View Full Version : Status LEDs for Transmit and Receive lines

Chuck Rice
04-26-2008, 04:21 AM
I have several protoboards (4) connected serially, using 51 ohm series resistors on the tx and rx lines, and a 10k pullup to 3.3v as shown in the Propeller Cookbook. I would like to add some reassuring blinky lights when data is transmitted.

Where/how would it be best to attach these without interfering with the 115,200 baud traffic?

Mike Green
04-26-2008, 05:35 AM
If you really want to attach LEDs without affecting the signal lines, you need to use some kind of buffer. Something like a 74LCX07 which has an open drain output that can drive LEDs and is compatible with a 3.3V supply.

04-26-2008, 05:37 AM
You could modify the FDS driver to turn on a pin when in the process of receiving or transmitting data and then attach the LED to that pin.

I am 1010, so be surprised!

Chuck Rice
04-26-2008, 05:55 AM
There does not appear to be a DIP version of that buffer. Would an inverting buffer such as this (http://www.st.com/stonline/products/literature/ds/1883.pdf) work as well?

I am not a true swimmer in the electronics "pool". I have to dog-paddle to keep my head above water.


Mike Green
04-26-2008, 06:09 AM
Yes, the 74HC05 will also work. Connect the LED's anode through a 100 Ohm resistor to the output pin and ground the cathode. Use the Propeller's 3.3V supply for the 74HC05. When the input pin goes high, the LED will light (with about a 10mA forward current).

Chuck Rice
04-26-2008, 06:23 AM
Perfect! Thanks.