10 December 2017
09 December 2017
08 December 2017
07 December 2017
04 December 2017
03 December 2017
I had the pleasure of attending a Dinkydoodle Designs cake making course today, it was great to go along and learn how to make the cake that goes around the droid heads I've been making for the last 2 years. Because BB9 has a flat top I had to rebuild the whole mechanism to make it fit into the available space, I took the opportunity to make some needed improvements, such as vacuum formed heads and bearings for the moving parts. I also decided to add an ESP8266 into the mix so that the whole thing could be controlled from my phone. I had a great day and would really recommend it for anyone who is looking for something different to do.
02 December 2017
30 November 2017
27 November 2017
24 November 2017
23 November 2017
The basic car framework needs a food safe covering to go between the cake and the electronics. Thankfully when I bought my vacuum former this project was already on the cards so it's a perfect excuse to use it. I laser cut a basic mould from ply and assembled it as a mould to form over. The cake needed to clear the wheels and provide turning clearance for the steering. These covers came out pretty well, the trouble is that vacuum forming works best with a 3 degree angle on the edges and the laser cutter cuts perfectly vertical so it was a bit of a struggle to get the mould out. This was when I introduced an air pump to the vacuum former. Applying a bit of positive pressure after forming releases the mould a little and made it easier to remove.
22 November 2017
18 November 2017
I bought some cheap LED clock kits a few years ago, they were good fun to assemble and left me with a perfectly serviceable clock for the kids rooms. I opted not to buy the (nasty plastic) case because it was laser cut and I could make my own, move forward a few years and today seemed like a good time to actually do that.
It's a simple wooden box really with holes in the correct places, the clock module slides in loosely from the front and wedges in place with some foam. The plastic panel slides into some grooves along the front of the box. It's transparent grey plastic so the LED's can shine through it. The grooves on the front mean that the burnt side is out on the top of the clock, I should have masked it to reduce those marks. Being wooden I can easily glue it to the underside of my shelves next to my hooky hooks (svg here)
16 November 2017
This is one of those little projects that didn’t need to be done, or could have been done very simply. Despite a lot of feature creep, everything just came together at the right time, and I’m really pleased with the results.
Firstly, a bit of background. My mum celebrated her birthday a couple of months ago, and the present my sister bought her was a geode that she picked up on a recent holiday to the United States. Not just a regular geode, though, but one that had a hole drilled out and a light attachment that went inside so it glowed nicely. Well, it would have glowed nicely, except that the US bulb was rated for 110v, not our full strength 220v electricity. So the millisecond it glowed nicely for was followed by a loud bang and the bulb turning black.
As I was there to see this, I figured I could easily get a light fitting and bulb from Ikea and we’re all good. Then I wondered if LED lights might be a better idea. At the same time, I was starting a project for the RC2014 involving a ESP8266 wifi enabled micro controller. Coincidentally, I’d just bought a couple of WS2812 based Zip Sticks. So, you can guess where I’m going with this, right? Yup, use the ESP8266 to set all the LEDs to a nice shade of blue, and job done!
Well, that would have been within my programming abilities. However, I wondered if anybody else had controlled WS2812 LEDs from an ESP8266. Would you believe they had? And not only that, but I found McLighting from Toblum on GitHub which enables wifi control from your phone or computer to not only set the colour, but a whole stack of pretty effects.
Time to fire up KiCad and get a PCB designed. I’d taken the rough measurement of the hole in the geode, and had a 40mm diameter round PCB to fit everything on. With a mixture of through-hole and surface mount components, and using both sides of the board, everything went on well. Because I only needed a very small run, OSHPark was the obvious choice. And at only $11.25, it was a real bargain!
A few days later and 3 lovely purple PCBs arrived. It wasn’t long before I got the soldering iron out and started to assemble them. I assumed that my selection of every single value of surface mount capacitor would have contained what I needed, but sadly it didn’t even get close, so I had to resort to bodging in a couple of through-hole electrolytic caps. I also noticed that I hadn’t indicated the +ve or -ve power terminals. A rookie mistake, but not the end of the world. Once it was all together, though, it worked just great!
