Science Buddies: "Ask an Expert"

HI
We were able to successfully complete this project. We have Science fair on Monday.When we did the last trial ,we realised that the pump starts without the conductivity sensor and Potentiometer,and the pump does not stop.After troubleshooting we could concluded that MOSFET may be shorted so we replaced it.IN Radio shack we could only find LM317T and 7812.We are not clear about the terminals .
https://www.sciencebuddies.org/science-f ... p040.shtml
The circuit did not work with first MOSFET.With the second MOSFET we tried our best guess for the terminals.The pumped was initially off.After adjusting the potentionmteter,we could normalized by placing both tubes in vinegar.The main problem is after putting the pipes in baking soda and vinegar,the pump did not start by itself in order for baking soda to flow in vinegar to neutralize the solution.Also based on the terminals we guessed ,we also changed the connection based on S,D, and G.
Its due Monday.We can not demonstrate like this.Please help us.Its not possible for us to get same MOSFET at such short notice.Please guide us with the MOSFET terminals and why we are not getting the results.How should we manage ?Also I think we need help with the concept of conductivity sensor.What else can we change?
Please help us ASAP.

Hello khadeejabaqui,

Sorry that you're having problems with your artificial pancreas model. I have no experience with circuit design and I don’t know if the replacement MOSFET you sourced from Radio Shack is equivalent to the one supplied in the project kit. The MOSFET “D” pin should be connected to the pump, the “S” pin should go to the negative voltage supply and the “G” pin should be connected to the conductivity sensor/resistor network. There’s a description and circuit diagram at this link:

https://www.sciencebuddies.org/science- ... shtml#help

The conductivity sensor is formed by two coils of bare wire with a gap between the coils. When the sensor is submerged in an electrolyte solution, movement of the ions in the solution will conduct current across the gap between the wire coils of the sensor. In an electrolyte the conductivity sensor has a low resistance but in water with no electrolyte, the sensor has a high resistance. The artificial pancreas circuit is designed to use this resistance difference to trigger the pump operation. This pump is on when the solution electrolyte concentration is high (low sensor resistance) and off when the solution is neutralized (high sensor resistance).

The circuit won’t work properly if the two conductance sensor wires touch as this would give a low resistance regardless of the solution. The conductance sensor wire coils must be fully immersed in a solution to function. Also, the circuit must be normalized in a neutralized solution as described in the project procedure.

I hope this helps. Please ask again if you have more questions.

A. Norman