The transmitting section of this infrared tx-rx is unusually simple but it works rather well: the infrared LED pulses at a frequency of 160Hz and its range, with its receiver, is between 2 and 4m depending on the transformer used and the setting of the 100k pot. With other receivers it may reach a range of 15m, without any lens but with a perfect alignment between tx and rx. The receiving section uses an infrared phototransistor and an additional infrared emitter is placed next to it in order to provide a light bias thus improving sensitivity.
The 100k pot. will also adjust sensitivity by setting the right operating point for the transistor. The supply should be well regulated in order to avoid self-oscillations. The audio transformer is a small output transformer recovered from an old transistor radio.