A quick test, and a run through of the settings and things were looking really good. But would it fit the geode? Time for a trip down to my parents house and try it out.
The PCB went in to the geode with no problem at all. If anything it was slightly undersized, but I’d prefer it to be a little bit too small than too big. A plastic ring that was a snug fit held it securely in place.
In the daylight, it looks absolutely awesome!
By night it looks even better… although getting a good photograph isn’t so easy!
12 November 2017
11 November 2017
07 November 2017
03 November 2017
02 November 2017
My current large project (details soon I promise) has produced some large and bulky parts that can't be laid flat on a surface but need supporting fairly equally all over. A quick run to the laser and I have parts that support in all the relevant places. I left a lot of gaps around the edges so sadly they would be better if they were a tighter fit. Next time I should check and measure rather than guesstimate but they will do the job and they only took 10 minutes. Now, back to my video editing....
01 November 2017
Every day is a learning day here where I try new things and make something new. Makevember is encouraging people to make one thing every day, I don't know if I'm going to be able to do that (especially when I'm exhibiting at the NEC for 3 days this weekend) but I would hope to start using some of my newer tools a bit more. In the meantime I thought I'd just post some of the things I've made today. This is a selection of all the things I'm allowed to show you, I've actually spent most of the day working on a large project (more details on Monday).
One of the important things about doing this every day is that you learn something from everything you do, I'll also briefly describe what I learned from these things.
Pictured above are some car shells for remote control cake cars. These were vacuum formed from 1mm HIPS and I learned a lot considering I haven't formed anything for a few weeks. Plastic sheets come in multiple sizes, buy them too small and they won't fit in the former. Plaster of Paris moulds absorb moisture and go mouldy unless you take extra car of them
30 October 2017
The laser is good for personalising any item, this pull along toy made of wood is a prime candidate. There are a whole range of toys that could be done like this perfect for little ones.
29 October 2017
I had a visitor come round and look at my laser cutters, he also bought some samples with him to see what could and couldn't be engraved. It's always fun to find new things to go in the machine. This purse was made from Poly Urethane, I set the power light and the speed fast and it lightly marked the surface, it's subtle but effective.
12 October 2017
09 October 2017
About 2 months ago I was talking about getting some parts vacuum formed so I had a quick look on ebay to see how much vacuum formers were and found one very local that was going cheap (comparatively). It needed to be gone by Monday morning, I found it Saturday morning, and it was a big industrial beasty that would probably need a van with a tail lift to move it. A few quick emails/phone calls later and I'd booked a van, spoken to the sellers, found a friend to help and things were all set for Monday morning. I'd gone from thinking about vacuum forming parts to owning a machine in the space of a few hours.
Moving day was fun, the machine is indeed big, heavy and very industrial. The machine had been unused for a while and the previous owner had it wired directly in to the wall because high currents kept blowing fuses. It was fair to say I was nervous about what I'd just bought but worse case I could just sell it again on ebay (and not being in a rush I would probably make a profit on it with a proper auction).
I ignored the warnings of the previous owners, whacked a plug onto the loose wires, ensured I had a 13A fuse fitted in the plug and connected it directly to the wall. The machine crackled into life literally, the heating elements were getting warm and making plinky plunk noises. The heating tray slid nicely back and forth, the lift mechanism was good and it seemed to form a seal around the plastic sheets. While we were out collecting the machine some plastic sheets had arrived and I was ready to vac form something. Thankfully I had a laser cutter to hand so could quickly and easily make a mould.
07 October 2017
I have recently been inspired by the work of Diego from the Engravers Dungeon (brought to my attention courtesy of the Laser Cut Stuff blog). He's been doing some amazing engraving of custom artworks, he's really managed to bring out the best of the wood using different colours but what really caught my eye are the two significantly different colours of laser cut. The large areas are engraved to create the typical 'brown' engraving where the wood grain can show through. The details however have this amazing 'black' engraving effect, he's understandably protective of his methods but it got me thinking about how it's done.
