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Create a simple touch-sensor using a transistor


NPN Transistor

A transistor is like an electronic switch (and can also act as an amplifier). It can turn a current on and off. A very common one is the “bipolar junction transistor” or “BJT”. It has three pins named base (b), collector (c), and emitter (e). If you have a small current flowing from base to emitter, you turn the transistor on. With no current, it is off. To get current flowing, you only need about 0.7V from base to emitter (for this NPN transistor).


Unlike a normal switch that only has two states (ON or OFF), the transistor can also be “partly on” by controlling the current that goes through its base. A bit of current on the base produces a current of maybe 100 times more (depending on the transistor) through the Collector and Emitter. You can use this effect to build an amplifier.


NOTE: Apple iPhone 15 contains 19 billion transistors

Building a Touch Sensor


Touch Sensor

In this project, the transistor does not let any current flow through the LED and resistor (R1) if there is no current flowing through the base. That means there is no light in the LED.


When you touch your finger to the touch-pad (for example two metal pins), your finger acts as a resistor, and lets a little bit of current flow down through the base of the transistor. The transistor “opens its gate” and lets about 100 times the amount of current flow through the LED and resistor, which makes the LED light up.


This touch sensor circuit is versatile and can be used to accomplish various tasks. For instance, it can replace mechanical switches in applications like lamps, toys, or control panels, where a simple touch can turn the device on or off. It can also be utilized in more advanced applications, such as creating interactive surfaces like touch-sensitive tables or walls. These surfaces are commonly used in museums, exhibitions, or even at home for interactive displays.



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