LED flasher for inspecting fiber bundles

The circuit in Figure 1 allows you to verify the fiber optic cable, especially when the link passes through several patch panels. You can see it at the far end. It uses two high-brightness LED flashers that turn off after about 50 minutes. The power consumption in the off state is negligible. The circuit works over multimode fiber at distances exceeding 1km. It also works with single-mode fiber, but it is more difficult to see at the receiving end. If the fiber is short, it is best to view it at a small angle at the far end because the LEDs are very bright.

IC IB is a Schmitt trigger oscillator running at about 5Hz, driving IC 2 4020 binary flip-flop. IC3A is the control flip-flop. Pressing pushbutton switch S1 sets the flip-flop, which starts the oscillator and enables the 4020 to begin counting from its all-zero state. It also enables gates ICIA and ICID, which control the PNP and LED driver transistors . The control flip-flop is reset by pressing pushbutton switch S2 or by the 4020 reaching the end of its counting sequence. IC3B divides the oscillator frequency by 2, providing a 50% duty cycle to the LED driver. The T1-3/4 LEDs fit nicely in the ST barrel connectors , which are glued in place. Yellow LEDs are also suitable, given the fiber attenuation. But if you choose to use two red LEDs, drive one at the oscillator frequency and the other at half the oscillator frequency. Component values ​​and placement are not important. The circuit shown in Figure 1 can be placed in a surface mount fiber optic jack box.