Touch circuit design and analysis

The working principle of the device application circuit is shown in Figure 4 (a) and (b). In (a), when no one touches the metal sheet TP, the transistor VT is cut off, the ④ pin of the controller DM is in a high level state, and the signal light H is on. Once someone touches the metal sheet TP, the transistor VT is turned on, causing the voltage of the ④ pin of the controller DM to drop, and H dims until it goes out. (b) is also a touch controller, and its function is exactly the opposite of that shown in (a). It is a touch-and-light type.