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Download BoBT - Transistor - Chesham Grammar School Moodle
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TRANSISTOR PROCESS COMPONENTS Weblinks: http://www.technologystudent.com/elec1/transis1.htm https://www.youtube.com/watch?v=MVAqfj9RTPA Transistors are electronically operated switches. They amplify a small input current to produce a much bigger output current. They have two uses in GCSE circuits: as an output driver - many process devices do not produce enough current to drive certain output devices they are commonly used with sensors in detector circuits such as: electronic thermometer, black ice detector – thermistor night or day detector in a security light – LDR automatic plant watering system, flood alarm, damp detector – moisture probes Circuit Diagram The basic circuit is: Transistor – it switches on (1) when the input is greater than 0.7V (1) Input – Process Device 555 Astable / Monostable Op-Amp PIC Logic Gate base Input – Potential Divider LDR + variable resistor thermistor + variable resistor moisture probes + variable resistor collector emitter Base Resistor – protects the transistor (1) by limiting the base current to a safe level for the transistor (1) DT@CGS: Electronic Products – EDEXCEL 2EP01 Output – LED (+ protective resistor) buzzer 555 Astable (+ LED / Buzzer) 555 Monostable (+ LED / Buzzer) SUMMARY The transistor switches on when the input (base) voltage rises above 0.7V. A small current will now flow into the base which causes a much larger current to flow through the transistor from the collector to the emitter and the output device therefore switches on. The transistor switches off when the input (base) voltage falls below 0.7V. No current therefore flows into the base so the transistor switches off.