Car Lock/Unlock Project (Latching relays are doing my head in...)

Yea one problem I should mention is the 9v-12v that the XAC v2 requires should not be used directly on a key fob. I believe OP is attempting to use relays to bridge the fob’s circuit board contacts, so I suggest the alternating pulse control two additional SPST relays that act as the fobs buttons.

Another problem I’ve noticed with the schematic revolves around the “button”/trigger of the latching relay. The latching relay’s trigger voltage needs to accept the XAC’s 9v-12v otherwise, magic smoke… If the latching relay has a button then the button would need to be replaced by an additional SPST relay. The whole schematic only works as long as everything is 9v-12v.

I appreciate that! Just trying to help in a timely manner :sweat_smile:

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Ok I’ve got an idea, might work… might not…

No means or skills to depict it, but maybe I can describe it

3 relays
1x latching
2x 12v NO relays

The xACv2 is in NO mode, 1 second mode
powered by vehicle 12vdc
When activated it closes a circuit and allows POWER AND SIGNAL to the latching relay,

Each Output of the latching relay is used as signal to each NO 12vDc relay, which are supplied with vehicle 12vDc

1 NO relay is wired to lock and 1 to unlock button on key fob, only completing button circuit, not applying voltage

When xACv2 Opens again, latching relay looses power… depriving NO relay of signal

I think this would require the latching relay that @Pilgrimsmaster mentioned earlier which can maintain state without power. I just thought it
was worth mentioning that this is not a feature of all latching relays. Possibly something for OP to plan towards because a latching relay that can maintain state would save battery life.

Hi all, I’ve made some progress with my circuit design, thank you all for your input!

I have something in the works involving the xEM Access V2, two switch relays and a flip-flop latching relay. Just trying to solve the issue where the trigger for the switch relay is a 12v high signal instead of a closed circuit. Sure something is out there but if I’ve found it I don’t know it yet lol.

Really appreciate all the suggestions here, watch this space! Order on it’s way with a NExT and a Spark 2.

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I decided to spend some extra time working on the schematic, so here it is! Totaling 5 relays including the Dangerous Things XAC.

Most “latching” relays I come across do not maintain their state after power is lost. These latching relays use a small flip flop circuit (usually activated by a button) connected to a standard relay coil. This means if the power is cut, this latching relay will default to the Normally Closed circuit, not remaining latched. I use this type of “latching” relay in the schematic above. (Edit: They are called Bistable Self-Locking relays) Also, the SPST Relays used in the schematic do not have a trigger in other words they need to actuate the instant 9v-12v is applied to them.

I feel like this video should accompany my schematic because I am not an electrical engineer.

I believe there is a way of removing one of the relays if you can determine whether the key fob’s buttons pulled the contacts on the circuit board high or low. Here is the schematic for that. Though I should mention this method should only be used by professionals as connecting the battery directly to the button contacts could be harmful to the key fob.

Edit: Forehead slap DPST Relays exist

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You have given me exactly what I’m looking for! Once the pieces arrive I will assemble this circuit and test it. If it works as intended I will be extremely grateful. Thank you so much :sunglasses::+1:

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From what I can read on the subject, magnetic latching relays will hold state without power

I think I need a double throw though, and everything is single throw

Edit,
Electrician friend recommended This

May or may not be fully correct, but I might pick it up to at least play with and see if I can make heads or tails of it to learn

I apologize @Eriequiet for the confusion, I was completely wrong. Amazon’s choice for a “latching relay” has led me to incorrectly refer to a “bistable self-locking” relay, as a “latching” relay. OP needs a bistable self-locking relay for all of the wiring diagrams above. I think I can set up a small demonstration of different relays to help myself aswell as any other future passerby. Please let me know if I’m misinformed.

Relays come in all shapes and sizes, all of the following relays function the same way: When their electromagnet is powered, a switch is actuated which closes (or opens) a circuit. The switch will remain in that state until power is removed from the electromagnet.

This SPST relay is one of the types used by Electroboom in the videos above. Since these types of relays are rated for high current, they are usually sold for automotive purposes.

This is an example of a standard SPDT relay mounted on a pcb, this type is connected to power at all times which allows for activation by a high/low level trigger. The SPDT relay can be thought of as a SPST Relay with the addition of a Normally Closed (N.C.) terminal for signal inversion.

This relay is what I incorrectly refered to earlier as a “latching” relay. It is actually called a “Bistable Self-Locking” relay which operates using a D Flip Flop Circuit not a physical latch. This means it will not hold its state when power is lost.

Tip: Inside the prominent blue boxes of each pcb relay is where the electromagnet and switch are housed. Since the bistable self-locking relays are manufactured with the same electromagnets and switches as the standard relays, it can be easily determined that neither will be able to hold their state without power.

Although the prominent box of this relay is a different color than the other two pcb relays, AFAIK it is the same standard electromagnet and switch configuration. What makes this relay interesting is that it has a dual Set/Reset trigger. This means it has the potential to work for both Eriequiet’s suggestion and the diagrams I drew up. The seller claims that the relay will revert to its last held state when power is reinstated. That might cause technical difficulties though if current can flow through the relay before the switch is able to revert. The effect would be a “glitch” similar to the edge detector featured in the “D Flip Flop Circuit” link above.

Edit: Changed fragile links into pictures and some other stuff. I’d also like to share this post I just found: Wiring Diagram Repository.

