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When we talk about input to the microcontroller unit, which may or may not be user input, it's not just ON/OFF switches or Push Buttons. In fact there are different methods for collecting data either from a user or the environment. According to the requirements, you can expect a user to press some button, toggle a switch, enter a username/password e.t.c. while various types of sensing devices are used to detect changes in the environment which will be discussed in my future posts. Today I am going to briefly introduce you to Keypad interface with the 8051 microcontroller. I have noticed many students having problem with this so I have tried my best to keep it as simple as possible while you will try your best to concentrate and understand it once and for all. READ MORE

Here are many ways to you will provide the protection you deserve. 4 Alarm Control Systems has been providing Fire and Security The Simple 4 Digit Alarm Control keypad Circuit Diagram must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13terminals. The matrix type with 7 terminals will NOT do. The Alarm is set by pressing a single key. Choose the key you want to use and wire it to `E`. Choose the four keys you want to use to switch the alarm off, and connect them to `A B C & D`. Your code can include the non-numeric symbols. With a 12-keypad, over 10 000 different codes are available. Wire the comm onto R1 and all the remaining keys to `F`. When `E` is pressed,current through D2 and R9 switches Q5 on.   4 digit Alarm Control keypad Circuit Diagram The relay energies, and then holds itself on by providing base current for Q5 throughR10. The 12-volt output is switched from the “off ” to the “set ”terminal, and the LED lights...

Description   This is a universal version of the Four-Digit Alarm Keypad . I've modified the design of the output section - to free up the relay contacts. This allows the circuit to operate as a general-purpose switch. I used a SPCO/SPDT relay - but you can use a multi-pole relay if it suits your application. Do not use the "on-board" relay to switch mains voltage. The board's layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage - mount a suitably rated relay somewhere safe - Away From The Board .   Circuit Diagram   Notes: The relay is energized by pressing a single key. Choose the key you want to use - and connect it to terminal "E". Choose the four keys you want to use to de-energize the relay - and connect them to "A B C & D". Wire the common to R1 and all the remaining keys to "F". The Circuit is easy to use. When you press "E" - curre...