Lesson 33 The Photo Transistor: You Can’t Do This with an LDR

Chapter Lesson 33 The Photo Transistor: You Can’t Do This with an LDR

Electronic Circuits for the Evil Genius Second Edition 64 Lessons with Projects
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Electronic Circuits for the Evil Genius Second Edition 64 Lessons with Projects

  • 124Section 9 ■How Do We Understand What We Can’t See?Lesson 33The Photo Transistor: YouCan’t Do This with an LDRNow is a good time to revisit the phototransistorand further examine its unique abilities. Manydigital systems transfer data wirelessly, through theair or through optical cable.Surprisingly simple substitutions are made inFigure L33-1 to change over to the infrared LED.Use the same values for RC2 as you did in Lesson31, producing a frequency in the 1kHz range, andkeep pins 12 and 13 connected directly to groundfor an uninterrupted signal.Remember that the short leg/flatedge of the phototransistorrepresents “C.”The speaker/phototransistor circuit is separate, atthe far end of the SBB. Figure L33-2 indicates theactual distance I’ve given between the two circuits.The power provided through a transistor willburn out our infrared LED, but we do want it as“bright” as possible. Also, we use the second LEDto tell us that something we can’t see is actuallyworking. We could add three more regular or IRLEDs in series to cut the voltage. Each LED usesabout 1.7 volts. That would work. But a single100-ohm resistor will eat up the remaining voltage.If you really want to see an IR LED working,look at a TV remote control’s output through adigital camera, and then push a button. The IRLED might appear bright or hardly on at all.1.Make sure the IR LED and phototransistor arelined up directly with each other. The bestresults come from having both componentslooking at each other through a straw.2.Open the Soundcard Scope and plug in yourscope probe.3.Connect the scope probe to TP1, shown inFigure L33-1.Figure L33-1NO TEFigure L33-2