In this circuit, the ringing voltage on a telephone line causes NE-1 to break over, triggering SCR1, which
in turn triggers SCR2. If a call has been received, depressing S2 will cause LED1 to light. Depressing SI resets the circuit. This circuit has the advantage of lower battery drain because LED1 is not left on continuously after a ring signal, but only when S2 is depressed.