48 kHz Infrareds Emitter and Receiver, Rangefinder

By Dillian Wong, 24/3/04 

ABSTRACT

Infrared is commonly used both in communication, control and object detection. Objects that generate heat also generate infrared radiation. Many sources of infrared radiation generate and around the environment. The sun, standard light bulbs, computer monitor, visible light LEDs and even animals and human body produce different level of infrared light. If a device simply relied on the presence or lack of presence of infrared light, the communication or object detection algorithm would receive false and noisy readings. The common way of making the Infrared signal more noticeable is using the modulation technique. That is the information of Infrared signals does not only depend on the signal magnitude, but depend on the variation of the signal. The noisy infrared radiation can be rejected, as the particular frequency of modulation technique is selected by the application.  

INTRODUCTION

Infrared LEDs are commonly used for object detection in robots. Blinking the LED on and off can improves long-range detection and reduces false triggers. In this project, I implement a 48kHz modulation frequency. Because the application of this project is measure the distance of the targeted object, no communication is required. Thus, only carrier is enough, the reflected back signal magnitude in this case is represented the distance of the targeted object. For the further stage sof the AD conversation, it is necessary to converse the sinusoidal signal back into state DC voltage. Otherwise, the AD converter output will be fluctuate seriously. And it is not easy to get the means values.

CIRCUIT DESCRIPTION

The following will explain the usage of the electronic components, and how they work to make a infrared RangFinder. This project mainly divide into 4 part; one is the oscillation circuit for the Infrared modulation, following is the emitter circuit, receiver circuit and the last is the envelop detector circuit. After the 4 stages the output DC voltage is expected. This DC voltage represent the distance of the measured target object distance.

Oscillator

555 as a astable multivibrator. Capacitor C2 charges toward Vcc through external resistor R1 and R3. The capacitor voltage rises until it goes above 2Vcc/3. This voltage is the threshold voltage at pin6, which drive comparator to trigger the flip-flop so that the output at pin3 goes low. In addition, the discharge transistor is driven on, causing the output at pin7 to discharge the capacitor through resistor R3. 

The capacitor voltage then decreases until it drops below the trigger level Vcc/3. The flip-flop is triggered so that the output goes back high and the discharge transistor is turned off, so that the capacitor can again charge through resistor R1 and R3 toward Vcc.

Infrared Emitter

Infrared Receiver

At the receiver side, the received signal a 48kHz sine wave. If the application of infrared is only used to turn on something when the object is within a certain distance, for simplicity, It can directly fed the carrier signal to a relay, when the magnitude of the signal is large enough, the relay will turn on. The reason is the signal is fast change enough that causes the relay do not have time to turn off. Therefore, overall the relay is always on when the magnitude large enough.

If the application require an analogy voltage to represent the distance, that means we measure the intensity of the reflected infrared signal, when the detected object is nearby the measured intensity is greater, and when the object is far the intensity is smaller, in this application an envelope circuit is a must. Without envelope circuit, the forward stage of the AD converter can be measure the DC voltage, or the output of the AD converter is fluctuating, the fluctuating level depend on the speed of AD conversion time and the carrier frequency.

Because infrared receivers amplify the signal to improve detection, electrical noise generated from the oscillator can leak into the receiver and trigger a false detection. Meanwhile, 9041 (after the stage of receiver transmitter) may be perfectly work in linear region. The following is the measurement of the receiver waveform. Electrical noise isn't a problem for VCRs or most consumer devices as they tend to contain either a transmitter (remote control) or a receiver (CD player), but not both. Use of decoupling capacitors and metal shielding helps to lower the noise.( LED in this circuit used to indicate the magnitude of the received signal, notice that the situation is as same the relay, LED is fast change enough that LED seems always on.)

The Envelope Detector

The simplest form of an envelope detector is a nonlinear charging circuit with a fast charge time and slow discharge time. It can easily be constructed using a diode in series with a capacitor. A resistor placed across the capacitor controls the discharge time constant.

