I recently went into my attic and pulled out my old 1990s 4-track Fostex X-26 tape recorder. I was hoping to transfer some of my old band tapes to the computer. Alas, it is dead. While it turns on, the motor appears frozen, and the device hums. (I don't mean there's line noise when you plug a speaker into it, I mean the device itself audibly hums, even when not using the tape controls.)

I have seen online that some people repair theirs. It's probably beyond my skill level to do so, though. (And I don't really want to do it myself.) I have contacted a few shops near me (Los Angeles), but have gotten no replies, unfortunately. I was wondering if anyone here knows of a good shop that would repair an older device like this.

I have also contacted some places that claim to do cassette to digital transfers, but none have replied to my questions about whether they can deal with a 4-track recording. Also, just looking at their prices, it seems like it would cost significantly more to have them convert my tapes than to get the recorder repaired. (And I'm not sure what's on all of the tapes, so I don't really want to hand over some embarrassing practice tape or tape containing some out-of-context banter between takes on it.)

I've looked online for a used one, but all the listings I've found have been "for parts only", which is what mine would be if I were to sell it. And knowing how these things go, they probably all have the same broken part. So any recommendations are appreciated!

Back when I was a kid, I had a radioshack 200-in-1 science fair electronics kit this one

I loved that thing, and it sparked an interest in tech that ultimately led me to a CS degree.

Id like to buy something similar for my much younger sister, but nowdays everything I can find is arduino or raspi based. Id love to get something like that eventually, but I think it might be better to get a kit that more focuses on individual components first... Does anyone know of one sold these days?

Alternatively, if anyone has one of these and would be willing to scan the book, Id love to build one of these as a breadboard-based system.

I’ve been wanting to get in to hobby electronics now that I have some free time, but I’m not sure what I want to start with. I am super interested in building an 8-bit computer but I’m curious what other types of things people have built or are building. Feel free to talk about any Raspberry Pi or Arduino projects as well even if they’re not strictly ‘hobby electronics’.

I recently came across a cool video on youtube. https://www.youtube.com/watch?v=dxj8JwdQ7Lk&feature=youtu.be - The guy added a 4g connector to his rc plane and I think some extra batteries and managed to fly to the next island over.

I love stuff like this. Do you guys know any other current electronics that can be modded like this? Sadly it seems like most new consumer electronics come with their own small walled garden and often enough stuff just stops working once the seller goes bankrupt.

Other things that come to mind are:
Raspberry Pi
Arduino
ESP
But do you guys know whole systems that are moddable like this?
The Ryze Tello is a programmable drone, which is pretty cool as well.
I also saw some people modding 3d printers to laser cutters

Let's do a little show and tell for our new toys :)

(I was thinking about this as a non-recurring equivalent to the listening threads and such but I probably worded it a bit too...fluffily for ~tech so I moved it to ~talk)

I am currently trying to learn how to fix things myself by learning how to solder and fix things like that. I also want to DIY some things for quite some time.

My current project is working through the book Make:Electronics , I bought it at humblebundle some years ago. Yesterday I powered a led via a power outlet for the first time, by connecting it to a cut off USB cable. I'll share a picture of the abomination later.

So: what have you built at home? What are you building? What are you planning to build?

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Bookmarkable meta post with links to all previous topics

Today's topic

Today's topic will be light emitting diodes, better known as LEDs. As the name suggests, we'll have to discuss light and diodes. We will find out why LEDs can only emit a single colour and why they don't get hot like other sources of light. Let's start by discussing diodes, in case you are already familiar with diodes note that I will limit the discussion to semiconductor (p-n with a direct bandgap) diodes as that's the type that's used in LEDs.

What's a diode?

A diode is an electronic component that, ideally, only lets electric current through in one direction. In other words it's a good resistor when the current flows in one direction and a really good conductor when the current flows in the other direction. Let's look a bit closer at how diodes function.

