1. http://www.arresearchpublication.com/images/shortpdf/1459435178_1159B.pdf
2. Automatic Tyre Pressure Control in a Vehicle
3. https://www.grundfos.com/service-support/encyclopedia-search/non-return-valve.html
4. https://en.wikipedia.org/wiki/Solenoid_valve
5. http://www.ques10.com/p/5298/prove-that-op-amp-can-be-used-as-current-to-volt-1/