What kinds of latching relays are there and how do you unlatch them?
There are two main types of latching relays - single coil and dual coil.
Single Coil Relay
Supply the relay with the rated coil voltage to latch, this can be steady or a single pulse. The datasheet should show the minimum amount of time the voltage is needed to activate the relay (usually in the milliseconds). The coil will stay latched after the voltage is removed. To unlatch the relay, another steady or pulsed voltage must be supplied to the coil in the opposite direction or polarity.
Dual coil relays have two coils. Supplying the first relay coil with the rated coil voltage (again this can be steady or a single pulse) will latch the relay. The coil will stay latched after the voltage is removed. Supplying a voltage to the same coil again will not affect the relay. To unlatch the relay, voltage must be supplied to the second coil.