In this instructable, I will show you how to use your finger to make a circuit to switch on an LED. (ONLY USE THIS TECHNIQUE WITH VERY LOW VOLTAGE CURRENTS 3.3 V OR LESS). Usually, the levels of electricity from your body are very low so in this instructable we use a technique to overcome this issue.
The principle is that your body naturally creates very small amounts of electricity and can act like a battery or capacitor in electrical circuits, e.g. touch screen TVs. I am using my finger to switch on a transistor which then activates a circuit to switch on a LED or small speaker.
I am using a BC548 NPN transistor which I am using as a switch (It can also be used as an amplifier). The NPN transistor has 3 pins. When you place the flat side down, as show in the diagram, you have the Emitter, Base and Collector, E,B,C respectively. Here, we have the main circuit connecting the LED with the NPN emitter and collector pins, and the power source. By default, this circuit is off until it is switched on. We achieve this by applying a tiny current from my finger to the NPN Base (middle pin) which "turns on the switch" for the LED circuit.
BOM - For this you will need,
- Breadboard and cables
- Power Supply - <= 3.3V
- NPN (BC548) transistor - usually cost less than a dollar
- Resistor (optional) . I am not using one in this case because of the low currents generated.
- Your finger (of course, lol)
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Step 1: Building the Circuit and Conclusion
Building the Circuit
The components should be assembled on the breadboard as per the photographs. Note, that to connect electrical components on a bread board, the two outer pairs of lines with holes are connected lengthwise. These lines are known as the + and - rails, respectively. In contrast, the inner holes on the main part of the board are connected crosswise, in lines which run from the outer rails to the central well. So, as you can see from the photographs, components in the main board are linked to each other by sharing the same cross lines.
When you have built the circuit, switch on the power supply then hold the wire which is connected to the middle of the transistor and you should see the LED lighting up.
Conclusion and observations
To summarise, the modus operandi of this NPN transistor circuit is that when no base voltage is present, the switch is off. When base voltage is present, the switch is on. The transistor is off when there’s no bias voltage ( i.e. to the base) or when the bias voltage is less than 0.7 V. The switch is on when the base is saturated so that collector current can flow without restriction. I learned that some lower voltage batteries, particularly button batteries, do not provide sufficient voltage to energise this circuit.
This is a simple example if harnessing the electrical energy of the body. We could develop this into other projects such as simple capacitive buttons etc. We could also amplify the signal and play music with it, for example. Anyway, I hope you enjoy this.