Thursday, November 13, 2014
555 timer bassed Electronic lock circuit with explanation
A very simple electronic key code lock circuit that require few external components can be constructed using this schematic diagram . This electronic key code lock circuit is based on a common 555 timer circuit and some other common components .
This low cost key code circuit use six switches that needs to be pressed to open the lock, but only two switches at a time. In many other , more expensive electronic circuits the key code is formed by pressing some switches one by one , not like in this case two switches . If you don’t like to press two switches in the same time you can eliminate one switch , but in that case the code can be more easy to guess by someone ells .
Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.)
An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the
timing components, before returning to the quiescent low state.
Pin 2 of 555 timer is the triggering input pin which, when held below 1/3 of the supply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC.
Three sets of switches SA-SC, S1- S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential
divider comprising resistors R3 and R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3.
Depressing switches S1 and S8 in same time, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds .
Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.
The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is reset and the output goes to its quiescent low state.
The given circuit can be recoded easily by rearranging connections to the switches as desired by the user.
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