Download BoBT - Transistor - Chesham Grammar School Moodle

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.