An op amp is used as a comparator and a sink for LED current. The output voltage of the amplifier changes about 1.4 V depending on the direction of the current. Only one transistor is on at any time. Maximum LED current is limited to 25 mA by overcurrent protection of the uA741.
If LEDs are not capable of carrying such a current or an alternative op amp is used and an additional resistor R^ is necessary.