NOTE: Familiarity with Vex microcontroller If you want to use the IR phototransi
ID: 2267948 • Letter: N
Question
NOTE: Familiarity with Vex microcontroller If you want to use the IR phototransistor circuit in figure 1 to detect IR light. If you were going to connect this circuit to an ANALOG port on a VEX kit, what woul d be the configuration port setting under the RobotC “Motors and Sensor Setup" options to obtain a val ue indicating the level of IR light incident on the phototransistor? If this port setting is used, what would be the value read by the VEX software if no light is on the phototransistor, and what would be the expe cted change in the value read from the sensor when the phototransistor is exposed to intense IR light as opposed to when it is not exposed to any light? How does the value of the resistance in series with the phototransistor affect the required light level to read a given numerical from the port? PAIP VEX Analog Port 1 Power VEX Power Analog or Digital Por 10002 Resistor E.X Analog Port 1 Signal Variable Resistor OP270 Infrared Light Emitting Diode PAIG PG VEX Analog PrtI VEX Ground Analog or Digital Port PDIS VEX Digital Port I Signal Visible Light LED EX Digital Port 1 Ground igure 1Explanation / Answer
Motor and Sensor setup
Operation of Circuit:
IR Phototransistor whose collector to Emitter Resistance is dependent on the intensity of Light falling on it. As light intensity increases the Resistance decreases. as the circuit is similar to voltage divider ciruit we use following anaolgy,
Vo = (Rvar/(Rvar+Rpt))*Vin;
where Vo is analog voltage at PA1S from circuit, Rvar resistance value of varible resistor and Rpt is the instantaneous resistance of Phototransistor.
As intesity of light incident on the Phototransistor increases the Rpt value decrease there by the value of Vo increases. The value output from the circuit Vomin to Vomax. Vomax is 5V if PA1P is 5V and digital value is 1023 because VEX contains 12bit ADC. But Vomin value depend on the value Rvar.
Vo range can be set by value of Varible resistance. The typical value Rpt when no light is incedent on it is arround 100K to 1M ohms. if we Rvar as 1M the Vo will varry from Vin/2 to Vin implies vex values are from 512 to 1023.
As Rvar value increase the lower limit values also increases.
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