Hi folks!

Just a quick progress update, I’ve started putting the circuit together, but I’m still waiting on the XAC reader to arrive. Latest shipping update said next Monday to Wednesday.

I got my hands on a bistable latching relay from China and paired it with a 4 channel relay board I had laying around from my previous project. I have wired it up based on the diagram provided by @AbbottSmith, minus some of the COM connections.

Each of these will trigger the circuit once it receives the +12V pulse generated by the XAC.

The key fob was very easy to disassemble, and with a slight modification to the membrane I was able to run the wires directly from inside out to the relays. The internal switches were a bit tricky to solder to as I only have one sized iron tip, but with patience and constant testing to make sure I didn’t break it, I have a working connection between the lock and unlock buttons. I’m considering potting the inside of the key with epoxy to relieve some stress on the solder joints, but I’m not sure yet.

I will post another update once the XAC arrives. In the meantime I need to think about a mounting position in the car and running a 12V bus to the area.

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Nice progress :+1:

I would put in some preventative measures around this termination, That doesn’t look too far from “Sparky shorty smoke ghosts” :boom: :tornado: :ghost:

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You can buy preterminated cables with du-pont connectors on the ends. These will match the pin header on that board perfectly.

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Thank you for pointing that out @Pilgrimsmaster. I have now resolved this issue. I will also be running a 10A fuse inline to ensure I don’t blow anything up :sweat_smile:

@enginerd I like the sound of that. I will do that if this connector does fail me :stuck_out_tongue:

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Alright lads, I have fully assembled the circuit using a fourth relay in place of the XAC for now. Please see the most painful-to-watch circuit test video ever from a very nervous Aussie boi.

Feel free to like and subscribe :wink:

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Nice work mate, Im looking forward to seeing your final assembly with the xAC.

Also interested to see the Total current draw when idle.

Here is a rough “guide” for you to consider if you dont use your car for a week or so.

  • Here’s a rough guide and why I am suggesting this.

A “normal” expectation for most cars, with no power mitigation, would be approximately 2-3 weeks (depending on the age and type of vehicle, the battery and condition, accessories etc.)

Your battery in your car is a 60Ah battery
Normal Parasitic draw of vehicle is approximately 60mA

The xAC V2 is also 60mA Idle (100mA Operating ( Theoretical ))
So you can half what time you would usually expect to be able to leave you car without starting or driving and the battery being OK to start, Therefore = 1 -1.5 weeks
Quick calc, not including your relays ?

Then you probably want to include A safety margin of approx 20-25% ( so you can still start your car, and not fuck your battery )

Basically
You might need some power mitigation strategies if your car ever sits unused for a Week or more.

#Rough Guide/ suggestion, feel free to ignore or do your own calcs

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Just to expand, if you were wondering…
For example, if you were leaving you car idle for a long period of time

  • Maybe Isolating the Negative terminal, ( switch / Breaker etc ) it will only discharge at approx 5% per month…You might want to check what your car does with no power eg. One of my cars automatically unlocks
  • A solar trickle charger. Check that the cigarette lighter plug will charge the battery (blah blah,circuit protection blah diode blah, whatever…it may not work, so you may need to go direct to battery) But remember to disconnect before you start your car ( add a switch, if you have gone direct)
  • add a momentary switch, (or similar)that only powers your circuit when pressed, this is not as clean, as you have to press when you scan in, but it removes all the other mitigation options needs…

Anyway, just some more STUFF for you to think about

Love the video! I’m slightly confused about the inclusion of an extra relay though. You should be able to actuate the circuit by pretending to be the switch inside the XAC, which touches the COM terminal of the bistable relay to the positive post of your battery for 1 second durations.

Seconded.

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@Pilgrimsmaster Thanks for the advice regarding power draw. Next time I’m at work I’ll pop the multimeter on it with the XAC system connected and check the parasitic draw rate just to make sure it doesn’t kill the battery in a few days. It’s very rare the car sits undriven for anymore than a couple days though, so I think I’d be more likely to go with an isolator terminal for the battery.

@AbbottSmith Looking back at the diagram now, I see what you mean. I think my brain went to “The XAC has an onboard relay so I should use the spare relay on the module to represent that”. Let’s just write that one off as a brain fart :wink:

Glad you guys like it though and I look forward to showing off the final version!

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Hey I hope the project is going well. Some information about the XAC v2 was recently posted in a different topic that I was unaware of, and I think is important to know if you do not already.

Since I’m unsure of the XAC v2’s default position for the S3 jumper, you will need to either:

1.Remove the S3 jumper completely, preventing the chance of the XAC v2 shorting the power supply to ground.

Or

2.Simplify the wiring diagram by using the S3 jumper properly, which effectively replaces the wire connected to the XAC v2’s COM terminal.

Hey, thanks for the info! I’ve installed it all in the car and will post photos soon. I did work this out very quickly when I connected 12v to it and a diode started smoking. Thankfully nothing else broke and I removed the S3 jumper.

It’s being a bit temperamental though, only unlocking after holding the chip to the sensor for an extended period of time, but I’m thinking changing the delay will fix that. It’s almost like the 4 channel relay board is going to sleep or something.

Anyway, I’ll post everything soon once I’m not so flat out at work.

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It should be connecting the COM pin to ground out of the box. But always check first, the board has markings on the back that should make this relatively easy.

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