The laser cutter has 2 main modes of operation it can trace out a shape and cut it out or it can engrave an area where it moves over every 'pixel' within an image and puts a single dot onto each spot. Fancy laser cutters are able to vary the power of this spot to produce greyscale images, the Leetro controller (what I have) is only capable of doing on/off engraving. The burn mark is either full power or off, this produces the 'brown' engraving seen here.
There is technically one other option with the laser that I love and have used to great effect many times before. I use it for Line art, it's a cut operation but with a very low power that only marks the surface of the material without cutting all the way through it. This would appear to be very similar to what is happening here, large areas of seemingly 'black' engraving. The stuff I have previously done is much more for outlining objects but I thought I'd give it a try anyway.
First things first, I needed a suitable image to work with. I showed Diegos work to some of my friends and one of them told me that Vesalius made some anatomical drawings that would work well in this style. A quick search led me to some works of his that were recently digitised, 5 minutes on there and I had some suitable skulls to work with. I vector traced the first one and cut it (image above) and it kind of works. The skull was painted white, the brown areas are engraved and there is lots of line artwork that is starting to look correct. It works particularly well when the lines are close together but it fails on the big gaps, I needed some method for filling the gaps.
Normally when I want to engrave in greyscale I have to convert the image with a halftone effect, this varies the dot densities to produce a grey engraving effect (like how newspapers used to work). More dots make a darker area and a pattern like this fills the whole area so would be better for this line art. I converted the original image using a halftone plugin for paint.net, most raster editors have a method for doing something similar and GIMP is my suggestion if you're not using windows.
This is the point where I got carried away and shared my intermediary success with the laser engraving and cutting forum, received a dozen requests to show my workings and ended up writing this blog post (wordy for me I know). There are more things I want to try with this method....
1. This halftone effect would probably work well, instead of lots of dots the noodle shapes would outline well.
2. It is apparently possible to make halftone images using filter methods in Inkscape these look like they would create more suitable images for engraving.
3. The Eggbot from Evil Mad Scientist using hatch filling to colour in regions on their pen plotter, this would be another way to fill a region and worth investigating.
Finally the main reason my 'black' engraving looks horribly burnt is because I have a 100W laser cutter than doesn't offer particularly fine control at the low end. If I had an RF tube like Diego I would have better control over the beam power. The Glowforge laser cutter is also showing great potential for this kind of line artwork, sadly it looks as though they're about to announce another long delay on international shipping otherwise I would have mine already and I'd be able to try it out (2 years delayed and waiting)
From my little bits of wargaming experience it appears that each of the models sits on a small base which needs decorating which can be done with little disks and there are trays to move whole units of models around at once (even if just for display). My idea was to combine those two things together and make bases and trays where the pattern continues, as though the whole unit were on a marbled floor. Not sure if this is a desirable thing, let me know.
06 October 2017
02 October 2017
Last Friday, I went to the ACE Study Day ‘Aspects of Publishing: Guide and Souvenir Books’ at the Geffrye Museum. We’re in the process of redeveloping our guidebook at work, and it was interesting to see how other organisations and museums had come to their decisions about the kinds of books they produce, and who they are for.
It was great to hear people speak openly about what had and hadn’t worked in their institutions, and how they dealt with external factors, such as placement within their shops and ticketing areas, and the ways that staff help sell them as part of a visit. It was also interesting to hear people speak honestly about the tensions between commercial and curatorial needs within the institution, and the strategies for overcoming them. The title of this post comes from a response to a question about the needs of museum shops to ensure they make money while maintaining the voice of the institution.
The range of publications was wide, and raised a lot of questions about where we might go with our guidebook, what its function might be, and who it is for:
- Does it need to act as a guide? Is its main function to help guide people around the exhibition and interpret the pieces on display? Does it need to contain a map? Is the intention that the book is used while at the centre?
Our current guidebook has a spread for each of our galleries, with key objects to see, as well as a map, and information about our other business activities, such as Education, Venue Hire and Weddings. We were shown examples of books which offered ‘A quick look’, and ‘containing 10 trails’, which were specifically intended to guide the visitor on the day.