Semiconductors

Diodes are made out of two different semiconducting materials. In everyday life we tend to classify materials as either conducting (metals being the prime example) or non-conducting (wood, plastics, rubber). Conductance is the flow of electrons through a material, a conducting material has a lot of electrons that can move freely through a material while an insulator has none. Semiconducting materials fall in between these two categories. They do conduct but not a lot, so in other words they have a few electrons that can move freely.

N-type semiconductors

We are able to change a semiconductor's conductivity by adding tiny amounts of other materials, this is called doping. As an example, we can take silicon (the stuff that the device you're reading this on is made out of) which is the most well-known semiconductor. Pure silicon will form a crystal structure where each silicon atom has 4 neighbours, and each atom will share 1 electron with each neighbour. Now we add a little bit of a material that can share 5 electrons with its neighbours (how generous!). What will happen? Four of its shareable electrons are busy being shared with neighbours and won't leave the vicinity of the atom, but the fifth can't be shared and is now free to move around the material! So this means we added more freely flowing electron and that the conductivity of the semiconductor increases. An illustration of this process is provided here, Si is chemistry-talk for silicon and P is chemistry-talk for phosphorus, a material with 5 shareable electrons. This kind of doping is called n-type doping because we added more electrons, which have a negative charge, that can freely move.

P-type semiconductors

We can do the same thing by adding a material that's a bit stingy and is only willing to share 3 electrons, for example boron. Think for a moment what will happen in this case. One of the silicon atoms neighbouring a boron atom will want to share an electron, but the boron atom is already sharing all of its atoms. This attracts other electrons that are nearby, one of them will move in to allow the boron atom to share a fourth electron. However, this will create the same problem elsewhere in our material. Which will also get compensated, but this just creates the same problem once more in yet another location. So what we now have is a hole, a place where an electron should be but isn't, that is moving around the crystal. So in effect we created a freely moving positive charged hole. We call this type of doping p-type. Here's an illustration with B the boron atoms.

Creating a diode

So what would happen if we took a n-type semiconductor and a p-type semiconductor and pushed them against one another? Suddenly the extra free-flowing electrons of the n-type semiconductor have a purpose; to fill the holes in the p-type. So these electrons rush over and fill the holes nearest to the junction between the two semiconductors. However, as they do this a charge imbalance is created. Suddenly the region of p-type semiconductor that is near the junction has an abundance of electrons relative to the positive charges of the atom cores. A net negative charge is created in the p-type semiconductor. Similarly, the swift exit of the electrons from the n-type semiconductor means the charge of the cores there isn't compensated, so the region of the n-type semiconductor near the junction is now positively charged. This creates a barrier, the remaining free electrons of the n-type cannot reach the far-away holes of the p-type because they have to get through the big net negative charge of the p-type near the junction. Illustration here. We have now created a diode!

How diodes work

Think for a moment what will happen if we send current* (which is just a bunch of electrons moving) from the p-type towards the n-type. The incoming electrons will face the negative charge barrier of the p-type and be unable to continue. This means there is no current. In other words the diode has a high resistance. Now let's flip things around and send electrons through the other way. Now they will come across the positive charge barrier of the n-type semiconductor and be attracted to the barrier instead. The electrons' negative charge compensates the net positive charge of the barrier on the n-type and it will vanish. This destroys the equilibrium situation of the barrier. The p-type holes are no longer repelled by the positive barrier of the n-type (as it no longer exists) and move closer to the junction, this means the entire barrier will fade and current can move through. We now have a conductor.

OK, but I don't see what this has to do with light

Now let's find out how we can create light using this method. When current is applied to a diode what happens is that one side of the diode is at a higher energy than the other side. This is what motivates the electrons to move, they want to go from high energy to low energy. If the p-type semiconductor is at a higher energy than the n-type the electron will, upon crossing the junction between the two types, go from a high energy level to a lower one. This difference in energy must be compensated because (as @ducks mentioned in his thermodynamics post) energy cannot be destroyed. So where does the energy go? It gets turned into light!

The energy difference between the p-type and n-type is fixed, meaning a fixed amount of energy is released each time an electron crosses the junction. This means the light is of a single colour (colour is how we perceive the wavelength of light, which is determined by the energy of the light wave). Furthermore, none of the energy is lost so there is no energy being turned into heat, in other words the LED does not get warm.

