Arduino TTL Voltage Levels

If I’m trying to connect 3.3V TTL serial devices to a Sparkfun RedBoard1 using SoftwareSerial, do I need a level shifter2 to avoid damaging the devices? I’m trying to connect the following items.

GPS Receiver: GPS Receiver - GP-20U7 (56 Channel) - GPS-13740 - SparkFun Electronics
RockBLOCK Iridium SatComm Module: RockBLOCK Mk2 - Iridium SatComm Module - WRL-13745 - SparkFun Electronics

One thing to note, I had actually hooked up the GPS before thinking about this question and did not damage it. However it only transmits data…it doesn’t have a RX wire, only TX. I’m guessing it was only transmitting at 3.3V to the Arduino, so it wasn’t in danger of being damaged by 5V signals coming from the board.

Good thing I realized this before trying to hookup the RockBLOCK…it’s a bit expensive!

1: Sparkfun Redboard, https://www.sparkfun.com/products/12757
2: Level Shifter, https://www.sparkfun.com/products/12009

Sending a 3V3 signal into a 5V Arduino is fine. Going the other way you need to cut it down. There is no need to use a level shifter. Just a 1K and 510R potential divider will be fine.

Ahh, gotcha. Thanks for the help!