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Joined: Apr 2010
30-05-2010, 08:45 PM
this project and implimentation is for designing a Obstacle Detector , Now with video of the sensor being used as a Mintvelt inspired object tracker! This sensor is a short range obstacle detector with no dead zone. It has a reasonably narrow detection area which can be increased using the dual version. Range can also be increased by increasing the power to the IR LEDs or adding more IR LEDs
IR LED's as a light source and two phototransistors in parallel for the reciever. You could use one of each .This setup works like a FritsLDR but with IR. It has a range of about 10-15cm (4-6 inches) with a note book as the object being detected.
and LEDs about 20mA. LEDs are capable of 50mA continuous and some LEDs are capable of 100mA (see "Getting the most from a LED").
this setup is a general purpose object advoidance sensor to prevent him backing into anything. you may get response with less than a volt when test object is up close and reflecting the IR and over 4.5V with no IR.
there is two IR LEDs with a resistor and transistor in series. The transistor allows the processor to turn the LEDs on or off. This is necessary to tell the difference between the ambiant IR from daylight and indor lighting and the reflected light from the LEDs that indicates the presence of an object.
and two phototransistors in parallel with a 1M resistor in series. You could use only one and may transistors will point in slightly different directions. If either one detects IR it will allow more current to flow. Since volts=current x resistance, even a small increase in current will create a reasonable increase in voltage across the 1M resistor. Unfortunately the low input impedance of many AD converters will act like a small resistor in parallel with the 1M resistor and dramatically reduce the output to the processor. This is where our BC549 transistor comes in to save the day. In conjunction with the 1K and 10K resistors it amplifies the signal so that the analog input on your processor gets a nice strong signal. The BC549 is not too critical, just about any general purpose signal transistor should do. this transistor had a hfe of 490 when measured with a multimeter. You should probably have a hfe of at least 200-300.
As you can see sensor is made from liberal amounts of hotglue. Click image for a bigger picture. This has the advantage that you can flex the leds and transistors outward to cover a larger area. This is Juniors reversing sensor to prevent him reversing into anything and as such will cover a wide area. I may make single Led/Phototransistor sensors for front left and front right. This will allow him to avoid crashing into obstacles when his rangefinder/object tracker is looking elsewhere.
Note that the phototransistors are slightly forward of the blue LEDs. This helps stop stray light from the LEDs being detected.
Using a simple test program that turns on the IR LEDs, stores the value of the ADC, turns off the LEDs, reads the ADC again and then subtracts the stored value from the recent value I was getting readings from 6 to 940. This was with the curtains closed and the lights off. When the reading was 6, test object was about 300mm (1ft) away. With the lights on the values ranged from about 60 to 940 with a value of 60 being with test object only about 150mm (6inches) away. Considering the max possible resolution with a 10bit ADC is 0 to 1023, I thought 60-960 with the lights on was a very good result.
test with heatshrink sleeves on the LEDs and phototransistors. The sleeves actually had a negative effect and reduced the range. After I removed the sleeves I did not get the same reduction in range with the lights on. I don't know if it is because during the first test it was daylight outside and the curtains didn't block it all or if it was the way I held the sensor but the second set of test gave an almost identical range of approximately 300mm (12 inches) reguardless of the lights being on or off. I'll have to try again tomorrow when it is daylight again
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