Thursday, 28 June 2012

1st Reprap prints

Got the Reprap printer going!

Some notes about this are below:

* Usefull g-codes are: G1 Xn Yn Enew Fnew: move to coord, new extrusion, new federate. G28 [X0 Y0 Z0]: home [X, Y, Z]. M104 Snnn: Ext temp, return. M109 Snnn: Ext tempo, wait. M140 Snn: bed temp, return. M190 Snn: bed temp, wait. M84: Stop after print job.

Calibrating the extruder. You have to make sure that the extruder consume the right amount of filament. So heat the nozzle up, then feed in filament until some comes out. Then give a command to feed in precisely 10mm and measure what actually goes in, say 13.8mm as ours did. Then you can set the Slic3r parameter Printer & Filament > Extrusion Multipler to 10/13.8 or about 0.75.

Running the printer. Noe: I use a MacBook, and Replicator Host and Slic3r software. MAke sure you have the latest versions installed. After setting up the various parameters in the slicer, and in Repetier Host software… Output an STL file from your CAD software (e.g. from Sketchup), Add this file to RH, check the objects orientation and that it is sitting on the bed, adjust if needed by rotating and shifting. Open Slic3r chose external version and load your configuration then Generate the g-code. Finally check that the right start and end g-code have been generated, and hit "Run".

You can monitor the bed and nozzle temperature as it rises, and then watch the printing happening. IF all goes wrong hit Kill or Emergency Stop.

These are our first attempts at printing as we learnt the Slic3r settings and played about a bit:

2012 06 27 22 31 59


2012 06 27 22 34 39

The results after various tries. Not yet perfect. The main problem we have is getting the object to stick on the print bed. Need to try different temperatures.

Tuesday, 26 June 2012

Using a computer as a scope

I have a MacBook. On the Mac there are a number of programs which can display audio signals as would an oscilloscope. One I like is called Signal Inspector which as well as displaying as an oscilloscope can also display spectra and generate signals. There is another very useful program called PTHVolume which gives you an audio routing selection and gain controls in the menu bar.

The range of the spectral display depends on the settings of the Audio MIDI program where you can chose the A/D sample rate for the audio inputs (up to 96kHz giving a 48kHz bandwidth).

But the audio input to the computer is fairly low level, around 100mV at maximum gain. If you want to check, for example the frequency response of an audio power amplifier then it can have an output of 100V Peak-to-Peak which will severely overload the computers input. On the other hand if you want to monitor low level signal down to just a few milli-volts them the computer is not sensitive enough.

This calls for a preamplifier. And here is one with switchable gain of on the input of 0/-40dB (approximately) and a gain of 0/+20dB, giving a total of 0-60dB range.

2012 06 26 11 20 50

Looking at the circuit, the input attenuator needs to be aggressive, for if an input of 100V P-P is to be tolerated and an output of 1V P-P (max) is needed then it should switch 0/-40dB so the lower resistor is therefore just 100R. This would give gain settings of -40, -20, 0, +20dB.

Amateur 40m receiver - will it work?

I have been dreaming again, about very simple but unusual receiver designs for CW and SSB reception. This one is the latest idea:

2012 06 26 10 48 56
It has three parts:

1 The input mixer which heterodynes the incoming RF signals with the local VFO to produce an audio signal.

2 A simple audio amplifier.

3 A simple Hartley VFO with electronic tuning using a varicap diode.

The input transformer is a wide band one, using a ferrite core. The diodes are schottky type for low voltage drop. The RFC is 47-100uH "resistor" type. The FETs are the BF245A, a very, very useful transistor which can be used with zero bias (it conducts 2-6mA at a Vgs = 0). The values given here for the oscillator are about right for 7MHz, but may need tweaking a bit to centre the band and adjust the tuning range: to do this a variable trimmer can be put across the 100pF capacitor to centre the thing range, and the varicap series capacitor of 47pF changed to limit the width of the tuning range.

What I want to try also is to connect the output to my MacBook and run an SDR program, this then hopefully will display a +/-48kHz range of frequencies and let me tune across and change modes to LSB/USB or CW. The program I have is DSP Radio from DL2SDR, which works well with my dedicated SDR radio front end, from Kanga products.

Sunday, 24 June 2012

Beautiful Painting

A new friend of Charlie's (my youngest son) is a fine arts graduate. I asked her to reproduce a painting - from a small plaque I photographed in Greece. Here it is:

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If you are interested in having Amberle paint something for you, please contact me. You can find other examples of her work on the web, just Google Amberle Dickson.

Apple PhotoStream vs Dropbox Upload

Dropbox recently updated to version 1.5. This introduced a new feature, iPhone Camera Uploads. And very good it is too.

Previous to this the way to get your photos from the iPhone to your computer was to use Apple's PhotoStream. This automatically uploaded photos from the phone to iPhoto app on your computer. But it had problems:

- it was very slow and uncertain in operation; often I found my pictures never got sent from the iPhone.

- it brought the photos directly into iPhoto, with no way (well only a complicated way) to get at the photo files without opening iPhoto and exporting them - for example to join a series together as a panorama (I use DoubleTake for this…)

- it only works on WiFi, not on 3G. I have an unlimited 3G contract and my 3G speeds are just as fast as my home broadband, so this is a stupid and annoying limitation.

So I have decided to switch Photostream off. A brave move you might say, but I don't mss it.

