 would equate to a frequency of 49.858Hz, stored as 49858.

So, a change of one count represents a change of one significant figure (of the scaled frequency), exactly what I want.

The frequency value is sent directly to the display, so the frequency shown is updated every two seconds.

There are two LEDs on the control PCB to display the up/down trend of the frequency. The frequency is stored in a ring buffer once a minute. A ten-minute rolling average is calculated, and the difference between the first and last readings is used to work out the trend. A difference of more than +/- 0.02hz is counted as a significant trend.

In order to find the “best” algorithm for this, data from the National Grid real-time display was parsed with the Beautiful Soup python module. I just experimented with various guesses until it looked about right.

Putting it all together

I tested the code on an Arduino with an opto-isolated mains input before designing the final circuit. The electronic design draws on a lot of previous work I’ve done. It has:

• The mains-frequency input from my Days-Accident-Free counter.
• (This clamps a 9V AC signal (from a small wall-wart transformer) to the 5V supply rails)
• The display connection and DS3231 RTC from the UNIX clock.
• A standard ATTINY84 microcontroller from a host of other projects.

The system block diagram

I was able to reduce the board size down to be the same size as a seven-segment display, which was nice. I ordered the boards from Ragworm, and everything worked first time, which was nice.

The only modification I made was to add a small “heatsink” to the 5V regulator. The input to the system is 9V AC from a small adaptor. Because the regualtor runs all 5 displays, it’s dumping about 1W of power into a rather small package. It gets pretty hot, so I added a heatsink made of lots of solder on a bit of stripboard. A bit of a hack, but it does help keep the regulator cooler than it would be.

The control PCB, with LEDs, microcontroller, power supply and “heatsink”.

Finally, I made a laser-cut case (as is practically standard for my projects). It was an adaption of the UNIX clock casing. For the up and down trend LED symbols, I tried using crayon wax melted into two laser-etched arrows. This worked pretty well, so I think I might use this technique on future projects.

The completed display, showing a “down” frequency trend.

The frequency trend of interest

Posted in Electronics | Tagged , , | 10 Comments

# A Problem

## The Arduino IDE tries too hard to be nice.

Here’s some Arduino code from a recent project of mine:

enum state_enum
{
STOPPED,
ACCEL,
RUNNING,
DECEL
};
typedef enum state_enum STATE_ENUM;
static STATE_ENUM currentState;
static void setState(STATE_ENUM newState)
{
/* Extra code omitted for clarity */
currentState = newState;
}
void setup()
{
/* Extra code omitted for clarity */
setState(STOPPED);
}
void loop()
{
/* Extra code omitted for clarity */
setState(RUNNING);
}

We’ve got a state variable that can be in one of four states, and a function to change state.
Looks OK, right? Well, try compiling it and you’ll get the following error:

sketch_dec04a:3: error: variable or field 'setState' declared void
sketch_dec04a:3: error: 'STATE_ENUM' was not declared in this scope

Which essentially means that on line 3 of my code, the compiler has found a datatype called “STATE_ENUM”, in the setState function, and doesn’t know what that means. But how can this be, when the function isn’t defined here, but about a dozen lines further down?

Turns out that the Arduino IDE (as part of its pre-compile process) scans your code looking for functions, generates prototypes for them and sticks them just above your actual code.

So, this line:

static void setState(STATE_ENUM newState);

Was being inserted above the declaration of the STATE_ENUM type, which causes the compiler to quite rightly say that it has no idea what STATE_ENUM is.

I can see why Arduino have done this: it means inexperienced programmers can put functions anywhere in their sketch and not have to deal with prototyping them.

## Possible Solutions

So, how DO you use user defined types in your Arduino sketch?

### Only use standard types

Well, the messy way is to avoid using functions that return or use those types. So, I could have altered my setState function to be

static void setState(int newState)
{
/* Extra code omitted for clarity */
currentState = (STATE_ENUM)newState;
}

This is a bit messy, as it hides the intent of the function input a bit.

Another option is to put the enumeration and typedef in a header file and #include it. The file is included before the auto-generated prototypes, so the compiler is happy.
This again is a bit messy, and not in the spirit of hiding implementation detail. If nothing else needs to know about your datatype, it shouldn’t be in a header file.

### Hide the function from the auto-generator

Finally, the Arduino website build process page gives us a clue how we can solve this:

“Also, this generation isn’t perfect: it won’t create prototypes for functions that have default argument values, or which are declared within a namespace or class.”

So, we can make the function have a default argument:

static void setState(STATE_ENUM newState = STOPPED)
{
/* Extra code omitted for clarity */
currentState = newState;
}

…which will successfully compile, but now risks being called without an argument, which has the potential to do harmful things.

