Tuesday, 15 June 2021

Workshop apron from leather scraps

Now that I've recently added a welder to my workshop, I need something a bit more protective than just the usual old clothes I use as workshop attire, so I've been planning to build a leather apron.

What makes this project a bit different though, is I have inherited a large box of leather from another leather crafter who sadly passed away. It's a significant amount of material, so I don't really want to go purchasing yet more for this project.

So the challenge that I set myself is to 'frankenstein' the apron together from the smaller pieces, but do so in a way that hides that fact - or at the very least, styles it out so it doesn't look like a patchwork bodge.

I'm using a borrowed canvas apron as a rough template, but will adapt where necessary to suit me.

The largest suitable piece I have is this, which covers most of the area, but the template comes in a bit short at the edges.








A second piece, although a different colour can attach to the bottom, extending it to a more suitable length. This covers the size of the canvas example apron.






I also created edge pieces for the vertical sides of the apron, to tidy it up.
When I've used aprons in the past, I've always found an annoyance of them is how the side bits often 'flap about'. To try and combat this in this design, I've enclosed a length of steel wire in there, which should strike a balance of being flexible enough yet maintaining some sense of shape.

There's still the scuffed portion at the top to deal with. This will be where I affix the straps, so have cut a couple smaller pieces of thicker leather here so which cover the problematic patch, and provide the mounting point for the strap. I did the same on both sides to maintain the symmetry.

There was also enough left over to add a central pocket, which I planned to divide into two.


 I wanted all the pieces to be a consistent colour, but not too dark, so I opted for a shade of brown just slightly darker than the pieces were currently.

After dyeing, I fixed the bottom and side sections in place with contact adhesive. This would serve as a placeholder while I stitched them, and add to the overall strength.



In the box of leather that I inherited, there were what appear to be, a number of several unfinished belts. These were all consistent in the type of leather, so were perfect to repurpose as straps for the apron. All I needed do is dye them to match the colour.


For the shoulder straps I used a single rivet, as this would also act like a pivot and allow flexibility. This was covered over by the top pieces I mentioned earlier.

I joined these in the middle of the back using a small piece cut from thick leather, from which another piece moves downward to join the belt portion, like a 'Y' shape.



Another 'junction' piece joins that to the belt in an 'upside-down T' shape.

The belt itself is simply riveted to the sides of the apron. I made the right-hand side particularly for the buckle, as I reasoned it would be easier to fasten/unfasten from the side of the body than trying to reach behind my back.

Finishing touches

I soon realised that the front pocket would just end up filling with dust, so I made a cover flap for that.

Also used some remaining scraps to add some padding to the straps near the shoulder.

Sunday, 9 May 2021

Dartboard Cabinet

I got a dartboard for Christmas, which I have on the wall in my home office.

Darts is a great excuse for getting up and stretching your legs between video meetings, or while thinking over a problem, rather than just pacing back and forth.

However up until now it's simply had an old piece of hardboard as a backing, so thought it's time it got a proper cabinet.

The design is to have two compartments, the main dartboard cupboard (obviously!), and below that a small compartment which can be used as storage for the darts and accessories, and will open to provide a platform to help catch bounce-outs.

The Frame 

The frame is made from a reclaimed iroko desktop - I believe it was from an old school chemistry lab. It's about 5cm thick, so ideal for covering the thickness of the dartboard.

This was ripped down into 4 lengths to make the sides of the frame.

These are jointed using lapped mitre joints, screwed through from the back for extra support.

The divider between the main dartboard compartment and the lower storage compartment is joined into the sides with pocket screws.

The backing of the main compartment is hardboard, reclaimed from some dismantled hollow-core doors, simply pinned into rebates in the frame.
The backing of the lower storage compartment is reclaimed laminate flooring, which helps to add rigidity to the structure, again, screwed into rebates in the back of the frame.

I layered some cork over it, to try and prevent bounce-outs and reduce impact noise, although in hindsight I regret that decision, as the thin cork veneer seems to disintegrate at the slightest provocation. But seeing as it's there to catch errant darts, it's pretty much a consumable anyway.

The doors
The doors are iroko, edged with oak to add some contrast.

The fold down door for the storage compartment is reclaimed from a previous project which didn't pan out, and is already built in the same manner.

It also has a leather lining, again from a previous project - I don't know how well that's going to withstand some dart bounce-outs, but I don't think it's chances of surviving being ripped out are much better, so opted to leave it in.

For the storage compartment, a leather lace is used to create a limiter to stop the door folding down too far. This simply threads through a hole drilled in the divider and is knotted.







A simple latch for this is mounted in the side, made from miscellaneous brass hardware from the junk bin.

On the door side, the lace is threaded through more brass hardware (I believe a Chubb keyhole cover), and knotted. The knot is recessed into the door to hide it, and the hardware screwed to the door.







The handles

The handles for the doors are made of leather. This is done by creating a loop as shown below.

A hole is drilled through the thickest part (the 3-layer section). Between the second and third layer only, a small screw is placed with a washer.

The first and second layers are stitched together, and the handle screwed to the door. The hole in the first layer provides access for the screwdriver, as the head of the screw is hidden.

When that's done, the hole in the first layer is hidden by the rivet.


Finishing touches

In the back of the cabinet, keyhole hooks were recessed into the top and bottom of the frame. The sides would've been preferable, but the placement on the wall would have collided with cabling behind the wall.

A whole was cut in the centre of the dartboard compartment, this is to allow the existing wall mount to pass through - so the dartboard does still have a direct mount to the wall, which aids with it's positioning on the wall - we can make use of the calculations we did when it was first hung.

