Challenge: Create an infrared (IR) monitor to detect the human pulse in a finger. Materials: circuit components, breadboard, plastic tube, and foam pieces. Performance evaluation: Measure student’s heart rate.
What is the need or problem that this product addresses?
What are the design criteria?
Write your answers in your report.
Infrared pulse monitors
Human flesh is partially translucent at IR wavelengths. A pulse monitor has an IR LED that radiates light across the fingertip and a photodetector on the other side that detects light passing through the fingertip. As the heart beats, arterial blood vessels expand and contract, blocking varying amounts of the IR light.
The image above actually depicts an oximeter, which is a device that also measures the oxygen content of the patient’s blood. The oximeter uses two different pairs of LED emitters and sensors: one at visible (red) wavelengths and the other at infrared wavelengths. Absorption at these two wavelengths is significantly different between oxyhemoglobin and deoxyhemoglobin (its deoxygenated form). The oximeter compares the two absorptions to determine the percentage of oxygenation of the hemoglobin in the patient’s blood.
IR LED circuit
Build the LED lamp and switch portion of the circuit. Test it by verifying that the LED goes on and off when the switch is pressed.
Build the QED (IR-emitting diode) portion of the circuit and the second LED lamp (and other 100 Ω resistor) portion of the circuit. The LED should be illuminated.
Build the QSD (IR-sensing diode) portion of the circuit. The QSD should be detecting light overhead, which will cause the LED to turn off.
Add series resistors and adjust the potentiometer so that the LED illuminates when you cover the sensor with your hand.
Point the QED and QSD at each other. Modify the resistance (series resistors and potentiometer) so that pressing the switch turns the sensor’s lamp on and off.
The QED (IR-emitting diode) and QSD (IR-sensing diode) are both directional; i.e., they emit or sense IR light in a beam pointed out of the device—not in every direction.
The LED lamps, QED, and QSD all have two wires that are the anode and the cathode. The short wire is the cathode and should be connected to the more negative voltage side of the circuit.
The transistor has three connections: emitter (E), base (B), and collector (C). The front of device #3904 is labeled “EBC,” showing that the emitter is the wire on the left, the base is the wire in the middle, and the collector is the wire on the right.
The potentiometer is a variable resistor. When you turn it, its resistance varies so that the circuit’s performance can be optimized.
Modeling the system
Place your finger between the emitter and sensor and optimize the circuit performance to detect your pulse. (Hint: Add one or more resistors in series and adjust the potentiometer.)
Create a design based on this circuit for an IR pulse monitor using the tube, foam, duct tape, and/or other common materials.
Construct the prototype of your IR pulse monitor using the equipment.
Measure your pulse. Is your measurement reasonable and easy to take?
How well did your design work? Evaluate its performance.
Based on your evaluation, improve your design. Prototype and test it.