Active 3-to-8 line decoder for address decoding and demultiplexing
The SN74LS138D is a 1-of-8 decoder/demultiplexer from the 74LS family, active in production and ROHS3 compliant. It accepts a 3-bit binary input and asserts one of eight active-low outputs, making it a standard building block for memory-mapped I/O address decoding and data routing in 5 V logic systems.
The device sources 400 µA and sinks 8 mA per output (high/low) — enough to drive one or two standard TTL loads directly, but not enough for high-fan-out buses without buffering. For a memory chip select line or a single LED indicator, the 8 mA sink handles the load without an external transistor. The surface-mount form factor suits high-density PCBs where through-hole decoders would consume more board area.
Sourcing and compliance for the BOM line
The ROHS3 compliance covers current EU substance restrictions, so no exemption paperwork is needed for new builds shipping into regulated markets.