Touch Switch illuminates LED

Posted on Mar 13, 2013

Our circuit is amplifying the current flowing through your finger (when it touches the touch plate) 10,000 times and this is sufficient to illuminate the LED. The current flowing through your finger (when touching the touch plate) is only a few microamps and this is not enough to illuminated the Light Emitting Diode (LED). We need an amplifier with considerable gain to get the LED to light and this is what the circuit does. You can alter the brightness of the LED by pressing lightly on the plate and this will show that the resistance of the plate changes with the amount of pressure you exert. You can also moisten your finger to see how this changes the resistance of the plate and observe the brightness of the LED.

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We start the description at the touch plate. We have already explained how the touch plate works. It consists of two interleaved tracks so that when you touch them with a finger, the resistance between the two tracks reduces. This action allows more current to flow through the terminals of the touch plate and since it is connected to the base of the NPN transistor, more current flows into the base. Three components are not fitted to this section for this project. They are: 100k, 10n and 10u electrolytic. These are fitted in the second project. Pick out the components listed in the parts list and lay them on the work-bench. Now you can start assembly. Tick each step as you do it: ( ) Bend the leads of the 47k resistor to 90° and push them down the holes identified by the 47k symbol. Hold the resistor against the board with a finger while soldering so that it stays against the board after soldering. Resistors can be fitted either way around as they are not polarity sensitive devices. ( ) Fit the 1k resistor in the same way. ( ) Fit the 22R resistor. ( ) Fit the red LED so that the longer lead goes down the hole near the edge of the board. The short lead is the cathode and is identified on the board by the line on the symbol. ( ) Fit the NPN transistor so that the C, B and E leads fit down the correct holes. Refer to the large diagrams corresponding to the type of transistor you are using. Push the...

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