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Electronics - Resistors

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					Electronics – Switching Process
A transistor can be used as an electronic switch. It is placed between the input block and the output block and is controlled by the input block. The input block will usually be a sensor, such as an LDR, in a potential divider circuit.

In the circuit shown below an LDR is used in a potential divider circuit. This forms the input to the transistor switch; the output is a Light Emitting Diode (LED) and its safety resistor. As the LDR darkens its resistance increases and the voltage at the transistor’s base rises until the transistor switches on.

The base voltage needs to reach about 0.7 volts or 700mV before the transistor will switch on. Notice that the current through the base of the transistor is much smaller than the current through the collector. The emitter current is the sum of the collector and the base current.

collector base emitter

The circuit above is a “dark” detector; how would you make it a “light” detector? ..............................................................................................................................

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Electronics – Improving Sensitivity
Getting a single transistor to switch at just the right point using a sensor is quite difficult. The answer is to use 2 transistors in a high gain (amplification) arrangement called a “Darlington Pair”. The circuit is shown below.

You can see that the base current now is very small but the circuit will still switch. This is because the two transistors behave as if they are one transistor with a very high voltage gain (10,000 or more). This means that the circuit is more sensitive so that it is easier to get sensors to work with it. You can also use the FET driver:

What voltage at the gate will turn the FET on? .....................................................

Try this…
Try building a night light using the systems blocks or Crocodile Technology. The light should come on when the sensor is darkened. How many LEDs can you get to light up using a PP3 battery? Now change it to a tuck box alarm. This should sound a buzzer when the lid is opened and light shines onto a light sensor inside the tuck box.

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Electronics – OpAmp Comparators
For even better sensitivity and control you can use an “OpAmp ” or Operational Amplifier. This is an integrated circuit which is designed to switch according to which of the two inputs is higher in voltage. It is often called a voltage Comparator. The circuit is shown below.

The switching point can now be set accurately with the potentiometer. The output of the OpAmp is “low” (near zero) if the - input is more positive then the + input. The output is “high” (near battery voltage) if the + input is more positive than the – input. There is also a system board labelled “Easy Adjust OpAmp”. This adds two resistors, one either side of the potentiometer. This makes it easier to set the switching point. Try the following sensor blocks and see what results you get.
Position Top/Bottom? bottom

Sensor LDR Thermistor Motor Loudspeaker Microphone Buzzer Water sensor Touch sensor Variable resistor Tilt sensor Push switch

Sensor activated by? darkness

LED lights? yes

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