Design and analysis of wire control circuit

The working principle of the device application circuit is shown in Figure 6. Its typical application is as a wire-break burglar alarm. X is a very thin enameled wire. When it is connected to the circuit, the trigger pole of the unidirectional thyristor VS is short-circuited to the ground. Therefore, VS cannot be triggered to turn on. At this time, the transistor VT is in a saturated state, causing the ④ end of the controller DM to be low level (lower than 1.6V), DM is cut off, the alarm bell DL is not powered, and the alarm does not work.

Once someone breaks the filament X, current flows into the trigger electrode of VS, so VS is turned on and self-locked, making the base of the transistor VT at a low level, VT is cut off, and the fourth end of DM becomes a high level, so DM is turned on, DL is powered on and an alarm is sounded.