The human body picks up and generates electromagnetic activity. Therefore it can transfer voltage and a very small amount of current to a sensitive circuit.
The trigger, threshold, and reset pins of a 555 timer are very sensitive to any voltage.
Those 3 input pins have high impedance. That means that they don’t let current through them, other than a small amount of leakage current. It’s such a small amount of current, that it’s almost always OK to just think of it as being no current.
An input that doesn’t let current flow (high impedance) is useful because it doesn’t alter the voltage of a relatively weak signal. It just responds to that voltage.
- Trigger pin (2) with 1/3 or less of the supply voltage (Vcc) across it sets the output high (as close to the positive supply (Vcc) as it can get). The one million ohm pull up resistor connecting pin 2 to the 5V supply holds it at 5V against very weak signals. Touching it causes a voltage oscillation that is sometimes below 1/3 supply voltage. Signals from your body are strong enough to overcome a voltage through a 1MΩ resistor.
- Threshold pin (6) sets the output low (pretty good connection to ground/0V) if it gets more than 2/3 of the supply voltage (Vcc) applied to it. The one million ohm resistor connecting it to ground (pull down) holds it at 0V against very weak stray signals, but is low enough resistance to allow the oscillating signals from your body to raise the voltage enough.
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