Implementing Infrared Object Detection

This page is going to describe the parts, schematic, software, and some tips on using Infrared for object detection. The parts used are typically the same parts found in most consumer electronic remote controls, and are widely available. If you have an old TV or other IR remote device, you have the makings of a nice IR detection system.

The Parts

The two basic parts for working with IR are the emitter and the detector. The emitter is typically an LED that emits near-infrared light. A typical wavelength is 880nm, which is just beyond the human eyes ability to see. Many suppliers, new and surplus, can provide you with sources of LED emitters. A typical IR detection system will use two emitters. By controlling when the emitters are active, the detection system can determine simple directional information: Left, Right, or Front.

There are several types of detectors for IR light. There are photo-diodes, photo-transistors, and a couple others. Usually, you buy IR emitter/detector pairs, which are tuned for each other. This is recommended when buying surplus, since chances are someone has verified they work together.

The other important detector for working with IR is a detector module, such as the Sharp GP1U5 (also the G1U52X) module. This module contains the IR detector and a small circuit that detects a 40khz modulation (i.e. the IR light is flashing on and off at about 40khz). The nice part about using a modulated light is the ability to reject noisy light signals. The world is full of light sources. Being able to detect a certain quality of a light, such as flashing at 40khz, means you can filter out much of the light you don't want.

Picture of infrared components

(Click on picture for larger view)

The components of the IR detection system

The above photo shows these parts installed on the front of a robot. The Sharp IR detector module is the square box at the top of the frame, and the emitters are enclosed in aluminum tubes near the bottom.

 

The Schematic

  To use the Sharp IR detection module, you need to have a circuit that can flash the light at 40khz, which is 40,000 a second (pretty fast!). This is called modulation. The circuit that does modulation is called an oscillator, and I have shown one below.

Picture of schematic

A simple 40khz modulation circuit.

(Click on picture for larger view)

The above circuit is an interesting one to look at. It uses a single 74HC04 (inverter) in several ways that you don't normally see, especially all at once. Lets take a quick tour.

There are three major parts to the circuit. The center (U1:A and U1:B plus desecrate components) forms a feedback oscillator. If you follow the flow of the logic gates, you see that the output of A is fed into the input of B, which is fed to the input of A. Now, there is a gate propagation delay factor that causes this circuit to continuously flip states. The important state is the output of gate U1:B, which is fed to the driver gates. The resistors and capacitor in the circuit are there to regulate the frequency at which this circuit feeds back into itself. Resistor R2 can be used to adjust the frequency of this circuit. The Sharp detector is fairly forgiving about the exact modulation frequency, but getting it as close as possible to 40khz is recommended. Note that the above circuit is tuned to work with the 74HC04, and is known NOT TO WORK with the 7404, 74LS04, and other TTL versions. Do yourself a favor and get the real thing.

The output from the oscillator is sent through two buffers. Here the gates act to isolate the oscillator section from the IR emitter section. The buffers also act as current sources for the IR emitters. It is important to isolate the oscillator from the LED's, since the timing would be affected by the particular types of LED you use.

The final gates are the U1:C and U1:D gates. These are the control lines that you would connect to the output ports of the CPU. They act to buffer the CPU from the current requirements of the LED's.

The interface to the CPU is done through pins 5 and 9 in the above circuit (gates C and D). These control when the emitters turn on. The only other variable is the resistor R2, which should only require initial adjustment to get the correct frequency.

The Software

Ask any engineer who has actually built a robust machine, and the engineer will tell you that the world is far from perfect. Few things work as cleanly as they do on paper. Sensors are especially sensitive to the outside world, and more often than not require some additional help in picking out the useful information from the 'noise'. This is where the software comes in.

Lets write a function that determines if there is an obstacle in the way. The function, CheckForObstacle() will return one of states: Nothing, Left, Right, Front. This indicates where, if any, the object might be. So, the function, easily enough, looks like:

int CheckForObstacle()
{
	int ReturnValue;
	TurnOnEmitter(LEFT);
	if( DetectorSeesLight() )
	{
		ReturnValue = Left;
	}
	TurnOffEmitter(LEFT);
	TurnOnEmitter(RIGHT);
	if( DetectorSeesLight() )
	{
		ReturnValue = ReturnValue + Right;
	}
	TurnOffEmitter(RIGHT);
	return ReturnValue;
}

The Tips

IR detection tool