Please point me in the right direction

[Billy10mm]

Okay, so here goes.

I'm a total noob to electronics. I just completed my first project, a 9v battery powered phone charger in an Altoids tin. (Yes, I saw Minty Boost, but figured that was cheating instead of learning). Anyway, I have plans for version 2 of said charger and have been stuck on a single feature - low battery power-off.

Essentially, I want to be able to read the voltage in the circuit and when it drops below a certain threshold, open the circuit. As a software developer by profession, my first thoughts were to a microcontroller-based piece of hardware like an Arduino but then figured that maybe I don't need the entire Arduino, maybe I can get by with just a small 4-pin microcontroller? Someone else suggested to me a MOSFET, but I'm unclear exactly what their use is and how I find the exact one that fits my needs.

I don't know, I'm really new and really lost, and I hope someone can point me in the right direction. Thanks

--Billy

[adafruit_support_bill]

There are a lot of ways to approach it. I'd recommend reading through the "Process" page in the MintyBoost tutorial first. Even if you don't plan to build the Minty yourself, there is a lot of great insight into Ladyada's engineering process, starting with her evaluation of alternative approaches to the problem.

[adafruit_support_mike]

What you want is a device called a 'voltage detector' or 'voltage monitor'. Here's a datasheet for an example:

http://www.onsemi.com/pub_link/Collateral/NCP300-D.PDF

You can build an equivalent circuit yourself if you want the challenge (Google "low battery circuit"), or use an IC if you want to save time and board space.

Looking through some example circuits will give you a general idea of how the thing works though.. the basic idea is to use a mosfet as a 'power switch' between the rest of your circuit and the battery, and then use the voltage detector to 'flip the switch', as it were.

[Billy10mm]

There are a lot of ways to approach it. I'd recommend reading through the "Process" page in the MintyBoost tutorial first. Even if you don't plan to build the Minty yourself, there is a lot of great insight into Ladyada's engineering process, starting with her evaluation of alternative approaches to the problem.

Thank you. The Process page looks really interesting, though I must admit that much of it is over my head. I am however, googling each new part/phrase as I come across it. Unfortunately I've been very busy with work and haven't yet gotten a full understanding of the entire thing yet, but hope to by the end of next week if time permits.

[Billy10mm]

What you want is a device called a 'voltage detector' or 'voltage monitor'. Here's a datasheet for an example:

http://www.onsemi.com/pub_link/Collateral/NCP300-D.PDF

You can build an equivalent circuit yourself if you want the challenge (Google "low battery circuit"), or use an IC if you want to save time and board space.

Looking through some example circuits will give you a general idea of how the thing works though.. the basic idea is to use a mosfet as a 'power switch' between the rest of your circuit and the battery, and then use the voltage detector to 'flip the switch', as it were.

Thank you for your help though, as it is, I'm not really sure about anything you've just said. I don't know what an IC is, the datasheet for that voltage detector might as well be written in Klingon, and although I was pointed at mosfet's by another forum ... I still can't figure out exactly what they do or how they do it.

Let's try this a different way .... for someone who is truly a newborn to this hobby, what's a good book or magazine to get started with the learning process?

[adafruit_support_mike]

Fair enough.. sorry for hitting you with a pile of jargon.

A mosfet is a specific kind of transistor, and for the purposes of your project, a transistor is an electrically-controlled switch. One pin (called the 'gate' in a mosfet) controls the flow of electricity between the other two. An IC is an 'integrated circuit'.. they're basically the same as any other circuit, just really small and encased in plastic.

For the feature you described -- low voltage cutoff -- you could imagine putting a physical switch between the battery and the rest of your circuit then having someone monitor the battery with a voltmeter. If the battery voltage gets too low, that person flips the switch and disconnects the battery.

The parts I talked about do the same thing. The mosfet does the same thing as the switch, and the 'voltage detector' IC does the same thing as a person with a voltmeter.

As books go, "Getting Started in Electronics" by Forrest Mims is a good introduction. Everyone will tell you that "The Art of Electronics" by Horowitz and Hill is *the* reference book.. and they're right.. but it's a bit more advanced. Many people have good luck with "Practical Electronics for Inventors" by Paul Scherz, which sits about halfway between the other two.

No collection of books takes the place of actually building circuits though. Mims has a whole series of little books full of "here, build this" circuits that are wonderful building-and-learning fodder. Get a couple of those ("Engineer's Notebook" and "Electronic Formulas, Symbols, and Circuits" are classics) plus a breadboard and some parts, then spend a few weeks working cover-to-cover building everything as you go. As you do, you'll pick up a thousand little bits of ambient knowledge that are really hard to explain but really valuable.

[Billy10mm]

Thank you very much!