Arduino Leonardo:
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5V  -------------------> 5V (Geiger Counter)
GND -------------------> GND (Geiger Counter)
D2  -------------------> VIN (Geiger Counter)

5V (or 3.3V)  -------------------> VCC (Dimmer)
GND -------------------> GND (Dimmer)
D7  -------------------> Z-C (Dimmer)
D12 -------------------> PSM (Dimmer)

RobotDyn Dimmer:
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IN  -------------------> AC Live Input
N   -------------------> AC Neutral Input
OUT -------------------> AC Live Output (to Lamp)