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We’ve made it to the 4th video and in this video so moving on from our last video investigating variable frequency filters using analogue switches, we now go into more advanced variable filters using multipliers. As per usual we go through the theory and then show some results from some real circuits, as well as leaving you with some other options for making filters variable.
We also cover some common errors that are made when making filters variable and look at techniques to make the ranges covered larger, expanding to several decades of range.
For some examples covering the initial design of fixed frequency filters, see some of the earlier videos and in the next few videos, we’ll get the PCB assembled and take some measurements on a system which covers multiple decades, working towards a useful bit of lab equipment.

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This is the third video in our filter design series and we’re finally looking at variable filters! We’re starting off with an overview of some basic techniques and then switched resistor techniques in detail, building up an example and measuring it’s performance.
To get started, you can use the spreadsheet that we’re covered in our previous video here to design it for the highest frequency, and then use the techniques covered here to make it variable.
In the next video we’ll cover some more advanced techniques for variable filters still moving towards a microcontroller controlled filter.

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In this, our second tutorial video about filter design, we build on the topics that we covered in the last video. This time we take you through the steps to design a state variable filter of a fixed frequency. As a bonus, we also cover some of the nuances of prototyping precision analogue circuits.
The spreadsheet we talk about in the video can be found here, and should get you started with designing your own filters.
Following on from this in the next video we’ll start to investigate some of the techniques for making variable filters, controllable by a microcontroller or from analogue electronics.

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This is the first in a few videos looking at analogue signal filters. We’ll be building up to designing filters controlled from a microcontroller, but firstly this covers some of the tools which’ll make designing filters easier, and go from basic RC filters to some high end active filters!

The books mentioned are available on Amazon below,

Active Filter Cookbook
IC Op-Amp Cookbook

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Here is a video in which we show you the build process of a new electronics bench, we also go through some of the decisions that were made.

We’d love to hear any comments.

Thanks for watching.

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Observant viewers may have noticed that there haven’t been any videos filmed in Dave’s workshop for a while. After a move a couple of years ago, the previous workshop was lost and I’ve been stuck without one since then.

However, the new house had a back garden. And who needs a large back garden when you don’t have a workshop!

From Dave's workshop build

The first step was to apply for planning permission-approval from the local government to build a structure. After submitting some basic drawings and fees, approval was granted. time to break ground!

From Dave's workshop build

Doesn’t look like much here, but lots of weekend later, basic foundations were dug. They consisted of a trench around the perimeter of the workshop, and a less deep area in the middle. For insulation, the middle section was then partly filled with expanded polystyrene. Where the ground was not level on the outside, wooden shuttering was added to contain the concrete pour.
The reason for having the foundations above ground was that I wanted this building to work as a garage later as well, so I wanted the right hand side to be roughly level with the road and driveway.

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This is the second video in our LED driver tutorial series. This video covers successful prototyping of switch mode LED drivers with a focus on using solderless breadboard

Part three coming up will cover designing and producing a PCB for a driver. We’ll also take some measurements of the design and see how well the bench lighting works!

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Here’s the first in a series of tutorials on LED lighting drivers.

We take you through the background to designing switching power LED lighting drivers and through to component selection. In the next videos, we’ll take you through successful prototyping of these circuits right through to getting PCBs designed, ordered & built!

We’d love to hear any comments, and watch out for part two coming soon.

Thanks for watching.

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On Workshops

As the mark 4 workshop nears its fitting out period, thoughts on laying out and designing workshops are currently occupying much of my waking thoughts.
This will be my fourth workshop and they’ve got a bit better every time. My first workshop consisted of a shed which was fitted out with a bench and a vise.

From Musings on workshop design
From Musings on workshop design

Apologies for the pictures, these were taken many years ago! As my skills and budget increased, electricity appeared and insulation was added. This was a welcome improvement after trying to work on electronics by tea light! However, at 2.4m’ by 1.8m, space rapidly became an issue. With a paid gap year, workshop mark 2 made it’s appearance!

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Another quick post before whilst we get our ChipKit tutorial finished today; we have been designing a circuit which is powered from a thermal battery. These have a fairly slow voltage ramp rate when they are first powered on, and so circuits need to be tested to make sure they work with this slow increase in voltage.
Repeatedly doing this with batteries gets expensive rapidly and we had a capture on the oscilloscope which describes how the voltage increases. We can save this capture as a comma separated values (CSV) file. We can output this CSV file from an arbitary waveform generator, so the obvious next step was to amplify the signal (increase the current) so we could run the circuitry from a power supply.

The easiest way to do this is to use a common collector circuit. This is a single transistor amplifier that can provide more current than the input (current gain), and outputs the same voltage-exactly what we want!

So lets pick some basic components and simulate

We’ll use a triangle wave input (in green), the output is in blue

The output looks basically good, but there is a voltage drop as well as some distortion at the lower end. We can re-run the simulation with a lower voltage input to see this distortion

As you can see, it’s not behaving itself. Let’s plot Vin vs Vout to see what’s happening.

It isn’t working as a (linear) current amplifier below 0.4V. This is because the transistor requires a certain voltage between the base and emitter in order for it to start turning on. For most applications, we can use a common potential divider biasing circuit as shown below.

However, this won’t work in this case as this common biasing circuit is AC coupled, and this amplifier needs to work for signals down to DC. What we can do to solve the distortion and biasing is to use negative feedback. Basically we look at what the actual output is and compare this with what we would like the output to be. The fastest way to implement this is the old favourite, the operational amplifier.

So lets put an op-amp in the circuit. This will handle biasing the transistor, as well as compensating for any non-linearity in the circuit. So the circuit will now look like,

So, let’s simulate with the same triangle wave as we had before (using an ideal op-amp)

And the Vin vs Vout plot

As you can see, the output now follows the input precisely (they are both on there, just overlaid!), and if we try it again with a different load (80ohms), the circuit should still amplify nicely.

Looks like it’s working, lets build it! The op-amp selection isn’t critical for this particular application as it isn’t running at a particularly high speed. The op-amp used was a MC4558CN. This has an 5.5MHz gain bandwidth product so this shall be more than fast enough for the application. It will be running off a single supply, but we don’t need to have an output near the power supply rails (0V & 8V), so we don’t need to use a rail to rail output amplifier.

Selecting the output transistor is also not critical. We are looking for an NPN transistor with a reasonable current gain. Also, as we are using this as a linear amplifier (class B amplifier, or one half of a push-pull output stage), we are expecting the transistor to dissapate a fairly large amount of power. A safe choice, and more importantly, one that was in stock, was the classic 2N3055 transistor. This has a transistion frequency of 3MHz, so we know at this point that our current gain will drop to 1. However, we are operating far below this frequency. Importantly, as this is in a TO-3 case, we can dissipate up to 115W of power into this transistor, assuming it is properly heatsinked.

A quick build on veroboard later,

The performance is pretty good. I put in a sin wave from the function generator and the output tracked the input really well.

It was tried with a 500 ohm load and a 4 ohm power resistor-performance was identical. But what about frequency performance?

If we take the freqency up to about 220kHz, then we start to get some distortion at the output.

Looking at a lissajous curve, we get

Below this frequency it works pretty nicely. For the last test, we have a look at the response to a step change in load. In this case I went from a 1k load to a 22R load and scoped the result. We could improve this with some capacitive terms in the feedback, but not too bad for a 20minute lash up job!