This is a voltmeter / Battery tester!
With the hole on the rights side, you can directly insert a 9V Battery.
I've a Youtube Video about this 2 in 1 Voltmeter:
Necessary materials:
- 2 parts made with 3D Printer
- Panel meter: https://www.gotron.be/paneel-voltmeter-4-5v-30vdc.html
- 9V battery connection
- Crocodile clips
- Copper wire