Make a Latching double pole single throw relay, using electronic components
There are many possible ways of 'latching' a relay electronically.
The method below is one of many - whether it suits depends on the detailed requirement. 

Input C will  


latch the relay on when pulsed high, 
allow the relay to remain in it's last "driven state when left floating and 
Unlatch / deoperate the relay when pulsed low


Or, separate latch and unlatch inputs may be used.

Input B will 


latch the relay on when pulsed high, and 
not affect the relay when floated or held low.  


Input A will 


unlatch / deactivate the relay when pulsed low, and 
not affect the relay when floated or held high.  


The "sense" / polarity of any input can be altered by placing a simple inverter (similar to R4 &amp; Q1) before the input.

Inputs A B C have to provide enough current to overcome the latching current provided by R3. D1 would ideally be a Schottky diode to allow input A to pull Q1-base low enough in all situations.

The relay is shown as SPST but may have other contact configurations as desired. 







simulate this circuit &ndash; Schematic created using CircuitLab

Turn on:

R4 input high turns on Q1.
Q1 on pulls R1 low, turns on Q2.
Q2 on pulls R3 and relay high.  Relay now on.
Q2 on via R3 turns or holds Q1 on.

Latch / float:

Any or all of of the following input states have no effect on Q1 which remains latched by Q2 via R3. Inputs :


C floating (high impedance),   
A high or   
B low 


Delatch / Off.  

Either of
 - A low or
 - C low
grounds Q1 base via R4 and as R4 is &lt;&lt; R3 turns off Q1. 
This turns off 2 and relay and pullup/latch path via R3.  

For proper turn-off with A low, D1 should be a Schottky diode.
Then R4-left say 0.3V. Vb_Q1 ~= 0.3 + (Vcc-0.3) x (R4/R4+R3)
For eg Vcc = 5V
Vb_Q1 = 0.3 + 4.7 x 1k/(34k) = 0.44 V = OK
R3 can be larger if Vcc is larger, eg 100K for Vcc=12V
(Vcc = 12V, R3 = 100k, Vb_Q1 = 0.42V.

At higher Vcc levels, Dopt1, Ropt1 can be added for extra delatch certainty if desired.