The first in a series of chips that are useful for level translation between 3.3V and 5V (and the other way) systems. This one consists of 4 tristate output buffers in a 14 pin package. Each of the 4 buffers has an input, an output and an enable (active low in this case). Because of this independence this part uses schematic subparts which means you can move any of the 4 buffers around your schematic by clicking on one and dragging it. They are also intentionally small (.2 in wide by .2 high). The 74ahc125 (and only the ahc version NOT the hc version) has 5V tolerant inputs so if you use a VCC of 3.3V it will accept 5V inputs and output 3.3V to your 3.3V device at full speed. The 74ahct125 (and the 74hct125 in this case) runs on VCC of 5V and accepts anything greater than 2.4V ) as the t in the part number stands for TTL input) as a high. Thus your 3.3V output from your device will drive the input which in turn switches the output between 0 and 5V which does the level translation the other way. When used as a level translator you will usually connect the enable input to ground so that the tristate output is always active. Because the output is tristate, if you instead connect the enable to a select signal (such as for different SPI devices) it will act as a multiplexer as well because of the tristate outputs. Note that too many subpart parts (I find starting around 10 and higher) slows Fritzing down substantially.
74x125.fzpz (9.6 KB)