The gnd pin on the IR LED is actually switched on and off using mbed's serial data output to eliminate the need for any external digital logic, and still permit direct use of the serial port hardware for 38Khz IR data transmission.
It is a bit more complex than appears at first glance, since the start bit must be zero for the 10-bit serial port input hardware to work, and the IR receiver inverts the signal.
Long right angle header pins might be a good idea on these IR breakout boards since they can then be mounted sticking up and directly facing each other.
At close range, your hand or a piece of paper will reflect back enough IR to transmit the signal in case you can't mount them facing each other on the breadboard.
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