- Does it need to focus on the collection and exhibits? Should the book be specifically tied to aspects of the collection and the exhibits we have in the space, or should it be used to open up wider themes?
In the case of the Space Centre, could we use impressive images from Hubble or NASA’s archive to frame themes we address in the exhibition, rather than focusing on objects we hold or display? This potentially gives the guidebook a longer life, as the exhibition changes over time.
The National Trust recently published Prejudice and Pride, which celebrates and focuses on LGBTQ Heritage across the National Trust sites.
By focusing on a theme in the context of British heritage, the book is not a guide to a visit, but rather a resource that reflects on hidden or previously untold histories as part of a wider discourse on heritage and what that means.
- Who is it for? Should the guidebook be aimed at the reader who wants more in-depth information to supplement their experience at the centre? Should it be a visually led document that triggers memories and discussion about the visit? Is it aimed at all visitors, or targeted at a specific reader? Is it for kids?
Our current guidebook is a mix of all of the above, which over time (and with incremental changes) has become a tangle of information, voices and intention. One of the triggers for this new redevelopment has been the recognition that we need to focus the guidebook and ask who and what is it for.
- Do we need more than one? Does it make sense to divide the guidebooks into separate smaller pieces which can appeal to different readers, uses and price-points?
We were shown great examples of institutions’ range of books, from text-heavy interpretations of the museum, to pocket-sized books selling at £5, to kids’ activity books, which were created in response to audiences needs.
A great example is the National Portrait Gallery’s 100 Portraits book, which is small (150mm x 150mm), image-focused paperback, and retails for £6.95. It sits within a range of other publications from the National Portrait Gallery and is the kind of thing that would make a great gift or souvenir.
As we approach our new guidebook, these questions will help us form our ideas and work out the best way forward. I’m hoping that eventually we will develop a range of books which enable us to tell our stories in different ways and appeal to different audiences with a credible and confident voice.
27 September 2017
25 September 2017
I’ve just signed up for the Museum Association’s AMA professional development scheme. I’m really excited about the programme, and looking forward to developing new directions and ideas over the three years. I’m also excited to be able to have a framework to support critical reflection and looking beyond the everyday demands and restrictions of the space I work in.
I’m currently working as Exhibition Designer at the National Space Centre. I’m not sure it’s the same for everyone who has a similar role, but it’s one I’ve found my way into, rather than having a defined career goal from the start. I started out after university making visual art and performance before getting excited by web design in the 90’s, and via a wiggly path involving digital arts projects, horror film festivals, motion capture, teaching, and performing as Britney Spears at Glastonbury, I joined the National Space Centre as a web designer in 2011. Through luck and meeting some great colleagues, the role has changed over the years, and these days I’m mainly designing spaces, interactives and graphics, as well as being involved in the Exhibition Leadership Team.
I don’t think it’s necessarily a bad thing to have had such a varied work history. The transferrable skills, approaches and ways of thinking are all valuable things to have, but I’m not sure it’s consistent, and it makes me anxious that I’m missing fundamental experience and understanding of aspects of the Museums and Visitor Centre sector.
I’m also, to be honest, not entirely sure what Exhibition Designer means! Should I be writing and proposing stories and treatments for the galleries and the visitor experience? Should I be walking round with a pencil behind my ear, or heavy rimmed glasses and a turtle neck jumper? Should I be providing detail drawings and material lists to contractors? Should I be writing interactive software, or briefing someone else to do it? Should I be researching audiences, trends and new ways to engage visitors? Instinctively, I think it’s all of these things, but I’m aware that as I look around at jobs with that title, it’s sometimes a tiny, specialised bit of that list that people are looking for, and sometimes all of it and more!
The National Space Centre opened in 2001, and in some places, you can really tell! That’s no criticism, it’s the reality of any museum or exhibition space. Knowledge develops and changes, stories gain or lose relevance, audience expectations change, and parts of the exhibition begin to look dusty and dated. We are lucky to have an exhibition team in-house who can continue to refresh and redevelop the centre, whether replacing a damaged graphic or reimagining a space to communicate an idea better, or to allow our visitors to experience it in a new way. As a team, we are developing new ways to make the exhibition more accessible, inclusive and participatory.
