Electronic – Driving a 74HC series chip with a 74LS series


I made a mistake and accidentally bought a 74LS AND gate: this one. My other chips are 74HCs. I need the output of one of the AND gates to drive the clock signal on a JK flip-flop: this one. Unfortunately, the LS chip floats like crazy and I get random clock signals. Is there anything I can do to make sure that the LS chip behaves (that is, doesn't float and cause spurious signals) or do I just need a new chip?

Best Answer

An HC chip needs >70% Vcc for the high voltage, which an LS gate is not guaranteed to deliver.

Use a pullup resistor on the output of the LS. Its output is rated to sink 8mA, so you can go down as far as a 620ohm resistor, though a higher value will use less current in the low state. 1k is a reasonable value to try. You cannot go too high in value, otherwise the pullup speed into load capacitance will be too slow and still not fix your multiple clock transitions. You will need to experiment with this if you want to use less current.