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24 January 2015

How to build a Dying Battery Simulator MARK 2

Hot on the heels of my last post where I knocked together a prototype dying battery simulator, I have more to share with you in the form of a Mark II.

Before I get any further I want to reveal that electronics snobbery is alive and well and it lives at See the above snippet...

"The non-professional nature of the 'free-form' construction method merits no further comment." ~ AllAboutCircuits 2015

Where would Rock and Roll be if Jimi Hendrix had had this attitude? Do you think we'd have made it to space if Leonardo Da Vinci had stuck with the "professional" conventions of his time? NOOOO! :-D

I expected to find a lot more of this type of comment as I've been researching my projects and to be honest this sort of thing really does seem to be in the minority. Of course, visit any forum and you get what you rightly deserve, but I've found that most of the learning articles are well written and capture that sense of adventure that Electronics seems to engender.

And yes, I am doing all sorts of things that will raise eyebrows and might even be dangerous. You know what? I don't care. Stop reading now if you are easily offended.

When we last spoke I talked about a few things that I wasn't happy about with the Mark I and I hinted that I might even try to figure out how to solve them. I can now confirm that further investigation has been done and I have progress to report!

The Mark I wired up the potentiometer in series. I wondered if wiring it up as a voltage divider might have a different effect. It does! It fixes the "backwards" effect of the pot!

The picture above shows you how to wire up a pot as a voltage divider. There's no magic to it; you have to connect all 3 pins. The introduction of a pot like this does not affect the current flowing through the circuit; it simply allows the voltage to be affected. I read a fantastic article by Electronics Tutorials called "Resistors in Series" which I think taught me why this should be.

What I think I learnt that my Mark I is behaving as it is because the pot it is simply increasing the circuit's overall resistance as I turn it up, which pushes the voltage down. That's the "backwards" effect I'm talking about: turning the dial clockwise reduced the voltage which I find counter-intuitive. Here, the overall resistance of the pot is important; Have too high a resistance and the effect of the dial will all be in that initial turn. My pot in the Mark I was 10 ohms and I was seeing a bit of this.

My Mark II on the other hand is dividing the existing voltage. The pot ranges from (furthest to the left) no division (i.e. no effect), through to fully divided (furthest to the right - i.e maximum effect). The result is that the pot effect works in the opposite direction and also I notice a difference in the taper (i.e. how much effect you get by turning the dial). Every bit of the turn reduces or increases the effect consistently. As a side note: I don't think the overall resistance of the pot has any relevance for this circuit - You could use any.

Here's the Mark II in operation. See the direction I am turning the dial and the result of this on both the display and the LED.

Notice that although the effect of turning the dial appears linear, what we actually observe is that most of the "action" happens in the first part of the turn. Also, trying to get to a specific voltage is actually quite tricky; And quite quickly, below a certain threshold, the pedal isn't getting enough power to light the LED. If you watch to the end of the video you'll see that the second time that I turn it down below 2V, the pedal actually switches itself off altogether.

So although I've solved some of the problems with the Mark I, the Mark II could still do with some tweaking.

Maybe it would benefit from being gated at the lower end so that I can't turn it down to the point of too little power? Maybe the dial should offer more precise adjustment in the range that will actually matter?

Yeah, maybe.


I forgot to post the new schematic for the Mark II. Here it is! 

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