Because instead I am using Dropbox. A new menu option at the bottom of the dropbox screen on the iPhone now includes "Upload"; hit this and immediately any new photos you have taken are uploaded, over WiFi or 3G. They arrive in a new Dropbox folder called "Camera Uploads" which is then accessible from anywhere.

I can run through the photos quickly (select the file and hit the Space bar to Quickview), delete any I don't like (⌘/Command - ←/Delete) and then either drop those pictures I want to keep on to the iPhoto icon in the dock to import them. Or put them together first as a panorama using DoubleTake (which has a very convenient button to automatically import the result into iPhoto).

An additional benefit is that screen shots made on the iPhone (Home+Power buttons) are also uploaded.

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There, quick and easy

Tuesday, 12 June 2012

3D Printing

May be I mentioned it before but an update won't do any harm. My son purchased a 3D printer kit, the ABS Prusa Mendel. We have now put this together and it looks like this:

IMG 2190

We have tried to make as neat a job of it as we can, since we have seen a lot of Heath Robinson printers on You Tube!

However its not yet printing. The available software is a shambles, largely because it is developed by hundreds of people and there are few, if any, commercial offerings. We are running both PC/Windows 7 and Mac OSX computers. So far we have not managed to get the PC to run the printer at all (strange since there are more packages available for the PC and more developers). But we have one good package working for the Mac, this is Repetier-Host, using the slicer STL file to g-code converter. It looks like this:

Screen Shot 2012 06 12 at 10 19 08

At the top are the basic buttons, for such things as connect to the printer, Run etc. On the right you can see the g-code that has been generated and an image of the object on the printer bed is on the left. As the printer runs around a red line is drawn on the blue image to show the current slice and ABS extrusion taking place in real time.

Sound easy? It certainly seems to work. But this is only half the story, the difficult bit is calibrating the software to correctly drive the printer - temperatures, dimensions, extrusion rates, layer thickness, and on and on. We have not yet solved these issues but a few hours would do the job.

Stay tuned.

Wednesday, 6 June 2012


I have been in the dark about the fundamentals of finance, especially about loans, leverage, credit, bubbles, derivatives and de-leveraging. What? you too?

OK here goes:


Credit is cheap and abundant, "you never had it so good", UK borrows millions, house prices keep rising. The good times?

You buy a house, for say £100,000 (a low figure I admit but just an example). You get a mortgage which needs a 3% down-payment (only), this means you pay £3000 cash and get a loan of £97,000. The leverage on this is 33:1, or 97,000/3000.

Now say you want to refinance the house to reduce your mortgage. The best offer you get is a loan of £100,000 but 20% down-payment. So you have to find an extra £17,000 cash and you mortgage is then £80,000. Th leverage is now 4:1, or 80,000/20,000.


You have £100,000 cash to invest, your bank gives you a leverage of 2:1 and you buy £200,000 worth of stocks. Whoopee!!!

But the stocks fall in value by £50000, this is cash you have lost and your collateral is now only £100,000 - £50,000, and your stock value is now £150,000. i.e. a leverage of 3:1. But margin accounts like this can have a maximum of 2:1 so you have to either find £25,000 extra cash to put in or sell enough stock to get back to 2:1 leverage.


So you have a portfolio of £200,000, if the assets rise to £300,000 your collateral is now the original £100,000 + the new £100,000 = £200,000. And with this you can buy £400,000 worth of stocks. Or house owners can buy more houses, etc.

Then the bubble pops, and the assets fall dramatically. Your collateral declines, the lender faces losses, houses are given back to lenders…

Look at it from the point of view of the bank. The bank keeps some cash around, called reserves. This is around 4% of the a value of the loans it has extended (cash is £1 for every £25 loaned!). If it has losses then they can eat up the 4% cash pretty quickly, if they do so the bank is insolvent and cannot offer any new loans. The bank is forced to sell some of its assets to get some cash.


Say the bank has extended £1M loans, but its cash lok likely to go down to just £1, a leverage of 1,000,000:1, then it must liquidate its liabilities and raise cash to restore the 4% (25:1) it needs, this means raising £40,000 cash. How?

Sell assets, even at a loss which it must absorb, e.g. it sells the house above which is now worth only £50,000 against a mortgage it extended of £100,000 and get £50,000 from its reserves. Or it can keep the loans on its books at full value (that is assume the house is still worth £100,000 or the market will bounce back). Or it can sell assets it has of gold, bonds, property…

But here we enter the world of derivatives. It sells derivatives against assets it does not own, for cash, but which it believes it can buy at a later date for less than it now sells them. It also hedges against possible losses by buying derivatives from someone else. Any difference in the performance of these derivative in the market place is a profit to boost its collateral, or a loss…

Now finally if its cash reserve is too low it is in a bind, it cannot declare the true value of its assets (houses) NOR can it sell them and take to losses as the cash reserve is too low and even negative, so it is in solvent. That a bind! You see the true collateral it has = assets at market value, i.e. what it could sell them for.


If the market goes down, the house asset is impaired and could lead to insolvency. The house may "go under water" with a mortgage > house value. Then there are two choices, continue to demand and get the "owner" to pay the mortgage and come up with the cash to meet mortgage - value. Or the owner declares bankruptcy (he is then most unlikely to ever get any new loans).

If a factory has outstanding debts greater than it cash + assets, then it cannot pay them off. It can sell some assets to "put off the day" and hope the market will rise again. Overall if the assets are sold for cash then cash rises in value, and the assets decline in a flooded market.

It's all a fiddle and very, very complex. Some find it fun, others think it a stupid way to provide money to make the world go round.