user7412: are there any USB to TTL with switchable 5v and 3.3v 
i need to use it for raspberry pi and arduino

user337: Many 3.3V FTDI based devices are 5V tolerant and will work with both:

Adafruit's FTDI friend
Sparkfun FT231X Breakout

Here is general information about logic levels: https://learn.sparkfun.com/tutorials/logic-levels

7412: <p>are there any USB to TTL with switchable 5v and 3.3v </p>

<p>i need to use it for raspberry pi and arduino</p>
 337: <p>Many 3.3V FTDI based devices are 5V tolerant and will work with both:</p>

<ul>
<li><a href="https://www.adafruit.com/product/284" rel="nofollow">Adafruit's FTDI friend</a></li>
<li><a href="https://www.sparkfun.com/products/11736" rel="nofollow">Sparkfun FT231X Breakout</a></li>
</ul>

<p>Here is general information about logic levels: <a href="https://learn.sparkfun.com/tutorials/logic-levels" rel="nofollow">https://learn.sparkfun.com/tutorials/logic-levels</a></p>
