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THE PRESET RESISTOR
V. Ryan 2002-2022
 
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Preset resistors are used in circuits when it is necessary to alter the resistance. Dark/light and temperature sensors usually have these components as the preset resistor allows the circuit to be made more or less sensitive (they can be turned up or down - reducing or increasing resistance).
 
 
 
 
A small screwdriver can be used to turn the centre part of the preset resistor, altering the value of the resistance.

The range of resistance varies, for example:
0 to 100 ohms
0 to 1M ohms
 
 
The two circuits below are sensor circuits. The one of the left is a temperature sensor and the one on the right is a light sensitive circuit. Increasing the value of the preset resistor by turning the centre with a small screwdriver makes the circuit less sensitive. For instance, the temperature sensor would require a higher temperature and the light sensitive circuit would need more intense light before they activated.
 
 
TEMPERATURE SENSOR   LIGHT SENSOR
 
 
 

CIRCUIT WIZARD SIMULATION OF A DARK SENSOR
(Notice the variable resistor / prEset resistor has changed places with the LDR
Also, notice the transistor is protected by a 100ohm resistor )

 
 
PICTORIAL ILLUSTRATION OF THE DARK SENSOR CIRCUIT SHOWN ABOVE
(The preset resistor is shown in position)
 
 
 
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