Tuesday, November 15, 2011

An electronics kit for Christmas?

About this time of year I am often asked by friends who have children about the same age as my daughter "Which electronics kit should I buy for my child?"

A good question as there is a bewildering array of them out there a very quick search on Amazon brings back over a thousand results, and add to that ebay, other online sellers and the offerings of the high street toy shops and it becomes a huge  number.

So - how to chose??

Well, You have to weigh up the level of experience your child has - if they are already past the basics and can solder then there is a wide range of specific purpose kits out there eg:

They are usually good value, Costing from a few pounds up to a few tens of pounds - (this particular  one  costs about £10  from here). Most of them are simple enough to get working and to stand some rough and ready assembly techniques. Lots to choose from  - but look out for well made kits with good documentation. A personal favourite manufacturer is Vellman who generally seem one of the better manufacturers, though there are many.

But... The chances are that if your child is already advanced enough  to be at this stage, they will be telling you what they want  rather than leaving you guessing. While these  sorts of  kits are a good introduction to soldering, its also important to note that actually these sort of kits actually teach you very little about actual electronics - much the same way as painting by numbers teaches you little about painting.  Its simply a case of finding the right component from the list and soldering it into the holes on the board.  Few kits go beyond a hurried circuit diagram and do not properly explain any of the underlying workings of the circuit.

On the whole - great for those wanting to learn to solder, or for the more advanced builder , but best to avoid if your child is a newcomer to electronics

Instead, aim for a multi project kit which includes good learning materials teaching the basics of electronic circuit design  in a progressive way - what is generally though of as an "electronics kit" rather than a "project".

Of these,  what I call the "wire - spring linkup " electronics kits are by far the most common.

They range from small, inexpensive  kits with a limited number of options such as this one shown on the left, to very comprehensive kits similar to the one shown on the right which can cost hundreds of pounds

I call them the "wire - spring linkup" type because of the way you construct the circuits you are building. The components themselves are in fixed locations  within the kit and to join them together you link them together by trapping the ends of insulated wires in spring loaded terminals which each component is equipped with.

These kits generally offer great value for money - though the maxim about getting what you pay for is still applicable. The better ones generally  cover the basic fundamentals of electronics - the beginnings of ohms law, how components work and how they relate to each other to make a circuit. The more comprehensive ones have some interesting circuits too - which is important as your child will want to see something useful at the end of all that wire linking.

They do however have a downside. Because the components are in fixed locations on the board, when you are linking them up, the resulting rats nest of wires will have no resemblance to the circuit diagram you are following. Fault finding can be difficult, and generally understanding what is going on is much harder.

If I personally had to chose a particular type of kit to recommend overall, it would be one like this:



Why??   Well it comes down simply to this, you lay out the components and the links between them EXACTLY as it looks in the circuit diagram:


There are a range if kits by Snap Circuits, and other manufactures of this type ranging from a few tens of pounds up to a few hundreds .

While I cant comment on other manufacturers, the manuals produced in the genuine Snap Circuits kits are some of the best I have ever seen and closely follow the learning work which my daughter does at school. They are designed to be used by both learner and tutor - which is important as you will be taking a semi-active roll in helping your child with this right??  ;)

NB: links and images  are not intended to endorse any particular product / supplier - they are just the most convenient way of illustrating  this post - if any manufactures  / suppliers object to my use of their images  etc please leave me a message and I will edit accordingly.

Monday, November 07, 2011

Haynes in the oven.

Not a good start to Monday, it seems I left the haynes manual for the 4x4 on the bonnet yesterday (perils of working on car till its dark).  98% humidity overnight, plus a light frost means I had to chip it off the bonnet this morning. Duh!

Friday, November 04, 2011

Medium term drift seems to be settling down a bit now. The mark and space on my signal is about 6Hz, so as can be seen from the 15min grab I took - compared to te datum line I inserted it looks as though it wanders +/- 2Hz over the time frame.

Carrying out a similar test against WWV frequency stanards  on assorted bands and against my 10MHz standard  shows a minimum  +/- 1Hz drift attributable to the RX itself so Im reasonably pleased with that.

Thursday, November 03, 2011

Just 48mW :)

My replacement 50R termination arrived yesterday (The magic smoke escaped from the old one, it didnt like 30 watts for some reason!) ;)

So I decided to re-measure the beacon output as the usual x10 scope probes dont always give a good reading from the dummy load terminals.

But, results were much as previous - Im seeing 4.4v p2p - which into 50R is just a shade over 48mW.

To give some perspective - thats about the same output as a bright LED!

But... its enough!

