ismith1984
New Member
Hey rob another question tell me what you think about this schematic the website provided. Do you think this ia suitable for what I am trying to do.
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That would work. The trick here is knowing exactly what you are trying to do. The distance between the transmitter (light or IR) source and the receiver (detector be it photo transistor or whatever) are important. Therefore if you were up/down counting people for example entering and leaving a room that distance could be a few feet or a meter. Your detector needs to work at that distance. The detector in this circuit needs to be chosen for the application.
IR emitter and detectors are popular because they are not effected by for example room ambient light.
Ron
OK, I bread boarded real quick a little setup and am attaching the drawing.
This was just the simple Radio Shack part number 276-142 IR Emitter and Detector pair. The specifications are on the package.
IR Emitter:
Reverse Voltage: 5 Volts
Continuous Forward Current: 150 mA
Forward Voltage: 1.3 Volts (1.7 Volts Maximum)
Note: The cathode is the flat side and the shIR Detector:
Maximum Collector Current is 50 mA
Note: The collector is the flat side and the shorter of the 2 leads.
I limited the emitter current with R1. It should be below 100 mA with the 47 Ohm 1/4 watt resistor I used so well below the 150 mA maximum but adequate. When you start with things like this it is wise to keep things below maximum ratings.
The IR detector is an NPN Photo Transistor so you don't need another transistor in there. Think of the base as where it receives IR light. That leaves the emitter which is tied to common and the collector which is tied to 5 Volts through a 1.5 K Ohm 1/4 watt resistor to limit the collector current.
Note: You need to limit the current through your emitter and detector or you will toast your components!
I slipped a piece of drinking straw over each, the emitter and detector to help shield against stray light. About an inch long or so. Not the best shield but adequate for this test. I simply aimed them at each other across the bread board.
No light obstruction I got .1 Volts at the line marked Output Signal and when I blocked the light I got 4.5 Volts on the Output Signal line. Those are fine TTL levels for driving the gates.
If you put it together exactly as I drew it it should work fine. I did notice in my original drawing I had the detectors polarity reversed. The cathode sides should have been to common. Also, the original drawing used IR Photo Diodes and not photo transistors.
Questions?
Ron