Christmas Tree Lights Battery Voltage Tester

After christmas you might have got some broken lamps that don't light up any more. You can use them for a lot of interresting projects like for example this one. Its 1.5V Battery tester that uses christmas tree lights as the display.

Step 1: Necessary Parts

This project doesn't require many parts, here is the list of them:

-Prototyping Board (or maybe even two Prototyping Boards)

-ULN2003 IC

-2 LM324 ICs

-2 DIP14 IC Sockets

-DIP16 IC Socket

-6 1KΩ Resistos (brown, black, red, gold)

-10KΩ Resistor (brown, black, orange, gold)

-33KΩ Resistor (orange, orange, orange, gold)

-7 Christmas Tree Lamps

-Some Wires

Step 2: Schematic

Schematic may look complicated but it isn't.

Circuit uses 2 LM324 OP-AMP ICs to compare input voltage with reference voltages set by the resistor ladder, then outputs of the ICs go to ULN2003 IC. This IC contains 7 transistors which can handle current of the lamps (it is around 200mA).

Step 3: Prepare Lamps

This is very simple process.

Step 1: Take the light out you should end up with single lamp like its shown in upper right photo.

Step 2: Bend the leeds like in middle right picture.

Step 3: Gently pull lamp from it's green case.

Step 4: Sand paper the leeds, they have isolation that make soldering very hard.

Step 5: Its done.

Step 4: Solder ULN2003 and Lights

Soldering lights is kind of hard but its possible, just make sure that its making a good contact and you didn't short anything. I made this circuit on two separated boards so I used the colorful wire which is shown on middle right picture. At the end of the cable I soldered GOLDPINs to be able to detach this board and be able to use it in some other project. If you are creating this circuit on one board you don't need this cable.

Step 5: Test the Board

Connect the board to 5V Power Supply, It should be capable of delivering at least 1.4A because lamps draw a lot of current (around 0.2A). We have 7 lamps so 7 times 0.2A equals 1.4A. When you connect 5V to input of the IC lamp should light up.

Step 6: Make the Board Last Longer

Clean the board with PCB cleaner. Secure the lamps with Hot Glue like its shown on the picture and secure bottom of the board with electrical tape like its shown on the picture.

Step 7: Solder Second Board

This board is quite easy to assemble but it can get messy like in my case. On the picture you can see a big maze of wires. Normally when there is nothing connected to the input all lamps will light up, if you want to disable this feature solder 1kΩ resistor between battery plus wire and ground.

Step 8: Secure Second Board

Clean the board with PCB cleaner and secure bottom of the board with electrical tape like its shown on the picture. Like with previous board.

Step 9: Test Both Boards

If you did everything right two things can happen. All lamps will be on or off. If you soldered additional 1kΩ resistor they should be off and if you didn't they should be on. At this point it is working and you can test some batteries.

Step 10: Glue Both Board Together

Very simple you can use either hot glue or some tape or anything you want. If you're using one board you can skip this step.

Step 11: Connect the Wire

Make sure that you connect it in right polarity but if you make a mistake you won't propably destroy anything. Then fold the wire and secure it with some tape or something.