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LOW
COST ELECTRONIC CLOCK
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This
circuit
can be used for the safety of precious and valuable items.
The low-cost electronic lock is suitable for use in homes
and banks for lockers etc.The circuit consists of three
thumbwheel switches, three DIP switch sets (each having
eight switches), three inverter ICs (7404), three quad-AND
gate ICs (7408), one quad-NAND IC (7400), one timer IC
(555), few diodes/transistors and other passive components.The
Dip switches are supposed to be hidden and the actual
code to be set on the thumbwheel switches must match the
Dip switch settings, which will result in enabling of
all AND gates (G1 to G11). The high output of the last
AND gate is used for energising relay RL1 via DPDT switch
S1, which in turn results in energisation of solenoid
coil from mains supply via its N/O contacts. This causes
the steel rod of the solenoid to be pulled against the
spring tension and the lock gets opened.
If the code selected via thumbwheel switches does not
match the DIP switch settings and DPDT switch S1 is put
on, the low output of AND gate G11 will result in setting
of latch formed by NAND gates N13 and N14 as well as sounding
of buzzer through transistor T2 which gets forward biased.
Resetting of buzzer as well as the latch is possible by
momentary depression of switch S2. It is therefore desirable
to install switch S2 also in a hidden place.
For proper operation of the circuit, switch S1 is initially
kept in off position and desired code is selected with
the help of the dip switches. This is followed by resetting
of latch by momentary depression of switch S2. Now the
system is ready for operation. Switch S1 is to be put
on only after correct code is selected with the help of
thumbwheel switches for opening of lock (for energisation
of solenoid), else it will result in sounding of alarm
(buzzer), as explained earlier. Diode D2 prevents the
positive voltage present at pin 1 of NAND gate N13 (when
switch S1 is off) from reaching transistor T1 base and
energising relay RL1 and the solenoid.
A lock using solenoid coil can be easily fabricated even
if it is not readily available. If required, the solenoid
used in electrical gong type of bell may be used for the
purpose. |
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