For the uber-paranoid, you could add an extra “invalid” value to the enumeration, make this the argument default, and test for this value right at the start of your function:

static void setState(STATE_ENUM newState = INVALID_STATE)
{
if (newState == INVALID_STATE) { return; } // Do nothing if called without a state!
/* Extra code omitted for clarity */
currentState = newState;
}

The main disadvantage of this is that ALL the arguments after the defaulted one also need to have defaults. For example, this won’t compile:

static void setState(STATE_ENUM newState = INVALID_STATE, int anotherArg)

Because anotherArg needs a default value. This might be annoying.

## The REAL solution

Ultimatley, the real solution is for the Arduino IDE to allow you to turn off automatic prototyping. Interestingly, an “Arduino for Visual Studio” application does exactly that.

There are LOADS of ways to do this, but I think a nice work-around would be a comment-style function decorator. Something like this:

static void setState(STATE_ENUM newState)/*NOPROTO*/

The Arduino build process would recognise the NOPROTO comment and suppress output of a prototype for that function.

This would avoid adding extra options to either the IDE or the preferences.txt file, and allow individual control of which functions are affected. A downside is that if you have a LOT of functions like this, the code gets littered with /*NOPROTO*/ everywhere.

## What do I do?

Well, for the specific project above, I just modified the function to use a standard type.

I don’t tend to use the Arduino IDE much, so I think what I do in future will depend on the application.

Remember the mantra: “Software development IS decision making.” Get it tattoo’d on your eyeballs.

# Shapes of Constant Width

I’d got my hands on a set of these shapes at a Festival of the Spoken Nerd (FOTSN) show in Nottingham.

I’ve simplified Wikipedia’s explanation of what these things are:

“A shape of constant width is a shape whose width (defined as the perpendicular distance between two parallel lines each touching shape’s boundary) is the same regardless of the rotation of the shape.”

Or you can watch the maths gear video about then:

This means that if you roll these shapes between two rulers, the rules will stay the same distance apart. Magic!

The shapes I got were laser-cut plywood. I’d been toying with the idea of mounting them in a display to show off their movement between two surfaces, but actually they sat unloved in a pile of other stuff for ages.

Then, last August I took a trip to the Edinburgh Fringe, where FOTSN were on. I took the opportunity to get the shapes signed by Matt Parker, Helen Arney and Steve Mould. I dashed off to Edinburgh Hackspace afterwards to get the FOSTN logo laser-etched onto the remaining shape. After this, they could stay unloved no longer.

Here’s a video of the stand I built. You can read a bit more about it below the video.

## Stand Design

My initial idea was to replicate the linear “between two rulers” motion, but I pretty soon changed to a circular design, where a central drive wheel would move the pieces round in a track, just by friction between the surfaces. Observe this rubbish exploded view:

An awful concept drawing of the shapes-of-constant-width stand

The stand holds the motor and any control electronics. A living hinge is a nice way to get it standing up. I had to divide the stand with a fancy wavy line (not a sine wave, that was a silly thing not to think of) into two to fit each piece into the laser.

The retaining ring is glued to the stand. The centre rotating piece was cut to allow for a rubber band to go around it to try and get good grip between the wheel and the shapes.

Everything was designed in Inkscape.

I was hoping that the shapes would rotate without needing holding in from the front, but they kept popping out from between the ring and drive wheel, so I had to add a front retaining piece to the design.

I didn’t just want to cut a boring circle of perspex, so keeping with the maths theme I thought about some sort of grid or spiral.

I settled on the following shape, with was constructed out of overlaid golden spirals:

1. Generating a golden spiral.
2. Copying and rotating it twice.
3. Copying the result, mirroring it and overlaying on the original.
4. Thickening and outlining the paths to allow for cutting.
5. Adding a small circle to the centre and adding the retaining ring.
6. Adding a 3mm centre hole for motor shaft and 3mm screw holes around the edge.

I think it looks quite nice.

Due to the tolerances of the design, and probably in no small part to my own ineptness, the shapes move too freely in some places and stick in others. I solved this by adding some foam to the outside edge of the track, which takes up these non-uniformities.

I also had to add some extra pieces to the cover to stop the shapes being pushed upwards and getting stuck.

THEN I had to drill out the centre of the motor shaft and epoxy in a screw and washer to finally stop everything going wrong.

## Motor and Drive Electronics

The best speed for rotation seemed to be about 10rpm, so I got a 12V 10rpm motor off eBay for about £10. In basic tests, it actually drives the shapes round comfortably at about 6V.

During initial testing, the shapes occasionally stuck, causing a pile-up and the motor stalling. I thought I might have to design some kind of stall sensing and recovery circuit, but luckily with the changes to stop them sticking, and sanding down the edges of the shapes, that wasn’t necessary. I just added a PWM speed control with an Arduino Nano. The Arduino also reverses the direction of rotation every 30 seconds, in the hope that this might average out and long-term drift of the shapes relative to each other.

I might experiment with side-lighting the perspex in the future, maybe just a single LED at the bottom of the piece to draw attention to it.