Both works on 5V only but HCT use 1/1000 of the power that 74LS does. A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2k is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used. The devices are pin compatible with many existing devices such as the 74TTL, 74STTL and 4000 series. 74LS vs 74HC vs 74HCT Page history last edited by Wayne Warthen 1 year, 7 months ago. 74LS Series of High Speed, TTL Logic Gate Chips including AND, OR, NAND Gates as well as counters, shift registers and multiplexers. The following is a list of 7400-series digital logic integrated circuits.In the mid-1960s, the original 7400-series integrated circuits were introduced by Texas Instruments with the prefix "SN" to create the name SN74xx. Working voltage, maximum frequency, threshold voltage for ones and zeros, fan-in, fan-out, power consumption. Now people are telling me that the 74HC series might be a better choice. A circuit which includes 74LS or 74HCT ICs must have a 5V supply. the original logic familyhad no letters between the "74" and the part number. G. ghall426 New Member. Wouldn't think there's a difference... 74HC and HCT are compatible, HCT being fully compatible and HC being HC-output to TTL-input compatible (and TTL-output to HC-input compatible with a pull-up resistor, if you don't mind it being slow; but HCT is better to just drop in). As you can also see, the 74HC can source and sink up to 15mA more than that of the 4000 series, which also means that it (74HC) is better at sourcing and sinking current as compared to the 4000 series. The high and low logic levels of 74HCT have been tweaked to match TTL where (with Vcc=5V), high is around 2.4V and low is around 0.4V. HC and HCT have identical outputs. The 74LSxxx series is low-power Schottky and the 74HSxxx series is high-speed CMOS. L.Chung. 74HCT (74HC + TTL) is compatible (within the limits of the power supply specs). However, it is a bit slower than 74LS (25MHz vs 35MHz). And there is the 74HCT series which is a near replacement for the 74LS series. Copy pasted from 7400-series integrated circuits - Wikipedia * Bipolar74 – Standard TTL. 74LS is bipolar transistor TTL. The 74HC consumption is considered to be more than that of the 74LS series, which means it can be used in higher amperage applications. Y'mean comparing new 74LS? 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. Choose one or the other, not both, for your "practical" project builds. 74LS Series. Google "74hc vs 74ls" for some informative links.Basically, as @kellys_eye said, they are functionally and logically identical. 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. 20mA * Large fan-out capability The 74HC family of logic devices are high speed, low power devices that use CMOS circuitry. * 2V to 6V operation * Very high impedance inputs * Can sink and source approx. When I started working with ICs, the 74LS series was common. 10 ns gate delay, 10 mW dissipation, 4.75–5.25 V, released in 1966. 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. The HCT output can source and sink 20mA while that of 74LS can only sink 16mA but source 2mA. Compare and contrast the 7400 series and 4000 series logic familes with reference to at least 5 characteristics e.g. OK, this probably needs to be moved to somewhere more appropriate. 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. The two types can be mixed in the same circuitry ONLY IF you know what you are doing, which you obviously do not. But I need some help figuring this out. Note that a 4000 series output can drive only one 74LS …