Lesson 11 The Phototransistor: Shedding Some Light on Another Component

Chapter Lesson 11 The Phototransistor: Shedding Some Light on Another Component

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

  • 38Section 3 ■More Components and Semiconductorsd. The volume of water is smaller and drainsfaster.8.Play a little. a.Replace R3 and the LED with the buzzer.Make sure the buzzer’s red wire is gettingvoltage from the 3906’s collector and theblack wire is connected to ground.b.Push button and release. What happens tothe sound as the capacitor discharges?_________________________________.9.Carefully describe in your own words howthis circuit works._____________________________________.Lesson 11The Phototransistor:Shedding Some Light onAnother ComponentAll transistors are light-sensitive. Shortly aftertransistors were invented, engineers recognized thattheir new toys gave different results as lightingconditions changed. Lots of work went intoshielding the guts of the early transistors. But at thesame time, lots of effort was put into understandingand finding uses for this phenomenon. The results of this research? The phototransistor.Figure L11-1 shows both the phototransistor(darkened glass) and its corresponding infrared(IR) LED (clear glass). Many of the available phototransistors havebeen developed to react to specific wavelengths oflight. This is important, because the intent is tocontrol the transistor, leaving nothing to chance.So how do we use this “tuned” phototransistor toour advantage? Here, I’m using a phototransistorthat reacts specifically to IR light with a wavelengthof 940 nanometers. Both sunlight and filament lightbulbs create IR that will affect our phototransistor.Fluorescent lights don’t have an infrared componentin their spectrum.So what to use as a light source? Hmmm? Whatcolor LEDs are readily available? Red, yellow,blue, green, and orange. Table L11-1 shows thewavelength of each color. Our eyes don’t respondto the wavelengths above 750 nm, but digitalcameras do respond to low-end IR.LEDs that emit IR are also available. We use onethat is specifically matched to the phototransistorwe’ll be using. Even the part numbers are the same,except for one added letter.Let’s start playing. The Parts Bin has a full parts list.Figure L11-1400nm475nm510nm570nm590nm650nm750 to 2,500nmVioletBlueGreenYellowOrangeRedInfraredTABLE L11-1We Need an Infrared LED for an Infrared Phototransistor