The pulse detector in Fig 1 provides a visible indication of positive and negative current pulses. The pulses' amplitudes can vary from 20 to 150 mA. The pulses' durations can range from 10 to 40 msec, and their repetition rate can span 40 to 180 pulses/minute.

In Fig 1, the junction of the resistive divider connects to a bridge rectifier comprising diodes D₁ and D₂, and the LEDs D₃ and D₄. An N-channel JFET, Q₁, is the load for the bridge. The JFET provides a constant-current sink for the LEDs. Obviously, the JFET must have an I_DSS of less than the maximum LED forward current.

D₃ and D₄ indicate positive and negative pulses, respectively. The LEDs' brightness remains virtually constant over the specified range of input-current pulses.
EDN BBS /DI_SIG #1353