The input signal waveform charges capacitor C to the waveform's maximum value during positive half-cycles of the input signal. As the input signal falls below its maximum, the diode turns off. This is followed by a slow discharge of the capacitor through the resistor until the next positive half-cycle. when the input signal becomes greater than the capacitor voltage the diode turns on again.

If this time constant is too large, the envelope detector may miss some positive half-cycle of the carrier and thus will not reproduce the envelope faithfully. If the time constant is much too small, the envelope detector generates a very ragged waveform, losing some of its efficiency. The resultant detected signal is usually passed through a low-pass filter to eliminate the unwanted harmonic content.

The calculation of envelop circuit component values is RC>= T. I used R=750k and C=10n. ie RC=7.5ms > 1/48k = 0.02ms. I take large time constant is because I do not want the ragged DC output. Also in my application, rang finder, the state DC voltage is important for the AD conversion.

MEASUREMENT RESULT

The following is the photo on the left side indicated how I measure the Rangefinder. I place a obstacle in front of the Rangefinder then measure  the DC output Voltage. In the diagram, we found the voltage is varied form 1.5V to 2.5V, to represent the distance from 12cm to 2cm. The range is more suitable for more application, when the object near by or a object in front of the sensor. Then devices will turn on something. The required distance can use a comparator, compare the DC voltage output with a desired reference voltage.

PROBLEM ENCOUNTER

1. I missed to place a resistor between the NE555 output and Q12 N2222 base. The barratry is consumed quickly. The reason is QN2222 NPN bipolar transistor accept the large base current and causes a large collector current pass though R4, disparate the power. In this case, I found transistor QN2222 and regulator 7805 is very hot.
   
2. Before a envelop detector circuit, at first, I place a 104 capacitor to block all the DC voltage, I find there have a negative voltage swing at the envelop detector input. And because of this, some of the sensitive ruined, as negative voltage cannot turn diode on. To solve this problem at that time I add a voltage divider circuit to shift up the voltage input. Of course, I found this is a redundancy. And finally, I remove the DC block capacitor and direct fed the sine/square wave signal to envelop detector.
   
3. Because of the inappropriate circuit design, the DC voltage that represent the detect object’s distance in the first version circuit was varies form 0.16 to 0.43V. I found it difficult to build a DC amplifier, as the voltage is too small that cannot drive any TTL and FET etc. Especially it is impossible to use uA741 as a DC amplifier, it is totally wrong! Why I know it the reason leave reader to guess.

CONCLUSION

I develop the circuit is for the range detection of the robot Hex-avoider competition. It is a six-legged robot. With a developed control program and some sensor, it can move independently and avoid obstacles. When the obstacles is in front of the robot, the robot can turn left or right, depend on the left/ right side sensor. In this project, I attempt to illustrate the important of using modulation technique to improve the information communication and detection, although this project do not consist of any signal communication between two devices. The low frequency oscillation circuit design, the emitter circuit principle and IR receiver detection circuit. and the envelope converter are illustrated. The last stage is important to extract the information contained in the infrared signal, in this project no information is injected to the IR signal, ie only a state DC voltage is measured. At the receiver side, the received signal a 48kHz sine wave. If the application of infrared is only used to turn on something if the object is within a certain distance, for simplicity, It can directly fed the carrier signal to a relay, when the magnitude of the signal is large enough, the relay will turn on. The reason is the signal is fast change enough that causes the relay do not have time to turn off. Therefore, overall the relay is always on when the magnitude large enough.  

EXTEND

I have already implement the modulation technique. For the digital communication, we can place a PNP transistor to control the power source of the emitter IR Led, the transistor is connect the digital input, ie the digital signal can inject to the carrier. For the analogy communication, we can place a transformer between the power source of the IR emitter and the audio signal input, this is the famous AM modulation, and in the detector side clearly use envelope is the simplex one.