Nowadays security is the prime concern in our day to day lives. Through this secure access system, only that person can enter into a restricted area who knows [[wysiwyg_imageupload::]]the correct code. This simple circuit is built around a transistor with a few more components like relay, diode, capacitor, switches and resistor.In this circuit, switch placement is such that switches 1 to 4, which when pressed in a sequential manner, energize the relay for a specific interval of time to unlock the solenoid-operated lock. If you press any of the buttons from switches 5 to 8 in between the correct code then it will put the transistor to ground and relay will not be energized. Read more to find out how the circuit is designed and how it works.