How To Build
SCHEMATICS - INFRARED EMITTER DETECTOR
The Infrared emitter detector circuit is very useful if you plan to make a
following robot, or a robot with basic object or obstacle detection.
Infrared emitter detector pair sensors are fairly easy to
implement, although involved some level of testing and calibration to get right.
They can be used for obstacle detection, motion detection, transmitters, encoders,
and color detection (such as for line following).
I highly recommend reading the color sensor tutorial to understand more about infrared.
Note: If you are willing to spend $10-$20 for higher accuracy and better range, considering
looking into the Sharp IR Range Finder.
Infrared Emitter Detector Basic Circuit
R1 is to prevent the emitter (clear) LED from melting itself. Look at the emitter spec
sheet to find maximum power. Make sure you choose an R1 value so that
Vcc^2/R1 < Power_spec.
Or just use R1 = 120 ohms if you are lazy and trust me.
R2 should be larger then the maximum resistance of the detector.
Measure the resistance of the detector (black) when it is pointing into a dark area and then
choose the next larger resister. This means Vout is close to maximum when there is no signal.
Or just use R2 = 11kohms
Or use a 20kohm Pot here in series with a 100ohm resistor for white line following calibration.
Infrared Emitter Detector Amplified Signal Circuit
R1 = 150 ohms (calculate as above)
R2 = 220 Kohms (calculate as above, or use Pot for white line following)
R3 = 4.7 Kohms
R4 = 10 Kohms
OP1 = Operational Amplifier LM358 package includes two op amps.
Vcc = +5 Volts
R3 and R4 determine the amplification of the op amp, gain = 1 + R4/R3.
An appropriate ratio can be determined by connecting up the circuit and measuring the voltage
entering the op amp and knowing the threshold value needed at Vout. Vout = (1 + R4/R3)Vin
so just solve for the ratio using the values for Vout and Vin.
- The LED and detector have very narrow emission and detection angles, so it matters a
LOT how you place them. Place the LED and detector an 1/8 to a 1/4 inch apart maximum,
basically parallel and almost contacting.
- Many objects are opaque to visible light (that means light doesn't pass through it,
like wood, black plastic, metal), but are transparent to IR light. Black plastic is a good
example. Many forms of black plastic are transparent to IR light, and therefore doesn't
make a good shielding material. Aluminum foil covered with electrical
tape works as a great shielding material.
- Most consumer video cameras can see IR light. This is really useful, since you can aim
your video camera at the robot, and see the emitted IR light. Many emitters are strong
enough that if you aim the robot at a white wall, and turn off all other lights in the room,
you can see how the IR light is projected from your robot. Good debugging aid.
- Another debugging aid is an IR detection card that is available from Radio Shack, and
other places. This little card has a material that changes the wavelength of IR light into
something you can see. When IR strikes this card, it causes the card to light up and sparkle.
- Output your IR values on your computer screen real time to optimize positioning and Pot
calibration (depends on if you want range detection or white line detection). You may also want to
read the sensor interpretation tutorial.
- Often people cannot remember whether the black or the clear LED is the emitter or detector.
This is the mnemonic I use to remember, dark colors absorb more light than clear, so the
dark LED is the detector. Please note that this isn't always true, as I've heard of a blue emitter
and clear detector sold by RadioShack. The easiest way to tell is point a digital camera at it, as most
cameras can see IR light.
- If you plan your robot to work outside, make sure sunlight does not interfere with your sensor readings.
The general rule of thumb with sunlight shielding is if you cannot see any data reading difference inside or
outside, your sheilding is effective enough to work. Film canisters
or electrical tape works very well. A modulated signal (such as in remote controls)
also reduces external interference.
- Depending on resistor values, your IR circuit can be tweaked to better detect color instead of
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