A simple way to build a battery-powered RS-232 receiver circuit is shown on the following schematic diagram. This circuit convert RS232 level to TTL level. Because the 100 KΩ resistor prevents device damage, the input voltage can be driven as much as +- 30V without adverse effects. The LTC1045 draws only 100 μA of quiescent current, and has a propagation delay of 1.2 μs with 1 MΩ RSET resistor at pin 12. This circuit requires only a single 5 V supply.
[Circuit’s schematic diagram source: seekic.com]