This is a over/under voltage monitor circuit. This circuit is used to monitor Any potential from 1 to 15 V. If there is a undesirable variation was occurs, two lamps will flash. The voltage differential fro lamp turn off and turn off is about 0.2V at any setting. Low and high set points are independent of each other. The type of SCRs must be the sensitive gate type. Here is the schematic diagram of  the circuit:

Over Under Voltage Monitor circuit schematic

The value of R3 depends on the type of SCR. To determine value of R3, adjust the R3 until lamp turns on when there is no signal at the SCR2 gate. The zener diode is connected in series with positive lead, so any 15-V segment can be monitored.The zener voltage plus 0.8V is the low set-point voltage. [Circuit's schematic diagram source: seekic.com]