doorbell memory

LJ doorbel) memory On occasion it may be useful to know when a visitor has called in your absence. This is especially true in the case of an enforced absence when a visitor is expected. Confusion reigns supreme on these occasions. The circuit here helps to rectify the situation by providing a "memory" for the doorbell. On your return a LED wilt advise you whether or not a visitor called. The circuit is powered by the belt transformer via diode Dl and capacitor Cl. This provides a d.c. voltage level sufficient for the "memory". Under normal conditions (with no one ringing the doorbell) transistor Ti will be switched off and T2 will be conduc- ting to provide a form of latch for Ti. Obviously LED D3 will never light under these conditions! Now our visitor arrives! With a joyful cry of "Avon calling" they press the doorbell — only to lapse into total embarrasment when there is no answer! However our circuit now leaps into action. Via D2 and R1, the doorbel) switch Si provides a bas...