This is a very interesting IC 555 soft touch switch door lock mini project. It is very easy to construct and useful for your school or college level mini project submission.
The actual size PCB layout of the project is also given here, so that you will be able to construct it within an hour if you have all the components readily available with you.
Working
As shown in above circuit, the IC 555 is used as bistable multivibrator circuit. Its pin-2 i.e. trigger pin is held to +ve through resistor R1 and pin-6 i.e. threshold pin is returned to ground via resistor R2. Due to this connection, the IC does not get accidentally triggered or RESET.
The two miniature (soft touch) switches are used to control the door lock condition. Switch SW1 is a LOCK switch and SW2 is the UNLOCK switch.
A heavy duty solenoid SL1 is connected between pin-3 i.e. the output pin of the IC and ground through diode D1. Diode D2 is connected in antiparallel combination with the solenoid to absorb any back e.m.f. whenever the solenoid is deactivated. Thus diode D2 also protects IC 555.
One RED LED is connected in parallel with the solenoid through current limiting resistor R3 to serve as a LOCKed condition indicator – preferably it should be RED in color…!
Now when supply is connected to the circuit, the output of the IC remains LOW and the solenoid remains in UNLOCKed state.
When we press SW1 momentarily, the IC gets triggered through pin-2 as the trigger pin is momentarily grounded. So the IC is triggered and its output at pin-3 becomes HIGH. So the solenoid is activated and it locks the door. The RED LED glows indicating that the door is locked.
Now to unlock the door, press SW2. When SW2 is pressed momentarily, the output of the IC becomes LOW. The solenoid is now pulled back due to its spring action and the door is unlocked.
I recommend to use the battery supply or any full wave rectifier circuit having at least 9V supply. Also use the solenoid with less operating voltage. Here we have used 6V solenoid.
Note: The circuit is practically tested in Vidyasagar Academy laboratory and works very fine. If you notice that when the door is locked and if you switch on/off some other device like a tube light or fan, and the IC gets accidentally triggered or reset, then we recommend to use one small 0.01uF capacitor between pin-5 and ground. This will absorb any noise created in the circuit and will avoid accidental triggering or resetting. For any query, contact us.
PCB layout
This is the actual size PCB layout of the above given circuit.
Vidyasagar Academy 