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23 January 2015

How to build a Dying Battery Simulator

If you've been following my antics on Google+ recently, you'll have realised that I've been getting ready to try my hand at a little electronics wizardry. It's not something that I've done before and not something that I know a great deal about... but I'm not going to let those minor details get in the way of a great adventure... and possible death!

As is often the way, I'm going to share my ordeals on this blog with you to scoff at. I will try and be as honest as I can. Please remember that I am a novice, so there are likely to be many mistakes and omissions, and I can almost guarantee to not follow convention. Let's begin...

Where to start? I have a number of projects up my sleeve, including trying to figure out how to get the pictured circuit working again. This is a picture of the innards of my Tascam Portastudio. It hasn't powered up in 20 years. I would love to get it working again, but right now, I haven't got a clue as to how to start this project.

Knee-deep in resistors, I realised that I needed to start with something a little less ambitious? Read on... 

I bought a breadboard to let me do some prototyping, and also it looked like a fun toy to play with. Perhaps getting this working could be my warm-up?

What a bloody nightmare!

There are still a few loose ends for me to tie off, but I got it working as you can see in the video. BTW - those are my son's hands. I'm desperately trying to spark his interest. Slowly, slowly, catchee monkey! ;-)

Having read up on how a breadboard works, I really didn't have any problems with this aspect of the project. The bit that threw me was the power supply. As part of the kit came a small circuit board called a YwRobot Breadboard power supply (MB-V2?). It can be used to power the breadboard and is itself powered by a USB cable or via a standard 2.11mm jack.

For the 2.11mm socket, it says on it that it requires DC input and I've read on the internet that it needs to be 6.5-12V. When I tried with a 9V DC power supply for a effects pedal, I got nothing. As I hunted around for a USB cable to try I found another adapter that I thought was a 10V DC. I tried this and everything worked fine. Wahay! Having checked it again with my glasses on I've since discovered that it is actually a 10V AC adaptor which shouldn't work and in fact should have probably killed me and burnt my house to the ground, but it does work and I'm still here, and so is my house. I'm not sure what is going on here.

Interestingly and probably all that matters here is that the power supplied to the breadboard seems to be pretty much as expected: either 5V or 3.3V DC depending upon how you set the jumpers.

The actual circuit in the video is simply me trying out an LED, a resistor and a pot to produce a dimmer effect.  You should have seen my boy smile when we tested it and it actually worked. I got a pat on the back for that one. ;-) Ha ha.

I decided to build a dying battery simulator as my first real project. Here's the video of the prototype I knocked up as a first attempt.

Before I talk about what I did, perhaps I should explain a little about why I did it?

It has been noticed by many people over many years how analog guitar effects pedals produce different sounds as the batteries that power them begin to run out. Sometimes, as the batteries die, you can hit a "sweet spot" where you discover a magic sound that you simply can't live without. All a dying battery simulator does is to allow you to find that spot a little easier, and to be able to repeatedly find it at will.

The inspiration and basis for my prototype was an article I found on the fantastic Beavis Audio site.

Here's a schematic (of sorts) for what you can see in the above video with the addition of a volt meter (which you can ignore for now - this comes later).

First of all I was flummoxed by how to wire the 2.11mm sockets up. There are 3 pins on each and we're only interested in two of them, but which two and in what order? I couldn't find any guidance on the internet that fitted my specific sockets so I ended up experimenting. I plugged a male-to-male cable into each socket and tested for continuity on the pins. One pin showed no continuity and I decided not to use this one. The other two paired up as I'd expected.

See the pot. This is connected using a "2 terminal" or "series resistor connection" which means that the 1st lug isn't connected. The pot I'm using is a cheap 10 ohm linear pot.

 The only thing that's worth pointing out here is that the pot seems to operate backwards to my mind. You turn it all the way to the left to get full power. Intuitively, I would expect it to be the other way around. You'll laugh, but I experimented trying to invert the behaviour of the pot by connecting the wires the other way around. I realise now that this was never ever going to work. The only way to do this would be to have a reverse taper pot, so this might be something worth exploring for when/if I do this for real.

I haven't experimented any further (yet), but I've noticed that the pot does most of it's work in the first part of the turn. I'd love this to be spread more evenly over the whole turn, but for now, I don't know how to do this. If you know how to solve this problem, then please drop me a comment.

One last thing: I wasn't sure which of the socket leads should have been connected to the pot, so just picked any. Actually, I experimented with both and it made no difference - both worked the same for this circuit.

Before I forget. Here's a link to a fantastic article I found called "The Secret Life of Pots" by R G Keen. It explains pretty much everything you could ever want to know about how pots work.

The only change that you will spot in this video is that I've added a volt meter display to the circuit as per the schematic above. It's a HUGE retro 10V meter, and it works! If I build a real version of this, I reckon I would go for a smaller digital display.

In this experiment I'm running a 9V DC effects pedal power supply into the circuit and back out into the effects pedal.

As you can see, on full, the effects pedal LED is at its brightest and the meter reads just a hair over 9V as we might expect. Then, as I turn the pot resistor up causing the voltage to reduce, we see this reflected in both the meter reading dropping and the dimming of the effects pedal LED.


Doesn't this look sad. Poor guy didn't last the night.

Remember, If you've got any hints on project ideas or how I could do this better, then please drop be a comment. I am all ears!

I stumbled across this video whilst trying to figure out what was wrong with my breadboard power supply. This video didn't help me, but I did enjoy listening to Thonain explain everything. He speaks French like a native! It's beautiful to hear.

Here's someone else who seems to be masquerading as a Frenchman. Yep! Our very own Ukulele Russ! I'd love to be able to embed the video in this post, but unfortunately... bastard FACEBOOK.

As you can probably tell, I am on a bit of a high following my first foray into electronics. I didn't die or anything! Ha ha.

I'm not sure what I'll do next: whether I'll try and solve some of the challenges I called out out with this project or whether I'll move on to something else. Phew.... so many choices! Until next time... Au revoir de l'assistant!

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