This transmitter uses a 741 op amp as a high-gain audio amplifier, which is driven by a microphone. The output of the 741 is coupled to Ql, which serves as the driver for a LED. Potentiometer R1 is the amplifier`s gain control. Miniature trimmer resistor R6 permits adjustment of the base bias of Ql for best transmitter performance. Gain control R1 can be eliminated if CI and R2 are connected directly to pin 2 of the 741.
For maximum sensitivity, increase the value of R2 from 1 to 10 and use a crystal microphone with a large diaphragm.