RFID Vehicle unlock and start for CHEAP !?! ( R&D Phase )

Hi all,
So here’s where I am at;
I have just received my Ultron Upgrade Kit ( Great value ).
My project is to install an RFID reader on my car and to use this to unlock my doors ( Central locking, No alarm) using a Flip-Flop latching Relay attached to the same circuit, this same equipment will then allow me to install a push to start button.
I have ordered 2 items from Ali-express for this project ( It will take 4-6 weeks to arrive ) in the interim I have sketched up a pictorial wiring diagram, which in THEORY I think will work… but here’s where I ask this generous community for help, (hopefully will assist a whole lot more people than just myself).
Where I am not certain is, where / how to include the Flip-Flop Relay into the circuit. I am HOPING that when I scan the RFID tag, this will provide 12V to the relay to trigger the central locking mechanism to alternate lock / unlock.

As I say, I am waiting for these items to arrive so can’t do any testing as yet.

My master plan is to do this to my whole families vehicles and therefore a shared key amongst us all.
If successful, I may try and introduce the concept to my work.


RFID Push to start

Thanks in advance for your time and any assistance


You will need a separate RFID reader for your door locks, like the xEM access controller.

They easiest way to do this is to get a key fob and wire an xEM access controller and a relay to mimic pressing the unlock button.

Here’s an idea, though it’s not ideal. With proper design, current draw could be minimized. The issue is your latching relay. It’s electronically latched, not mechanically, so when in one state, the relay is energized …meaning you could end up with a dead battery. The way it is drawn, the relay will draw current when in the unlocked state in the assumption that if it’s unlocked, most likely the car will be running and the battery won’t be draining.

This also assumes that the door lock wires you find need a positive pulse to actuate. They may need a negative pulse and in that case, change the 12V on the com of the regualr relay to ground. This could also be used to control a two button key fob.

Have you verified the type of FOBs the pushbutton start uses? There’s no guaranty that the antenna will be strong enough to pick up an implant.


@turbo2ltr with a handle like that, and how active you are on the forum, I thought you would be “the car guy” to help out on this.

Firstly, apologies if my question should have been in “Support” rather than the “Projects” area. I suppose it is a little bit of both???:grimacing:

Thanks Mike, you have confirmed a few things ( assumptions ) for me:-

That this is a great community willing to help out others and share knowledge and experience.

I was also concerned about the latching relay drawing current, I have never used one, merely just aware of their existence. I was hoping that at a resting state in a Normally Open position it was mechanically held open. but I suppose this is more of a standard relay function. I guess it was just some wilful ignorance, I was actually trying to convince myself and ignore the fact that is how it works.

This also confirmed that my initial sketch I did, may have been closer to the answer, whereby I did use a standard automotive relay, with the thought ( and again hope ) that upon scanning the FOB it would energize the circuit/relay for as long as the fob was in range; so scan with one hand and open door with the other, stop scanning and the door auto re-locks. After scanning, I believe the Push to start stays active for 15 - 30 secs which should be plenty of time to get in and the car started.

I was also assuming/hoping that I could get this to work as a 2 in 1 system using only One RFID system. (Access AND Start (activate start/stop))

I have not yet been able to find out the FOBs operating frequency, but I will do this once it arrives.
( i might be in the market for another implant )

The claim is a read distance of 20-50mm, and I will do some testing through the car windows when the equipment arrives. The kit I purchased is the newer version with an 88mm reader rather than the 67mm so I’m assuming this will increase the gain and therefore my chances of a good read.

Re: positive or Negative switching I will find this out when I actually get the door panel off…

Thanks very much for the diagram, this will come in handy when the bench testing begins. and your idea will be the first setup I try.

Speaking of which, you have also planted a seed:-
I will do some measurements on current draw of the latching relay and I may isolate the latching relay system to run on its own battery with a Low Voltage Cut Out and connected to a VSR for charging. I’ll need to put a “hide a key” on the car as a backup…I’ll put some more thought into this.

Thanks again, and now the thinking and waiting begins.


Couple things…

The size of the antenna has little bearing on how well it will read an implant… Might help with their keyfob tags, but the implant has such a small antenna that a smaller antenna is actually desirable. But the differences between the old and new loops is negligible as they are both way too big to be optimized for small implants. Not saying it won’t read, but the size difference doesn’t really matter in this particular case. The antenna on the xEM controller is much better optimized for the implants.

