Universal Four-Digit Keypad-Operated Switch - Construction Details

  
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The key you`ve connected to `E` is used to energize the relay. And the four keys you`ve connected to A, B, C & D - are used to de-energize it. All the other keys on the keypad are connected to `F`. = When `E` is pressed - current from the `Com` terminal flows through D2 and R9 - into the base of Q6. This switches Q6 on - and the transistor connect
Universal Four-Digit Keypad-Operated Switch - Construction Details - schematic

s the negative side of the relay coil to ground. This causes the relay energize. Q6 also connects the green LED to ground. This provides a visual indication that the relay is energized. = Q5 & Q6 are wired to form a complementary latch. When Q6 switches on - it also connects the base of Q5 to ground. In so doing - it switches Q5 on - and current through R10 & Q5 flows into the base of Q6. Thus, Q6 uses Q5 to supply itself with base current. In this way - the relay remains energized after "E" is released. For a detailed account of how a complementary latch works - click on the link below. = To de-energize the relay - you have to press A, B, C & D in the right order. The IC is a Cmos 4081. It has four two-input AND gates. These gates only produce a high output when both inputs are high. = To begin with - pin 1 is held high by R5. This enables gate 1. When "A" is pressed and pin 2 is taken high - the output of gate 1 - at pin 3 - goes high. This output does two jobs. Firstly, it latches itself on by holding pin 2 high through R2. In this way - pin 2 will stay high after "A" is released. Secondly, Pin 3 enables gate 2 by taking pin 5 high. In other words - pin 3 is doing for gate 2 - what R5 does for gate 1. = Now - in exactly the same way - when "B" is pressed and pin 6 is taken high, the output from gate 2 latches itself on through R3. And it enables gate 3 by taking pin 12 high. = Again - in exactly the same way - when "C"...



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