Author Options:

How do I connect the phototransistor and emitter in this tutorial? Answered


I'm currently working through the above tutorial and am struggling with setting up the Phototransistor and Emitter. There is a circuit diagam on the third page of the tutorial but I cannot work out how to connect the Emitter and Phototransistor to the rest of the circuit.

If anyone could advise me further I'd be very grateful.

Kind regards,




8 years ago

Answer :-Phototransistor and Infrared Emitter
The light falling on a phototransistor creates charge carriers in the base region of a transistor, effectively providing base current. The intensity of the light determines the effective base drive and thus the conductivity of the transistor. Greater amounts of light cause greater currents to flow through the collector-emitter leads. Because a transistor is an active element having current gain, the phototransistor is more sensitive than a simple photoresistor. However, the increased sensitivity comes at the price of reduced dynamic range. Dynamic range is the difference between the lowest and highest levels that can be measured. The RoboBoard analog sensor inputs have a range of 0-5 Volts, and relatively small variations in light can cause the phototransistor output to change through this range. The exact range depends on the circuit used.

As shown in the phototransistor is wired in a configuration similar to the voltage divider. The variable current traveling through the resistor causes a voltage drop in the pull-up resistor. This voltage is measured as the output of the device.

Infrared Emitter
The light emitting element (an LED) uses a resistor to limit the current that can flow through the device to the proper value of about 10 milliamps. Normally the emitter is always on, but it could be wired to one of the LED output ports if you wanted to control it separately. In this way you could use the same sensor to detect the starting light (using the phototransistor with the emitter off) and then to follow a line on the board (normal operation with the emitter on)