The circuit provides audio-visual indication of on-hook, off-hook, and ringing modes. It can also be used to connect the telephone to a cid (caller identification device) through a relay and also to indicate tapping or misuse of telephone lines by sounding a buzzer.
In on-hook mode, 48V dc supply is maintained across the telephone lines. In this case, the bi-colour led glows in green, indicating the idle state of the telephone. The value of resistor r1 can be changed somewhat to adjust the led glow, without loading the telephone lines (by trial and error).
In on-hook mode of the hand-set, potentiometer vr1 is so adjusted that base of t1 (bc547) is forward biased, which, in turn, cuts off transistor t2 (bc108). While adjusting potmeter vr1, ensure that the led glows only in green and not in red.
When the hand-set is lifted, the voltage drops to around 12V dc. When this happens, the voltage across transistor t1�s base-emitter junction falls below its conduction level to cut it off. As a result transistor pair t2-t3 starts oscillating and the piezo-buzzer starts beeping (with switch s1 in on position). At the same time, the bi-colour led glows in red.
In ringing mode, the bi-colour led flashes in green in synchronisation with the telephone ring.
A cid can be connected using a relay. The relay driver transistor can be connected via point a as shown in the circuit. To use the circuit for warning against misuse, switch s1 can be left in on position to activate the piezo-buzzer when anyone tries to tap the telephone line. (When the telephone line is tapped, it�s like the off-hook mode of the telephone hand-set.)
Two 1.5V pencil cells can provide Vcc1 power supply, while a separate power supply for Vcc2 is recommended to avoid draining the battery. However, a single 6-volt supply source can be used in conjunction with a 3.3V zener diode to cater to both Vcc2 and Vcc1 supplies
Simple Code Lock
The circuit described here is of an electronic combination lock for daily use. It responds only to the right sequence of four digits that are keyed in remotely. If a wrong key is touched, it resets the lock. The lock code can be set by connecting the line wires to the pads a, b, c, and d in the figure. For example, if the code is 1756, connect line 1 to a, line 7 to b, line 5 to c, line 6 to d and rest of the lines—2, 3, 4, 8, and 9—to the reset pad as shown by dotted lines in the figure. The circuit is built around two cd4013 dual-d flip-flop ics. The clock pins of the four flip-flops are connected to a, b, c, and d pads. The correct code sequence for energisation of relay rl1 is realised by clocking points a, b, c, and d in that order. The five remaining switches are connected to reset pad which resets all the flip-flops. Touching the key pad switch a/b/c/d briefly pulls the clock input pin high and the state of flip-flop is altered. The q output pin of each flip-flop is wired to d input pin of the next flip-flop while d pin of the first flip-flop is grounded. Thus, if correct clocking sequence is followed then low level appears at q2 output of ic2 which energises the relay through relay driver transistor t1. The reset keys are wired to set pins 6 and 8 of each ic. (Power-on-reset capacitor c1 has been added at efy during testing as the state of q output is indeterminate during switching on operation.) This circuit can be usefully employed in cars so that the car can start only when the correct code sequence is keyed in via the key pad. The circuit can also be used in various other applications.Automatic School/College Bell Project using AT89S8252
This Project takes over the task of Ringing of the Bell in Colleges. It replaces the Manual Switching of the Bell in the College. It has an Inbuilt Real Time Clock (DS1307) which track over the Real Time. When this time Equals to the Bell Ringing time, then the Relay for the Bell is switched On. The Bell Ringing time can be Edited at any Time, so that it can be used at Normal Class Timings as well as Exam Times. The Real Time Clock is displayed on four 7-segment display. The Microcontroller AT89S8252 is used to control all the Functions, it get the time through the keypad and store it in its Memory. And when the Real time and Bell time get equal then the Bell is switched on for a predetermined time.
