Central locking system conversion to remote power switch (intro to the bistable relay)

I a continued effort to re-purpose un-needed items into generic solutions to common problems I have converted this no-name-brand aftermarket central locking system into a remotely switchable power strip.  The system was given to me since the only part of it that was needed for a friend’s hack was the solenoid actuators, with only the transmitter/receiver pair left I thought this would make it at least somewhat useful.

a most generic box


how it originally looked


So, this system used one solenoid and applied voltage one way to lock and another way to unlock.  There are two remotes and the receiver module has spots to wire up buttons for lock and unlock.  The first thing I did was pin it out, the lock and unlock signals are labeled in the above picture.


The inside of it wasn’t too hard to figure out.  Power and found were red and black, 12 volts DC since it was meant for (modern) automotive use.  Brown and white were wired to themselves on the board which left the question as to which was control and which was output.  I checked the bottom of the board and verified with a meter that when actuated to lock blue and green source and sink 12 volts momentarily, and unlock flips the polarity.  Now, the question is how to latch the output so one button closes the circuit and another opens it.

One solution would be to use a flip-flop and set, or reset it depending on the signal.  It might even be beneficial to trace back before the relays to the logic level signals and use those directly.  Then you could simply use the output of the flip-flop to run a fet, SSR, or relay to control your device, but I think I have found not only a simpler solution, but a more efficient one.  You may not think that more relays would make the system any better, but there’s one type that they don’t usually talk about that solves this problem perfectly.

When I was thinking about home automation I wanted to use electronics to flip lights and fans on and off to conserve power, but I had a problem: if I use a relay then it’s always burning power in one of the two positions.  If I hook up a computer in addition to the conventional light switch then either you can turn out the lights, or my sensors or timers can, but then the efficiency gets eaten into by the relay coil burning away the watts silently heating the wall.  I theorized that you could make a relay that is stable in two positions, one that has springs that hold it closed, but coils that pull it to one relaxed state or another.  This would be rather fiddly to build, but it’s not a complex idea.  Another design could use a ratchet to increment along a series of contacts making a kind of electro-mechanical rotary switch.  I really didn’t want to build either of these, but I couldn’t believe they were new ideas.  A trip to google later and I got the term ‘latching relay’ or ‘bistable relay‘ and a couple weeks later ebay delivered me a few to play with.  As an aside, the other design (the multi-pole one) is called a stepping relay.


There are multiple types of latching relay, I chose the type with one coil and flipping the polarity would change the state of the relay (since that’s what the system I had did).  In reality I could have treated both signals individually and used a common ground, but I had this available to me so I used it closer to how it was originally designed.  Now I have a single pole single throw relay contact as an output of an rf remote.  I packaged it all up and the result was this nice neat box.


Now I still didn’t have a use for it in this state, so I threw it in a box, connected it to mains, and have a wireless remote power strip.  I may not have much of a use for that either, but now I can just plug it in and go.

One Response to “Central locking system conversion to remote power switch (intro to the bistable relay)”

  1. ESP8266 Bistable Relay | Evan's Techie-Blog Says:

    […] their own these are not that useful.  This article is possibly most true in that department.  I have mentioned how much I like bistable relays before, and I mostly mention it to get more people to buy them because right now TE Connectivity seems to […]

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