Low Budget Conductivity Tester

Hey folks, this is my very first own design. So if you have any comments on how to improve it, I'd be glad to hear them! 

In chemistry lessons, there are often situations in which the conductivity of solutions or solids is tested in order to explain the results.

There are “low budget” conductivity testers on the internet, but they are still very expensive for the components used - for example, this page is selling one of those “low budget” things for 10 €: https://www.microscaleandmore.de/Microscale-Leitfaehigkeitspruefer/091005.1 

So I built a kind of ghetto engineered version and constructed a case for it.

You need to print the following parts: 

• 1x lid conductivity tester
• 1x cable clip conductivity tester
• 1x body conductivity tester
• 1x 5mm LED holder by firstgizmo (https://www.printables.com/model/625992-5mm-led-holder)

You also need the following parts:

• 5mm LED 
• Resistor (10 Ω or - if you want the tester to be less precise but more “robust”:  100 Ω)
• Switch (prevents the testers from discharging each other when the cables touch each other in the storage box) - I took these because I had them left over: https://www.amazon.de/dp/B0825R16JP?psc=1&ref=ppx_yo2ov_dt_b_product_details 
• 9V battery clip
• 9V battery
• Ferrule for the probes (or something equivalent)
• Two-wire cable that acts as a probe cable

The circuit is relatively simple: the switch and LED are connected to the positive terminal, the resistor to the negative terminal. 

As you can see, this even works with my very poor soldering skills. 

Have fun!