I’m lucky that work is giving me the time and space to work on the AMA, and to be part of a team that values the need for research and reflection. I’m looking forward to asking the tough questions about what my role is and should be, and what the National Space Centre is and can be. I’ll be posting my thoughts and questions here as I go (I’ll tag them as AMA), it would be great to hear from you!
22 September 2017
- What is the most efficient order for assembly?
- Do parts have to be specifically aligned to each other?
- Where does the glue have to go and are there places where the glue shouldn't go?
- How do you remove the part from the jig once it is assembled?
21 September 2017
20 September 2017
19 September 2017
17 September 2017
I made these story disks for Vale, each one has an encrypted message spiralling around the disk from the inside out. The encryption is made even harder by the fact it is displayed in the vales own font, 26 runes representing the 26 letters of the alphabet.
15 September 2017
14 September 2017
13 September 2017
11 September 2017
I've been making a range of items for the Leicester Guildball Group recently because I'm local and have a laser cutter. It's all fairly standard stuff but it's nice to see people using the things you make.
08 September 2017
I make a lot of things out of wood because it laser cuts incredibly well, it's cheaper than plastics and you can glue it together. I've tried quite a lot of glues and I really like this Weatherproof Exterior glue from Evo-Stik. I've been buying it in smaller bottles with applicators but recently discovered that Rapid Electronics sell it in 2.5 and 5L bottles, suitable for refilling all of my smaller bottles. The glue grabs within a few minutes and sets solid and clear overnight, I'd definitely recommend getting some.
07 September 2017
05 September 2017
I’ve just finished working on a set of Scientist Top Trumps for the British Science Association : Brighton and Hove Branch. They were launched on the 5 September, as part of the British Science Festival 2017, with different cards being placed in venues across the city, encouraging collectors to visit the different spaces and take part in the talks and activities there.
It’s been a really fun project to work on, and it was great to have the opportunity to work with the BSA. I really liked the diversity and range of scientists who were featured, and enjoyed reading their stories. The colours and fonts were chosen to fit the BSA branding guidelines, and I made an icon pattern for the back of the cards. The portraits were found on Wikipedia, with the background headers coming from stock, or public domain libraries.
Here are some of my favourites. If you’re out and about in Brighton, try to collect the set!
04 September 2017
I have a few relatively big cutting jobs to do over the last few weeks. They keep the laser busy while I'm free to answer emails, sort paperwork and all the other boring jobs that occur as part of life. Hopefully many of you will have noticed the not so subtle changes happening to the blog too as I try to get this sprawling monstrosity of 1200 posts into slightly better order of things. I'm not allowed to mention most of these projects yet but the posts are written and waiting for specific dates.
I have however been cutting some more Hex Terrain Toolkit which seems to be a very successful project for Ross.
31 August 2017
Earlier this year, I worked with the Eden Project on their new summer holidays exhibition, Journey Into Space. The exhibition formed their summer programme, and included an Alien Encounter, Astronaut training, a Solar System Safari and a VR Theatre, alongside talks and activities.
The key areas we were involved in designing were the Solar System Safari, and the VR Theatre.
The Solar System Safari is a walkthrough exhibition which takes the visitor through the Solar System, starting from the Sun and returning to Earth at the end of the journey, via ten interlinked spaces. Each space in the exhibition uses elements of sculpture, lighting, audio and video to give the viewer an experiential view of the planet or Solar System object.
The process of working with the team at Eden involved collaborating (mostly remotely) to fix ideas, scope the needs of the individual rooms and to design the piece to make a coherent journey. It was important that each room gave the viewer a different experience, and that the story of returning to Earth was key to the exhibit.
Our initial process involved using Mural to throw together visual ideas and inspirations. The advantage of using Mural, especially in remote meetings was the potential to update live and add new elements as they were discussed. From this early collaboration, we were able to map out the space, and to begin to design the elements of each space.
