mains voltage monitor

It is often desirable to know at a glance whether the mains voltage is at the low side; for instance, when you are about to work on a computer program. The danger is, of course, that when it is already low, further loads may cause the mains to drop below an acceptable level. The supply for the present circuit is taken direct from the mains, which exists across R1and P1. The 15 V stabilized voltage produced by R2, C1, C 2, Di, and D 2 provides two reference voltages. These voltages are com- pared in Aland A2 with a fixed pro- portion of the mains. If the mains is below 210 V, D7lights, and when it is higher than 250 V, D8 lights. When neither D, nor D8lights, Ti switches on and causes D4 to light, indicating that the mains voltage is within acceptable limits. The mains voltage limits are set with P1with the aid of a multimeter and a variac; where perfectionism is not required, red 1 )4 mains voltage monitor N 21VT 400 m14/ 100, 25 V 0 40 V D3 1N4001 81 1W Cl. 680 n 400 V R3 C2 I=1 ...