A board which provides isolation for a TTL serial TX / RX pair.
This board design is Open-Source Hardware. It is licensed under the MIT License. It was designed by Jason Pepas.
Initial board design.
PDF.
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.