CMPSCI 503(591C) -- Project #4

Project #4 - IR bumper

Often times, when transmitting signals into a noisy medium, it is a good idea to add a tag to the data that uniquely identifies the signal. This is how radio works. It is also a commonly used means of frequency encoding active sensors. Active sensors broadcast "test patterns" into the environment and then observe the signal reflected back. A simple example that is relevent to your team projects is the IR reflectance sensor that acts as a non-contact bumper. It transmits a 40 KHz IR signal into the environment and a special receiver then tries to identify that frequency in the electromagnetic soup around the robot. In this lab, you will build the oscillator presented in Section 5.3 of "Mobile Robots: Inspiration to Implementation," 2nd Edition, page 128 and then verify that it works by measuring its frequency using an oscilloscope. You will build a metastable oscillator that employs inverters to flip from logic 0 (gnd) to logic 1 (5 VDC) and RC circuits that govern the rate at which they will do that. Check out AN-118 and AN-140 for some insight into oscillators that can be constructed by cascading inverters.

Start this project by breadboarding the oscillator. We will use this oscillator constructed to modulate an Infrared LED and detect the modulated IR signal with an Everlight IR detector. The Everlight detector responds optimally between 35 and 40 KHz---tune your oscillator to this range. Use one more of the inverters on the 74HC04 hex inverter chip to drive a transistor that then drives the IR LED as shown in the figure. Q1 is able to switch larger currents to the infrared LED (up to 600 mA vs. only 4 mA from a 74HC04). Infrared LEDs are typically driven with 5-20 mA of current. When Q2 is turned on from logic high on the Handy Board this grounds the base of Q1 and inhibits D1 from turning on.

The Everlight IR detector is the three pin device illustrated. A capacitor is required that bridges the +5V and ground inputs on the detector. It's a low pass filter that will influence the noise on the signal line during transition. The spec sheet for the Everlight 8420 can be found here. Fabricate your IR bumper and solder it together on one of the small prototyping boards. You may very well use a bunch of these bumpers in your Trinity projects later, so build them carefully and document your bumper well. Provide a four pin edge connector where you will put the inhibit input, +5V, GND, and detector out signals. You may reuse the oscillator circuit for up to four IR Bumpers. Interface your board to the Handy Board. Write a simple program that polls the input port and beeps when there is a obstacle within range.

Materials:

a breadboard
hex inverter chip (you will need 2 of the 6 inverters per oscillator built)
resistors, potentiometers, and capacitors shown in the figure
two 2N4401 NPN transistors for each IR Bumper
an IR LED
a small prototype board

Grading:

demonstrate your circuit using the beep program
Report:
- One cycle of the oscillator should be proportional to twice the RC time constant, 2RC. Compute and record, for your components, what frequency you expect to find.
- measure and record the actual frequency using the oscilloscope
- tune the oscillator by measuring and recording the observed frequency as a function of the resistance in potentiometer, R2
- identify the best value for R2
- draw the circuit as built and identify components and values.
- plot the range of the IR bumper as a function of the resistance that controls the brightness of the LED
- what effect does oscillator frequency have on range for a fixed IR LED brightness?

Notes:

http://www.seattlerobotics.org/encoder/jul99/brb.html
http://users.frii.com/dlc/robotics/projects/botproj.htm