TTLSerialOptoisolator

A board which provides isolation for a TTL serial TX / RX pair.

License

This board design is Open-Source Hardware. It is licensed under the MIT License. It was designed by Jason Pepas.

Board design v1

Initial board design.

Gerbers

gerbers.zip

Schematic

PDF.

Design notes

LED series resistor calculation

Can we find an LED resistor value which works for both 3.3V and 5V TTL levels?

Lite-On LTV-817 datasheet: http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTV-817_827_847%28M,S,S_TA1%29.pdf

The forward voltage of the LED is typically 1.2V.

• 5V - 1.2V = 3.8V
• 3.3V - 1.2V = 2.1V

Voltage	100Ω	150Ω	220Ω	330Ω	470Ω	680Ω	1kΩ
5V	38mA	25mA	17mA	12mA	8mA	6mA	4mA
3.3V	21mA	14mA	10mA	6mA	4mA	3mA	2mA

So 330Ω would be a reasonable choice.