I mapped out the space in Sketchup, which allowed us to get a real sense of the space, and to begin to work out the sizes and technical needs for the projections, as well as the set elements.
Our team at NSC Creative was responsible for creating the media elements for the exhibition, which included an animated Sun, volcanoes on Venus, a Pepper’s Ghost projection of Saturn, and a five screen animation of the Earth as seen from the ISS.
In early July, Kyle and me headed down to Eden to spend a week installing the media and helping to get the exhibition ready for opening.
The Solar Safari is built on the stage area of Eden, a temporary exhibit space which is used for Eden Sessions, holiday exhibits and special events. This meant that the exhibition was installed in a week, from start to finish! There was a large team working throughout the day to build, install, troubleshoot and tidy the space. The atmosphere was great, and the team was fantastic.
It was great to see the exhibit forming around us, and to be involved in such a concentrated burst of activity.
It’s been an honour to work on such a large and fun project, and to spend time working with the team at Eden. It’s been great to see people’s reactions to the project too!
30 August 2017
I read on the internet about a few different techniques to infill acrylic with a different colour. I've done some previous painting where I engraved the protective cover off the acrylic. I found that engraving that protective sheet can be a bit hit and miss, sometimes it engraves fine but other times it just appears to melt so I prefer to peel it off now. I tried to apply some vinyl transfer tape which engraves better, this was effective but it seems silly peeling off one sheet and applying another.
I settled on a compromise between the two. Since all the numbers and words around the outside are cut using Hershey text, instead of engraving they cut cleanly even through the protective sheet. The engrave in the middle of the item is done by first outlining the engrave with enough power to cut the sheet and then peeling the sheet off from the place that's going to be engraved. The idea is to do it in a single piece so they will be no weeding, the engrave can then be done in the gap that remains. The item can then be painted and once dry you can peel the mask off and you only need to clean up the engraved area.
29 August 2017
24 August 2017
23 August 2017
20 August 2017
One project that’s been in my mind for ages is to revisit some of the circuits from my childhood Philips X40 kit. I finally got around to putting “Experiment 5 – Telephone pick-up and amplifier” in to my circuit simulator. This is the first of several audio amplifier circuits in the manual, and they all follow the same design pattern. Similar circuits are also used in other educational kits. I had tried to use this circuit design in my own electronics experiments as a kid. Mostly it didn’t work, but sometimes it did. As an adult, I was interested to understand its characteristics to help explain where it’s appropriate.
Looking at the simulation you will notice the amplifier is non-linear (compare the shape of the top of the sine wave out to the bottom in the output of the first stage). This isn’t unexpected given that transistors have an exponential relationship between the base voltage and the collector current. The biasing on the second stage also looks bad, meaning that the output signal is clipped on the positive side. The simulation result raises several questions in my mind:
- Why is this type of amplifier so popular in basic electronic kits?
- Are they circumstances in which the amplifier is linear?
- What are the characteristics of this type of amplifier anyway?
To answer these questions, let’s look at the basic building block of this amp. It’s a single transistor common emitter configuration. The biasing is from a high value resistor between the collector and the base. Wikipedia informs me that this configuration is called “collector feedback bias“. The signal is coupled in and out of the stage through capacitors.
Single Stage of Amplifier
Looking at this circuit you can guess why it’s popular in these kits: the component count is very low. Hence, it is easy for kids to build. If you have an intuitive understanding of transistors, you might also guess that resistor between the collector and the base creates negative feedback giving the circuit the ability to find a suitable operating point across a range of conditions.
As all the textbooks tell you, transistor amplifiers require that the transistor(s) are “biased” correctly to work. The aim of biasing is to set the DC conditions around the transistor so that it is in the “active zone” of operation. The active zone means that you are past the threshold for the base voltage required to start to turn the transistor on, but not yet at the base voltage where the transistor is fully turned on and saturated. The DC conditions that the biasing establishes are called the “operating point”.
