The DiversiTech TT-120 Tattle-Tale Control Monitor is a diagnostic monitoring device designed to indicate control circuit activity in HVAC systems. It connects to low-voltage circuits and provides a visual alert when voltage is present, helping technicians quickly identify intermittent or hard-to-trace control issues. Commonly used on air conditioners, heat pumps, furnaces, and air ha... https://www.partshnc.com/tt-120-diversitech-monitor-tattle-tale-pk-of-3