Many ‘busy’ indicators for use in telephone systems present undesirable loading of the telephone line. Some circuits are very simple indeed to the extent of only loading the line when it is not in use. The downside is that a (usually green) LED lights when the line is not occupied. The author feels that a LED should flash when the line is actually in use by another extension and that the circuit should present a minimal load of the line. The circuit shown here fulfills both requirements. We should, however, not forget to mention its only drawback: its needs to be powered from a battery or an energy-friendly battery eliminator (a.k.a. wall cube or mains adapter). If a high-efficiency LED is used then the current drain from the 9-volt supply will be so small that a standard (170-mAh) 9-V PP3 battery will last for months. Considering that the LED is powered at current of 2 mA by T1, theoretically some 85 hours of ‘LED on’ time can be obtained.
If, for some reason, you wish to change the flash frequency or on/off ratio (duty cycle) then do feel free to experiment with the values and ratio of R1 and R2. The effect will also depend on the brand of the 4093 IC, its exact logic High/Low switching thresholds and hysteresis.
The circuit is not approved to BABT standards for connection to the public switched telephone network (PSTN). Please check local/national regulations.