Conclusion

So now we know why the LED is so power-efficient; it does not turn any energy into heat, it all goes into light. We now also know why they only emit a single colour, because the energy released when an electron crosses the junction is fixed.

Next time

I think next time I will try to tackle the concept of wave functions in quantum mechanics.

Feedback

As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

Addendum

*) Yes, current flow is defined to be opposite to the flow of the electrons, but I don't want to confuse readers with annoying definitions.

Album link

So we have had this popcorn machine for ages and just recently it completely stopped working. Being a fan of bigclive on youtube I though why not try fix it and maybe get a sense of how it works.

The guts

The insides honestly are pretty underwhelming - both the impeller and heating elements are quite small considering it only takes a few minutes to make a bowl of popcorn.

Circuit board and Another shot

Considering how few components there it was not too time-consuming to draw up a diagram. Please excuse the crude drawing.

From what I can tell it uses a bridge rectifier with a some caps to convert DC and even out the signal - and what looks like two resistors to further drop the voltage to the motor, maybe two offered better cooling? idk why they didn't use one. It's also kind of interesting how they used two separate coils for the heating with one of them in series with the motor, that will drop the voltage right?

They also have overheating protection here, with a an sefuse and a thermo switch, it actually has the piece of metal and a contact that will bend away when it's hot, I've only seen these in a textbook. Not sure why there are two types of thermo fuses?

The fault

Finding the fault ended up being quite easy, the main heating coil that comes off blue wire had melted. A piece of corn, oil, bits of salt or a mixture had found their way in and somehow melted the wire in multiple places. It cut the circuit so not even the motor was working which was my initial though when it stopped. I wound some solid core wire around the two ends, hopefully it will not unfurl or melt in the heat.

To summarize, I am annoyed that there are two different standard for 4-pole audio connectors. For those curious I mean this.

You have OMTP and CTIA, the difference is they swap the mic and ground pins. This is irritating because Apple vs Android use them differently. This becomes especially annoying when you want a feature like an inline mic mute switch (one designed for CTIA for example will disconnect the ground pin on OMTP instead of mic)

This has been an ongoing frustration for me for a while. I really enjoy a good pair of headphones because I use Discord and I work from home which necessitates using headphones for extended periods of time to listen to music, take calls, chat on discord.

I just want there to be a device that does OMTP/CTIA swapping AND include the ability to physically mute the mic. Like this but with something that will break the mic pin. Im currently designing something in fritzing that will allow both direction switching as well as selective muting.

/rant

Has anyone else had any similar experience or frustration with this problem?

Someone recently asked me to replace the battery in their old iPod, and I found myself wondering what I should do with the old battery. It still works, but has less capacity than when it was new. So I looked around my workshop and found some of these surface mount LEDs and decided to test the limits of my soldering skills and make a flashlight out of them.

These LEDs are very hard to solder, since they're surface-mount and the pads are on the bottom of the LED. They were never meant to be soldered by hand, but rather placed by machine onto a specific amount of solder paste, which is then baked in a fancy oven at very specific temperatures for very specific times. To solder these by hand, you need to create a liquid puddle of solder and sorta float the LED on top, while being careful to not short the pads which are very close together as well as not overheating the LED. The temperature the plastic melts at seems to be only a few degrees higher than the solder melts at.

I wired up 5 of the LEDs in parallel, each with its own 6.8ohm resistor wired in series with the LED. This should limit the current to 150mA per LED. I hot glued this in place, as well as a lithium battery charging circuit I got off ebay for a dollar. Here's one such listing.

I slapped on a pushbutton, and Bob's your uncle! It worked first try!

Here's a blurry picture of the finished product. I'm pretty proud of how it came out, considering how tiny and fiddly the soldering was. And, I think I'll actually get some use out of it too. The battery ought to last at least an hour of runtime, and the thing is seriously bright.

Anyone here into electronics as a hobby?

Edit: Better-ish pic: https://i.imgur.com/Kxqy1jg.jpg

^{No potatoes were harmed in the making of this photo.}