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Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.

74HCT family: The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.

Outputs can sink and source about 4mA if you wish to maintain the correct output voltage to drive logic inputs, but if there is no need to drive any inputs the maximum current is about 20mA. To switch larger currents you can connectatransistor.

Supply: 5V 0.25V, it must be very smooth, a regulated supply is best. In addition to the normal supply smoothing, a 0.1F capacitor should be connected across the supply near the IC to remove the ‘spikes’ generated as it switches state, one capacitor is needed for every 4 ICs.

Inputs ‘float’ high to logic 1 if unconnected, but do not rely on this in a permanent (soldered) circuit because the inputs may pick up electrical noise. 1mA must be drawn out to hold inputs at logic 0. In a permanent circuit it is wise to connect any unused inputs to +Vs to ensure good immunity to noise.

Outputs can sink up to 16mA (enough to light an LED), but they can source only about 2mA. To switch larger currents you can connectatransistor.

Fan-out: one output can drive up to 10 74LS inputs, but many more 74HCT inputs.

Gate propagation time: about 10ns for a signal to travel through a gate.

Frequency: up to about 35MHz (under the right conditions).

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