The human body picks up and generates electromagnetic activity. Therefore it can transfer voltage and current to a sensitive enough 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 any current through them. That’s not completely true though. They do let a small amount of leakage current through them. It’s such a small amount of current though, that it’s almost always OK to just think of it as being no current.
- Trigger pin (2) with 1/3 or less of the supply voltage (Vcc) across it sets the output high (as close to 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 until you touch it and cause a voltage oscillation that is sometimes below 1/3 supply voltage. Signals from your body are strong enough to overcome 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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