60 Hz AC Presence Detector (Arduino Digital Input)

What this circuit does

A 60 Hz AC source is switched, half-wave rectified, divided down, filtered, and clamped so an Arduino digital input can tell whether AC is present.

• SW1 stands in for the mains connection. State = 1 is closed (AC present), State = 0 is open. Open the switch and the sense node collapses to 0 V, which is the state the original sketch was drawn in.
• D1 half-wave rectifies, so only positive half cycles reach the divider.
• R1 / R2 divide by 10k / (22k + 10k) = 0.3125.
• C1 holds charge between half cycles. Discharge is through R2 alone (t = 10k x 1uF = 10 ms) against a 16.7 ms period, so the node still swings — the Arduino sees a 60 Hz pulse train, not a steady DC level.
• D2 clamps the sense node to +5 V + one diode drop. In normal operation it never conducts; it is there to protect the input against surges.
• U1 is the Arduino input stage. VTH = 2.5 V matches a 5 V logic input.

What to look at

• Sense node (analog) is the conditioned voltage at the Arduino pin.
• Arduino input (digital) is what the pin actually reads.

The one assumption I had to make

The original sketch only labelled the source "60Hz" with no amplitude. V1 is set to 24 V peak (about a 17 Vrms transformer secondary). At that level the sense node swings 1.5 V to 4.0 V, which straddles the 2.5 V threshold cleanly and leaves D2 idle as pure protection.

This is not a direct-mains circuit as drawn. At 170 V peak (120 Vrms) the sense node never falls below 2.4 V, so the input stops toggling, and R1 has to dissipate over a watt. If a direct mains connection was the intent, R1 needs to be roughly 1 M and the divider re-proportioned.