4081 AND gates


Posted on Feb 4, 2014

The inputs of the gate must be connected, either to LOW or to HIGH, and must not be left open circuit. This is the function of the input switches with their pull-down resistors. To avoid loading the output of the gate, a transistor switch indicator circuit should be used. It is good practice with CMOS circuits to insert a decoupling capacitor, 47


4081 AND gates
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µF or 100 µF, across the power supply. (This helps to prevent the transfer of spikes along the power supply rails. ) In the prototype circuit, it is not essential to make connections to the unused gates. However, in any final circuit, all unused CMOS inputs must be connected either to HIGH or to LOW. Make it an absolute rule that CMOS inputs are never left open circuit. Click on the diagram and confirm the latching action of this circuit. When you click the SET button, the output goes HIGH and stays HIGH. This is one of the stable states of the bistable. Nothing further happens unless you click the RESET button, making the ouput LOW once more. Click the diagram to open a Crocodile Technology © simulation of the circuit. The four AND gates operate in sequence and are connected to the push button switches included in the code. The remaining, unselected, switches are connected in parallel and pressing any one of them RESETS all the AND gate bistables. With a real circuit using 4081 AND gates, the 1 µF, 100 nF and 10 nF capacitors are found to be necessary to eliminate a `glitch` caused by pressing the next switch in the sequence. Without the capacitors, pressing the next switch can RESET the previous AND gate. With the capacitors, the circuit operates reliably. This is an unusual way to make a combination lock but the components needed are cheap and easily available. With no repeat digits, a 10-switch keyboard will give...




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