I have duplicated the circuit below where the TTL side is a XBee Radio. The LEDs on TTL TX and RX are the same part number, the resistors are the same value and they are being driven by the same VCC level. The problem I am having is that when they are biased, the TX is much brighter than the RX, which is barely noticeable. I have searched and have not found an answer...maybe I'm not searching for the right thing?

Can anyone explain why they are different in brightness?

enter image description here

  • \$\begingroup\$ You have no control over the input voltages, so connecting an LED to them directly is not wise. \$\endgroup\$ Jun 17, 2016 at 23:54
  • \$\begingroup\$ The input voltages, according to the XBee datasheet is 3.3v. \$\endgroup\$ Jun 18, 2016 at 0:01
  • \$\begingroup\$ The nominal value doesn't help you. You need it to be guaranteed to be at least a particular value given a particular load. Did you measure it with the LED loading it? Was it 3.3? \$\endgroup\$ Jun 18, 2016 at 0:04
  • \$\begingroup\$ Would there be a better setup that will accomplish the same task of indicating status? \$\endgroup\$ Jun 18, 2016 at 0:06
  • 1
    \$\begingroup\$ @SolveEtCoagula07 Ideally, you would use a transistor to light the LEDs with the TX/RX line connected to its base. \$\endgroup\$ Jun 18, 2016 at 0:55

1 Answer 1


Your problem is Q2, the buffer transistor on the RS232 Tx/TTL Rx line. Both Q1 and Q2 provide signal inversion, but only the TTL Rx line is being driven by your buffer.

RS232 uses a default high voltage level to indicate a logical zero, so ordinarily (when there is no data being transmitted) TTL Tx and RS232 Tx are physically high. Since TTL Tx is high, there is little drop across the LED and resistor, so the LED is dim. The TTL Rx, though, is being pulled to ground by Q2, so there is full voltage across the LED.


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