This office bell with IN/OUT indicator circuit utilizes three 7-segment, common anode displays rather than a couple of LEDs for the in/out indicator as is done commonly. As soon as the calling bell pushbutton is shoved this indicator displays alphabetically ‘ln’ or ‘Out’, as it is instructed physically. If you are inside your office room, retain the rotary switch in ‘A’ position.
Whenever your visitor arrives and pushes the bell this individual receives pleasant surpris to notice the word ‘IN’ shining brightly upon the wall. If you are out, retain the rotary switch in ‘B’ position to be able to point out to your visitors that you are not inside. Now let's talk of the circuit. Once the pushbutton is pushed, the lCl (555) generates an audible tone and at the same time this ‘high command’ created by the pushbutton is inverted by a NOR gate (N4) to activate the monostable IC2 (555).
The pulse length of this monostable is established by l.1 x RC and also the pulse activates the relay which consequently triggers the displays. Any segment in the displays may light only if it is applied with a logic low pulse. Table l displays the logic levels of each of the segments for every of the two situations.
Additionally, a few of the segments must continue to be constantly at logic high state and some at logic low for each of the cases. The ‘always high‘ state is accomplished by immediately providing with the positive supply rail and this is inverted by a NOR gate (N3) to obtain the ‘always low‘ Status. Once the word ‘ln’ is to be shown, X and Y must be at low and high state correspondingly.
Therefore in that case a minimum of one ofthe inputs ol’ the NOR gate (N1) needs to be high. Likewise for ‘OUT’ to become viewable. both of the inputs needs to be low. The display stays on so long as the monostable logics a high output. After the exact time-duration, the relay becomes de-energized and the display shuts down.