Thats my 48.4mW into a 1/2wave zepp at about 6mAGL - with 10m of RG8 - so probably closer to 45mW at the antenna - arriving over with Bill - W4HBK in Florida.

Grid SquareEM60kj
US StateFlorida
US CountySanta Rosa
Bearing285.7° WNW (from G7NBP)
Distance4355.8 mi (7010.1 km)           

Tuesday, November 01, 2011

Sporadically active on 10m

OK - Ive slug up a very rough horizontal zepp for 10m  - main lobes roughly E/W from here.

Freq currently is 28.000810 MHz - ie tune to 28.0MHz USB  it will be a 810Hz tone

Currently 12wpm call in CW - followed by 10x callsign in QRSS6 FSK

and the results are in...

Well.... 3 hours of ten minutely data gathering done.

Results are.... interesting!

First the graphs:

And now the raw data:

time ambient C xtal C diff C 10 min change C freq Hz relative drift over 10mins total drift from power-on
09:30 23.4 27.6 4.2
0 0
09:40 23.8 28.2 4.4 0.2 727 0
09:50 23.9 29.8 5.9 1.5 737 10 10
10:00 23.9 31.1 7.2 1.3 754 17 27
10:10 24 32.1 8.1 0.9 761 7 34
10:20 23.9 33.1 9.2 1.1 771 10 44
10:30 24 33.8 9.8 0.6 777 6 50
10:40 24 34.4 10.4 0.6 784 7 57
10:50 23.7 35 11.3 0.9 790 6 63
11:00 23.6 35.3 11.7 0.4 793 3 66
11:10 23.5 35.8 12.3 0.6 797 4 70
11:20 23.4 36 12.6 0.3 801 4 74
11:30 23.4 36.2 12.8 0.2 802 1 75
11:40 23.3 36.3 13 0.2 804 2 77
11:50 23.3 36.5 13.2 0.2 807 3 80
12:00 23.3 36.6 13.3 0.1 808 1 81
12:10 23.3 36.7 13.4 0.1 810 2 83
12:20 23.3 36.8 13.5 0.1 810 0 83
12:30 23.3 36.8 13.5 0.0 810 0 83


After 3 hours the temp seems to have stabilised out as does the freq, and the temp is lower than in previous tests.

Its still however showing better part of 100Hz drift over 3 hours from power on until it settles down.

Thats fine if you are going to be leaving the beacon running for days at a time, but if you are going to be turning on and off for  a few hours at a time thats going to get somewhat annoying. It also means that during warm up you could potentially end up crashing into someone elses signal.

Based on this, as I was discussing with G0CER yesterday, I think the trick here is NOT to switch off the beacon. (Duh!)   Yes I realise the licensing implications here, what I suggest is leaving it running 24:7, but when not present in the QTH to switch the output into a dummy load - that way no (in theory!) RF radiates beyond the boundary of the property whilst the beacon is unattended.

Im also now looking at active cooling - ie popping a heatsink onto the lid of the box and then running some PWM proportional to the temp into a fan to try and hold the temp at a lower constant temp - though as others have commented on the GQRP list - it would be better to address the electronic design first with negative co-efficient caps etc.

I think Ive gone about as far as Im going to go for now with thermal tests. The bottom end of 10m isnt exactly crowded with QRSS sigs, so I guess I can afford to let it wander a bit for now.

Next priority is actually getting a signal out - so Im just cutting a very hurried 10m Zepp to sling out for now. It will be horizontal pola and be strung up roughly north south. Expected main lobes should (fingers crossed!) be heading in roughly the right directions.

More later.

Xtal insulation

Polystyrene added around the xtal. This is of course an imperfect solution as the thermo-couple is in contact with the diecast box and will thus be not only reading the case temp but will be radiating some of that heat inside the polystyrene cube.

Tests underway now with this mod and bead jacket between inner and outer box. Ons at 10 min intervals.

Graphs and stats later.

28MHz QRSS beacon - further tests

Sadly no time last night for anything particularly scientific, but having observed temperatures  about a dozen times last night and again this morning it looks like this:

Condx  = 7.3v supply, in diecast box, in outer box, NO beads, sitting on rack.

probe temp holds at around 7C higher than ambient temp.

As the room temp in here varies between 23 - 26C depending on if lights and monitors are on - the resulting swing on the after an hour or so from power on is 30.4 - 33.8 (from observation).

Heating appears to be significantly less than with a 12v supply.

What I plan to do now is:

a) cut a polystyrene block to fit around the xtal inside the diecast box to reduce temperature changes inside the box from any convection etc

b) re-add the beads to provide thermal insulation from room changes.

.....   Oh... and try and get a wire antenna up today :)