Infrared Emitter and Decoder

Introduction

One encounters infrared (IR) emitter/detector circuits in daily life. For example, there are doors that automatically swing open when a person walks closeby or passes a gate. In essence they operate as follows: The emitter passes an infrared beam which is detected by a phototransistor. When a person walks by, he "breaks" the beam. Upon this event, the phototransistor no longer can detect infrared light and another event is triggered - namely the door opens. Such a circuit is described here. This circuit can be easily made within a couple of hours. One can use simple point to point soldering or wirewrap. Because, of the versatility of this circuit and its small size factor, I created printed circuit board (PCB) artwork. The artwork measures approximately 2 inches by 2 inches. Typical copper boards at stores or surplus are large enough so that one can quickly made 4 of these IR boards at the same time.

One quick note about PCBs. I never had good success with iron-on transfer methods. I thus used the rub-on transfers from Radio Shack. I also imagine you can use the resist-ink pen too, since the trace lines are not thick and component spacing is not critical.

My motivation is to use such a circuit for motor speed detection. The photos that follow show the applicability of this circuit for such purposes. This article is broken down as follows:

Parts List with Potential Vendor Source
Theory of Operation
Construction
PCB Artwork

Motor Speed Applications

Parts List and Potential Vendor Source

Below is a parts I used for my construction. The beautiful thing is that all parts (except perhaps crimps and housings) from Radio Shack.

TABLE 1A: 8255 CARD PARTS LIST
PART DESCRIPTION	VENDOR PART	PRICE (1995)	QUANTITY
Infrared (IR) Emitter	Radio Shack #276-143	1.69	1
Infrared Phototransistor	Radio Shack #276-145	0.999	1
7414 Hex Inverting Trigger	Radio Shack #276-2808	0.89	1
NPN 2N2222 (or MPS2222A) Transistor	Radio Shack #276-2009	0.59	2
220 Ohm Resistor			1
1000 Ohm Resistor			1
470 Ohm Resistor			2
4 Pin header/housing (Optional)			1
On/Off LED (Optional)			1
220 Ohm Resistor(Optional)			1
Heat Shrink tubing (Optional)
2 Pin header/housing (Optional)			1

Theory of Operation

If the emitter and detector (aka phototransistor) are not blocked, then the output on pin 2 of the 74LS14 will be high (apx. 5 Volts). When they are blocked, then the output will be low (apx. 0 Volts). The 74LS14 is a Schmitt triggered hex inverter. A Schmitt trigger is a signal conditioner. It ensures that above a threshold value, we will always get "clean" HIGH and LOW signals. Not Blocked Case: Pin 2 High Current from Vcc flows through the detector. The current continues to flow through the base of Q2. Current from Vcc also flows through R2, and Q2's Drain and Emitter to ground. As a result of this current path, there will be no current flowing through Q1's base. The signal at U1's pin 1 will be low, and so pin 2 will be high. Blocked Case: Pin 2 Low Current "stops" at the detector. Q2's base is not turned on. The current is re-routed passing through R2 and into the base of Q1. This allows current to flow from Q1's detector and exiting out Q1's emitter. Pin 1 is thus high and pin 2 will be low.

Construction

I suggest point to point soldering for this circuit. It is simple enough and takes less than an hour. Component spacing is not critical. I do recommend using heat shrink tubing when soldering the wires to the ends of the phototransistor and emitter. This will prevent any possibility of shorting. Electrical tape can be used instead but is not as "graceful". Care should be used in the polarity of the IR LED and phototransistor. For the IR LED, one can use a ohmmeter. IR LEDs work like diodes. If one hooks up the positive clip of the ohmmeter to the positive (anode) side of the LED, and negative clip to the negative (cathode) side of the LED, then the ohmmeter should read approximately 0 ohms. If it is in the megaohm range, then you know you have the polarity reversed. For the phototransistor, you can use a similar test. First, connect your ohmmeter clips to the ends of the detector. Make sure there a desk lamp is not shining directly on the detector. The resistance should be high. Now, shine light on the dectector. The resistance should go low. If not, then you have the polority reversed. This "quick and dirty" method works because white light has some infrared light embedded in it. Lastly, make sure your 2N2222 NPN transistors are in the correct polority. One cautionary note: I rarely trust Radio Shack markings. I have a nice digital multimeter from Radio Shack. It has a transistor detect feature which quickly allows me to tell the polorities of Base, Emitter and Drain. If you don't have such a meter, then you have to do some "trial and error" with an ohmmeter. There should be little resistance between a NPN transitor's base and emitter. There should be a large resistance between the drain and emitter. Lastly there should be a moderate resistance between the drain and base.

Printed Circuit Board (PCB) Artwork

Solder Side

Component Side

Component and Silk Screen Side

Motor Speed Detection Application

The above photos show how the IR emitter/detector board will be used for motor speed determination. The black circular disc has one hole. The phototransistor can detect the IR emitter's light source though this hole. Thus, only at this point will U1's pin 2 output be HIGH. Since the disk is black (opaque) the phototransistor will be blocked and pin 2 will always be LOW. I used the 99 cents 1.5 to 3.5 VDC motor from Radio Shack (273-223). The black disc mounts on the motor shaft. The motor is mounted in a plastic motor stand. The stand is mounted on a piece of wood. The motor stand was milled and drilled by our Mechanical Engineering machinist Walter Kahn. He did an excellent job. It gives the motor a stable base. There are 2 shafts on either side of the disc. The emitter slides into one shaft. The detector slides into the other. In the future, I will show how one can interface the IR emitter/detector board to a PC (via the 8255 PPI board or other commercial digital data acquisiiton board) or embedded controller (Intel 8051 for example). In the fuutre I hope to introduce the reader to the following:

Speed Dectection using the IR Emitter/Dectector Board
Control Systems Theory and Radio Shack: with only high school math, human intuition, and cheap parts from Radio Shack, I will teach university level control systems. Everything from motor system identification, PID control, and Kalman filtering!
Motors and Embedded Control: using the Intel 8051 8-bit embedded controller, I will develop an motor control system
Mobile Robot from hacking the Radio Shack R/C cars Click here to go Paul's Main Page
Click here to go Columbia University's Robotics Lab
Click here to email me