In this Instructable you will be building an LED battery tester for any 1.5 V batteries.

Connecting this circuit to voltage of greater than 1.5 V will cause failure of the LED. Thus you might want to use a battery holder instead of just test points to test the battery so that you do no accidentally insert 9 V or 12 V batteries. There are also old fashioned 4.5 V batteries that are no longer in use.

Warning! Do not connect multiple batteries under test in parallel. There is small chance that the batteries will become hot and explode.

Supplies

You will need:

- one battery holder for one 1.5 V battery (power supply - AA or AAA),

- a few battery holders for 1.5 V batteries (for battery under test - AA, AAA, C, D),

- a few LEDs (you only need one but you might accidentally a few),

- Electrical box (shown in the picture),

- 100 ohm resistor,

- wires,

- drill, - screw driver,

- scissors,

- pencil, - solder and soldering iron,

- 0.9 mm or 1 mm metal wire,

- small bit of matrix board (optional),

- wire stripper,

- power supply (optional),

- multi-meter (not really needed).

Step 1: Test the LED

Use power 2 V power supply to the LED. You can see on the photo that the longest LED pin is the positive terminal. Increasing the voltage to more than 2 V will burn a typical LED instantly.

Step 2: Connect the LED

Use soldering iron to connect the LED to wires. The length of wires should only be about 10 cm.

Use metal wire to secure the wire to matrix board.

You can just connect the LED to wires without matrix board. Then there is not need for the metal wire in this Instructable. However, this would not be a reliable connection if the LED is not secured to box due to hole made for the LED being to large. The movement will eventually break the connection between two terminals.

Step 3: Drill a Hole for the LED

Mark the hole with the pencil as shown in the first photo. Note the blue line showing the required position of the hole because the LED needs to placed on top of the plastic fixture.

Use electric or normal drill to drill a hole for the LED. The diameter of the drill depends on the size of the LED. There are very small LEDs available on https://ebay.com or https://aliexpress.com You might never predict the size until they arrive in the mail. It is best to make a very narrow and then widen slowly with scissors.

Warning: Do not rush with the scissors. You might cut yourself.

Step 4: Build the Circuit

PSpice software was used. The LED is modelled with three general purpose diodes because the LED component is not available.

This is how I calculated the resistance:

R1 = (VPowerSupply + VBattery - Vled) / LED current = (1.5 V + 1.5 V - 2 V) / 10 mA = 100 ohms

I used two 180 ohms resistors in parallel which gives 90 ohms. You can also used two 220 ohms resistors in parallel or one 220 ohm with one 180 ohm in parallel.