This system is meant to take bottled DI water and squirt it onto a pig eye for external corneal hydration during experiments. It is intended to automate a practically cumbersome process that previously was performed by a medical fellow or engineering student who needed to remain beside the eye and squirt water onto the eye every ~30 s to maintain proper hydration. This system interfaces with the robot control software so that the operator, during robot-controlled surgical procedures can hydrate the eye at the click of a button.
Like any assembly, we start with a CAD assembly to design the system. To save development time, the tygon tubing is modeled as simple cylinders (yellow).
The electronics box takes special consideration since it will be 3D printed and must interface with specific electrical components.
The finalized box has two inputs: 12 VDC power and a SIG (signal) line from the controller; and one output: voltage to actuate the solenoid valve.
The ON/OFF control is through a simple Darlington transistor pair (Sziklai pair)—no need to draw this up in Eagle. The sketch illustrates the idea and transistor choice: 2N555 for the NPN and MJE2955 for the PNP. Applying a "high" signal (3.3 V) from the controller to the "digital input pin" causes the ground-side of the load (solenoid valve) to connect to ground and complete the circuit, turning it on. Supply voltage is 12 VDC.