Bipolar relay with single supply

When a bipolar relay is to operate from a single supply voltage, normally two coils are needed. When, in Figure 1, the push-button RESET switch briefly connects the second terminal of coil 1 with +Ub, the relay is in one position. Pressing the SET switch briefly causes the relay to switch over to the other position. The relay can, however, change over only if the currents through the coils flow in opposite directions.