MAX232 RS-232 Level Translator
An iconic charge-pump chip turns 5 V TTL UART into ±12 V RS-232 — talk to legacy GPS, modems, and industrial gear from any modern board.
Difficulty: Breadboard. Estimated build time: about 45 minutes. Estimated parts cost: about US$4.20. 3-line bill of materials. Compare supplier offers when available. An RS-232 level translator built around the MAX232, the 1987 chip that single-handedly killed the dual ±12 V power supply in serial-port designs.
Wire the four charge-pump capacitors
DIP-16, notch up. Pin 1 = C1+, pin 3 = C1-, pin 4 = C2+, pin 5 = C2-. Pin 2 = V+ (positive pump output), pin 6 = V- (negative pump output). Place a 1 µF cap from pin 1 to pin 3, another 1 µF from pin 4 to pin 5, a 1 µF from pin 2 to VCC, and a 1 µF from pin 6 to GND. Use ceramic or film, NOT electrolytic — polarity matters here and the pump rapidly inverts polarity. The datasheet allows 1–10 µF for these caps; 1 µF works for most cases.
Hook up power and one TX channel
VCC (pin 16) → 5 V. GND (pin 15) → GND. Add a 100 nF decoupling cap from VCC to GND right at the chip. Wire your MCU TX line to T1IN (pin 11). T1OUT (pin 14) is the RS-232 level output — connect that to pin 3 (TXD) of a DB9 connector going to the legacy device.
Hook up one RX channel
DB9 pin 2 (RXD) → R1IN (pin 13) on the MAX232. R1OUT (pin 12) is the TTL-level output — connect that to your MCU's RX pin. Don't forget DB9 pin 5 (signal ground) → your MCU GND. With this, your MCU UART can transmit and receive at any standard baud (9600, 19200, 115200) and talk to any device that expects classic RS-232 levels.