As I mentioned, in this circuit the base resistor provides negative feedback which tends to stabilize the operating point. When the circuit is switched on there is no current through the transistor so the collector voltage is at the power supply voltage and the base is pulled high though the two resistors. This rising base voltage turns the transistor on and this pulls the collector voltage down (because of a voltage drop through the collector resistor) which in turn reduces the base voltage – tending to turn the transistor off and reduce the collector current. In practice, these effects will almost instantaneously reach equilibrium with the transistor in a partially on state – i.e. in its active region.
Wikipedia gives a pretty good explanation of how to calculate the operating point so I won’t repeat it. One thing to note though, for the resistor values used in this configuration, the operating point will depend on the gain (β) of the transistor so the operating point will change for different transistors. Here is a graph showing how Vc depends on the gain (β).
Theoretical Vc (Y axis) Vs Gain (X axis) for Circuit Shown Above
So, that just about wraps-up the situation for the DC bias. It works just about OK as long as you aren’t fussy and don’t expect a big output voltage swing I guess! What about the AC characteristics of this circuit? What is the gain? What about this non-linearity effect?
I checked a few standard text books, and the web, and couldn’t find a theoretical treatment for this circuit configuration so I had to come up with my own. Let’s start by assuming that the impedance of the input source through the coupling resistor is low compared to the input impedance of the amplifier. In that case the input signal becomes just a change in voltage on the base of the transistor (ΔVBE). In that case we can use the Ebers-Moll equation for the transistor to work out the impact on the collector current (and hence the collector voltage). Ebers-Moll says:
IC2/IC1 = Exp( ΔVBE / VT ) Where VT ≈ 25.3mV at room temperature
If you write IC2 = IC1 + ΔIC, and convert the collector current to the corresponding voltage you can get the solution:
ΔVC = ( 1 – Exp( ΔVBE / VT) ) . ( VCC – VC1 )
Where VCC is the power supply voltage and VC1 is the voltage at the collector at the quiescent operating point
That equation fascinates me for two reasons:
- Clearly, this is an exponential amplifier – which explains the non-linearity
- The gain depends on the collector voltage at the quiescent operating point. The graph above which shows how the operating point changes with β translates to changes in the stage gain.
Can we imagine this amp could be approximately linear? Well, as my high-school physics teacher said “everything is linear if you look at a small enough range” (cue sound of 1,000,000 mathematicians groaning). So, we know by Taylor expansion that:
Exp( x ) = 1 + x + x2/2! + x3/3! + …
So, if ΔVBE << VT we can approximate the exponential using just the first two terms above.
ΔVC ≈ – ( ΔVBE ) . ( VCC – VC1 ) / VT
In other words, for very small input signals (<< 25mV) it is kind-of, sort-of, a linear amplifier of gain ( VCC – VC1 ) / VT . Here are some handy plots from Mathics of gain Vs β, Rc and Rb.
Stage Gain (Y axis) Vs β (Top), Rc (Middle), Rb (Bottom)
Actually, the stage gain in this configuration is quite high, though very dependent on the β of the transistor. I might also remark that the output impedance is poor (limited by Rc ), though, without having done the calculation, I think the input impedance is probably OK.
So, if you want a high gain, single stage, small signal amp with very few components, and, you aren’t too worried about predictability of the gain OR linearity OR output impedance then this circuit just about does the job. This actually fits the bill for these beginner’s electronic kits pretty well. If your requirements are more stringent then better avoid this circuit and look for others. Just one thing still seems odd – why is the second stage bias set where it is on the original circuit?
For me it’s been interesting to look at how much information you can extract about this, fairly trivial, circuit and it’s certainly refreshed my understanding of transistor fundamentals. I hope you find it interesting too.
11 August 2017
I was approached with a design for a coaster using a Reuleaux triangle in the middle. It is a shape of constant width so it is able to rotate within a square just like a circle would. I made some tweaks to the design like putting in a lip on the middle layer to stop the shape falling out. It works and is fairly functional but it's not very smooth and it squeaks as it turns, I've yet to come up with sensible solutions to those problems.
07 August 2017
06 August 2017
05 August 2017
04 August 2017