Assignment 8, Part 3: add flow control and test your device

From Course Wiki
Revision as of 13:21, 30 October 2018 by Juliesutton (Talk | contribs)

Jump to: navigation, search


Overview

The next step is to set up the reservoirs, tubing, and valves to control the flow through our device. The fluid from two large reservoirs will be connected to the PDMS device with thin tubing. We also need at mechanism to drive fluid flow through our device. While some devices use carefully regulated air pressure or syringe pumps to drive flow, we will take advantage of gravity. By raising the fluid reservoirs higher than the outlet tubing of our device, we create a difference in potential energy that will push the fluid from the reservoir, through the tubing and device, then out into the waste. A pinch valve (that does exactly what you think) will allow us to choose which reservoir will provide flow to the device.

Schematic of flow setup

The solenoid pinch valve has two slots for tubing – one is normally open (NO), which will be used for the regular (low salt) medium, and one is normally closed (NC), which will be used for tubing connected to the high salt medium. Applying 12V to the solenoid will switch the flow from low to high salt. We will use a short length of flexible silicon tubing that can be easily pinched by the valves, and connect it to the device and reservoirs using a stiff less-expensive tubing made by Tygon.

Pinch valve operation schematic

Assemble the fluidics caddy

  1. Grab one of the L-shaped black acrylic fluidics caddys and secure it to your vertical P14 post.
  2. Secure two 50ml conical tubes to the leftmost side of the acrylic using two routing clamps, a 1/4-20 screw and a wing nut.
  3. Secure the pinch valve to the acrylic nearest the P14 post using a two line routing clamp.
Fluidics caddy

Using a data acquisition card to control pinch valve and LEDs

In order to oscillate our flow at a particular rate, it would be nice to interface the valve with the computer so that we can use MATLAB to do the work for us. While we're at it, wouldn't it also be nice to turn on and off your LEDs using a computer signal? A data acquisition card or (DAQ) allows us to send and receive electronic signals from the computer. These are powerful cards, however, we'll only use them to send a digital +5V signal to turn something on, and a 0 V signal to turn something off.

Unfortunately the +5 V signal from the computer cannot provide enough power to run our LEDs at 1A of current, nor can it provide the +12 V necessary to control the solenoid pinch valve. Should we give up and go home? No! Let me introduce you to the cornerstone of digital electronics: the Transistor. We won't go into detail about how these incredibly useful and versatile circuit elements work (this video goes through an awesome explanation if you're interested). When implemented in the following way, we can think of a transistor as a voltage controlled switch:

Using a BJT transistor as a switch.

We will build three of these circuits, one for each of the LEDs and one for the solenoid pinch valve. We can then use the +5 V DAQ signal to control each of their on/off state.

Build the DAQ interface circuits

On an electronics breadboard, build the following circuits:

Interfacing LEDs and pinch valve with DAQ

Note the following things:

  • Each circuit will be connected to its own power supply. The blue and green LEDs will use the first and second channels on the lab power supply as usual, and the pinch valve will use the 12 V output from a computer power supply (below). Also shown below is a Molex connector that you can use to connect the power supply to the breadboard.

Navigation

Back to 20.309 Main Page