Most of us have probably seen these battery testers at the dollar store, if you haven't, its a pretty simple make. when I bought mine, it served well; then it started breaking being the dollar store item it was. I couldn't buy a new one because they stopped selling them (it's cheaper to buy them then it is to make them so this is really directed to those who don't have these testers to buy at a cheap store). so I reverse engineered the circuit and got a wonderful drawing on a piece of paper.
-General purpose NPN transistors x3 (3904, 9014, etc)
-2.,2k ohm resistor x1
-200 ohm resistor x3 (could also be 100, just needs to limit current)
-5.6k ohm resistors x2
-510 ohm resistor x1
-220 ohm resistors x2
-330 ohm resistor x1
-100 ohm resistor x1
-56 ohm resistor x1
-20 ohm resistor x1
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Step 1: That Wonderful Paper I Was Saying..
The schematic is quite simple to read so just look at the picture. I'm not 100% sure as to how the transistors are placed (emitter and collector) but I believe emitter connects to ground. And also I'm showing the setup of my design for those who really wanted to know how it looked.
How you build it is really up to you.
Step 2: Ending Notes
So hopefully, this helps those who want a basic battery tester but don't have one to buy, as stated in the beginning, its cheaper to buy (where I live at least).
Also, if anyone out there could further investigate this circuit, I wish to add a 3 volt measure pin installed along with the 1.5 and 9 volt installed. It would be kind if you posted the resistor values in the comments.