New Look for: mpsa13 datasheet

mpsa13 datasheet

In an on IR object detection sensors I mentioned that they could be used for object tracking. If the object moves to one side then one sensor will get more IR than another and the servo turns to correct the imballance. The tracking software reads the ambiant light and then turns on the IR LEDs to read the refected light. It subtracts the ambiant light from the reflected light to stop iR coming from a window causing the tracker to turn towards the light. The problem I believe was that light from the LEDs overlapped at different distances depending on where the object my hand was relative to the unit. My newest array, with navy blue IR LEDs works better as I've gone back to a single phototransistor for the sensor instead of two in parallel. The domed end of a LED or a phototransistor is a lens that focuses the light sent or received. The IR LEDs I'm using have a viewing angle of about 30 degrees. Check you datasheet of both LEDs and phototransistor as it can vary. After looking at it shining IR onto a sheet of paper through my camera I relised that once again I was getting light from different LEDs reflecting back to a phototransistor depending on how far away the paper was. First thing I want to change is the way my LEDs and phototransistors are mounted as they are at a fixed position and angle. Up close the phototransistor only gets light reflected back from the LEDs next to it but at a greater distance it starts to recieve light from nearby sensors. If you need more than one LED to get more range then cluster them as close as possible in the centre and mount your sensors around the outside facing outward slightly. Below is the LED array driver circuit. The current is limmited to about 50mA per LED so that I didn't have to worry about damaging the LEDs if they stayed on too long due to a fault in the software while debugging. Boosting range: So far I have used more LEDs to increase range but this has caused troubles with callibration when the LEDs were not clustered together. The LEDs I've used do have a peak current rating of 1.2A but at this intesity they could be damaged if left on for more than 100uS. I would probably use a 555 timer to control the pulse if I resort to that method of increasing range as this would protect the LEDs from faulty code. The biggest problem with this idea is that the LEDs can only be on for about 100uS. At this stage I've ordered some High intensity IR LEDs that will handle 100mA continuous. Hardware protection: This is a circuit I've designed to protect the IR LEDs if you want to push them to their limit. This uses a 555 timer to control the timing of the pulse to my IR LEDs. With the component values shown it should pulse the LEDs for just under 100uS and then canot be retriggered for at least 200uS. This means that the LEDs cannot be damaged even if the signal from the processor stays high due to a fault in the code. This turns on the FET to power the LEDs and begins charging the timing capacitor via a diode and the 1K3 resistor. The LEDs are turned off and the timing capacitor discharges by the 2K7 resistor and it's diode. One reason you might choose to use a circuit like this, even with good code is that if an interupt occurs during a pulseout then it could cause the LEDs to be on longer than normal. The maximum current through the IR LED is only 25mA although the multimeter registers less than 10mA total current draw since the LED is only on very briefly.
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