Finally, a couple of coats of danish oil were applied to bring out the pattern of the grain.

Friday, 19 March 2021

Sound-activated switch for a set frequency

Clap switches are an old automation gimmick from the 1980s. Basically the circuit hears a noise above a given volume (amplitude), and activates.

This project is an attempt to refine that idea, to create a switch that does the same, but responds only to a given sound (frequency).

This was inspired by watching my partner fail miserably playing South Park - The Stick of Truth.

In the game during battles, there is a timed button press during an attack that increases damage. Timing these button presses was not going well...

That noise triggers at the time the button press is required, so I started thinking about how that noise could be listened for, and the button press triggered.

Band-pass filters

This system relies on the use of a band-pass filter. There are plenty of explanations around about these and how they work, so I won't reinvent the wheel here.

For the purposes of this project, the key point is if the input has a frequency between the low and high thresholds, it is allowed through. Frequencies outside of those thresholds (bands) are rejected.

These can be built in hardware as circuits, but also in software on regular computers.

Fast-fourier Transform (FFT)

This is a well-known algorithm that, in simplistic terms, takes input over time (such as audio), and breaks it down into the frequencies it's composed of.

I'll admit, my understanding of FFTs is similar to the relationship most people have with their household appliances - know how to use it, but can't really explain how it works under the hood. There's plenty of detailed explanation for the more mathematically inclined.

So the basic concept is this

  • Pipe the audio input into the FFT
  • The FFT converts it into the frequency domain
  • Zero out all the values for the frequencies that fall outside of the range we're 'listening' for.
  • Do an inverse FFT transform, which turns the frequency domain data back into time-domain (i.e. back to real audio). This gives us sound where everything except for the range we're listening for is muted.
  • This can then be passed into a regular 'clap-switch', where we trigger if the volume of the sound is above a given level.


Finding the target frequency

This part of the process can be trickier than it initially seems. Sounds are composed of many frequencies, so it is necessary to select a frequency range that is unique to the target part of the audio.

To start with, I extracted the audio from the above video clip using FFMPEG, and opened it up in Audacity. This initially shows the audio waveform.

Select the area containing the sound, and select Tools, Plot Spectrum.

This will show the frequencies that exist within the selection. However, this doesn't give us all the answers. Save the plot (I just took the below screen-shot and used that). Then select another parts of the audio and repeat the exercise. Then basically it's a case of spot-the-difference, looking for a frequency spike that appears in our target audio but not the other samples.

A sample spectrum from elsewhere in the audio.
The segment containing the target sound. Circled are frequency spikes not seen in the rest of the audio,


To run this switch I'm going to use the Next Thing Co CHIP. This is the same system I used for the TV desk project years ago. Unfortunately these are now discontinued, but there are still many ARM-based SoCs running Linux out there.

Potentially this could be distilled down further on to a smaller microcontroller, although I'd have reservations about how far you could reduce the resources until the processing time introduces enough lag to make it too slow to use.

As well as effectively being a 'proper' computer, the CHIP has general purpose IO pins, like most microcontrollers. This can provide the interface for the output of the 'clap' switch. In this case, for the sake of example I'm just hooking up a simple LED that will blink on detection of the given sound.

Although the CHIP does have microphone pins and the ability to switch it's video pin from the jack to be an audio in, for the sake of prototyping, I found it much easer to just use a cheap USB adapter which has microphone and headphone sockets.

Power comes from a standard USB phone charger.

Control is done via a serial connection to my PC, using Minicom.


As the CHIP is a full-blown Linux distribution, there's lots more flexibility in the software that we use. I ended up using Java and the Apache Commons Math library.

The basic OS was pre-installed, Java 8 JDK was installed from here.

The Java code listens to the microphone input, and allows the user to load a JSON file containing details of the filter to apply - this made testing and refining the filter easier. The values are the start and end of the frequency range to listen for, and the threshold of amplitude to trigger the output (This figure can be a bit of trial and error based on how loud the input is, and can vary if the input volume varies.)

It can also be controlled via command line to either pass-through the audio to the headphones as-is, or post-filter - i.e. you hear what the 'clap' circuit would hear.

Code is on GitHub here.


Some configuration was required to enable the GPIO pins to be activated on boot. To do this, the necessary commands (below) are wrapped in the bash script 


echo 1023 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio1023/direction
echo 0 > /sys/class/gpio/gpio1023/value
chown chip:chip /sys/class/gpio/gpio1023/value

(Refer to the CHIP docs for what exactly these mean)

This is set to run on boot by adding the below line to /etc/rc.local

sh /etc/init.d/preparegpio.sh

Finally, for the Java code to trigger blinking the LED, it triggers another SH script. I went this route as I intend to develop the Java code into a more general purpose audio tool, so didn't want to tie the code too closely to the hardware I'm using for this project.

It also has the benefit that the audio processing doesn't wait for the GPIO operation to complete, thus reducing the lag.


echo 1 > /sys/class/gpio/gpio1023/value
sleep .5
echo 0 > /sys/class/gpio/gpio1023/value


While it certainly does respond to the input audio as expected, there is definitely some processing lag, as can be seen as the video progresses.

This isn't entirely unexpected, and could be overcome by throwing more computing power at the processing (The CHIP is a 1GHz processor), or possibly further optimisation of the code (or porting it to C or similar).

That might be the subject of a follow up project at a later date, but for now, this demonstrates the idea.