This USB to serial (TTL) converter project is easy to build, it is simple and inexpensive. It is based on the PL2303SA USB to USART bridge from Prolific. The PL2303SA chip is not required an external crystal as the internal clock oscillator is continuously tuning up to USB bus frequency. Having chip in SO-8 packaging does not require special soldering skills to assemble the project. Please note: the TX and RX signal levels are 3.3 Volts.
Another low component count USB to serial converter module is based on the FT230XS from FTDI Chip. The FT230XS is outfitted in SSOP-16 packaging. The first incarnation of FT230X chips got a nasty bug when the chip inadvertently goes into suspend mode triggered by certain byte sequences. The FT230X releases A, B and C were affected, see FTDI Chip TN_139 Technical Note. The resistor R1 connected to CBUS3 pin is providing workaround, keeping the chip awake. The Eagle projects files are here.
For more detail: USB to Serial Breakout Board for Prolific PL2303SA