Now getting a read on an implant through glass is certainly going to be a challenge. I’ll be eagerly awaiting your testing results.

The issue I see with the “two in one” idea is as far as I can see, all the RFID does on the module is let the button work… When you scan the tag, this enables the push button for 30 seconds or whatever. But when you scan, there is no output that you scanned the tag, it just internally “disarms” it so you can press the button to start the car. The outputs are only when you press the button. So I’m not seeing a way to get that same module to control the door locks.

Maybe you could find an internal signal that goes high or low when the tag is validated…

I did find some mechanical latching relays on amazon. (search for bistable latching relay) but they had one input for each state, it wasn’t a flip flop. You could design a flip flop circuit with a flip flop logic IC.


Fantastic, thanks again for your input, time and knowledge.

Indeed the plan was to dissect the box and find a high / low signal change when the tag / implant is scanned.

I was also hoping the higher gain antenna was going to help, so cheers for that clarification; I will use the xLED Field Tester to locate optimal position and orientation. (I may need to become a contortionist).
Depending on the compatibility of the frequency/ system, I might have to buy another xEM controller, and I may be able to swap out and solder in the xEM scanner.

I will check out the amazon bistable latching relay, thanks.

I will indeed keep yourself and the community up-to-date with any progress/ success’/ failures when the gear arrives…in 4-6 weeks

Once again, thanks very much.:vulcan_salute:


Quick update,
All good news with more to come hopefully.
My bench test actually became a direct install ( just to make testing more difficult for myself :roll_eyes: )
Push to start is installed

  • The fobs that the el cheapo Start / Stop system work perfectly with the xEM Access controller and vise-versa, so that is great news for compatibility.
  • The reader antenna ring it comes with, reads the fobs it came with through windscreen at approximately 60mm.
    I haven’t got my implants, implanted yet because my primary use will be my work access and I’m still trying to get that sorted so I’m yet to commit to what will go where. However, with a 60mm range, I am hopeful.
  • Once I have my implant in, if it doesn’t read through the windscreen I have 2 immediate alternatives to try.
    Swap out the ring for the xEM reader antenna and/or install the antenna into the wing mirror. or the @TomHarkness antenna when that is available.
  • The following needs to be confirmed for more accuracy. There is a voltage difference from ~2.3V (Idle) - ~4.4V ( fob within approx. 100mm ) Idle current appears to be around 10mA

My plan is still to find a way to use the Start/Stop system to unlock the car also; therefore more work to be done around the voltage/current to find a solution.
I never actually lock my car, so this is more of a “doing it because I can”

A bit more work to do when I get some spare time.

Progress updates to come


Short update,
My Flip Flop Latch Relay Module that I ordered arrived and I have done some bench testing.
It does what it say on the box
It is good for the proof of concept but the circuitry doesn’t fit this application.
The one I have is a 5V version; during the testing I did when it was “triggered on” and I then dropped the voltage it would cut out at 3.5 Volts, When the voltage was increased it would switch back on ( into its previous state ) at 4.5 Volts.
When triggered it would switch the 12V relay ( In theory the door locks )
I believe / hope if I get a 3V version this would be triggered by the voltage variance ~2.3V - ~4.4V. my testing above was done with a Momentary switch ( Normally Closed ) so to confirm I would need to start playing with resistors.
HOWEVER (Now the hurdles)
The trigger on this particular module is on the ground ⏚ , I am pretty sure the antenna coil is +ve, so I need to confirm this.
On the bench I happened to have bought a spare module so thought “why not?” and tried to reverse the polarity. pretty much knowing what the result would be.
I found out 2 things:-

  • there is no polarity protection
  • how to turn a voltage regulator into smoke (if anybody wants to learn how to do this, I can write them a tutorial )

The Module also draws approximately 10mA so combined with the Start / Stop system we have 20mA, Quick calc would be approximately 1Amp per 2 days.

I am currently looking for further alternatives and will order a few 3V modules to test in the interim.
Bench test Photo below if you are interested.
The module is “as is” 12V Relay on Left and 5V trigger on the Right
*The LCD display represents the 12V Load ( what would be the Central locking )
*5V terminals represent the antenna coil ( Would be connected to the antenna coil )
*The Momentary switch represents the FOB/ xEM when in close proximity to the coil causing the voltage fluctuation which Turns on/off ( Locks / Unlocks car) as the button is pushed ( Fob / xEM scanned )

Obviously, some more testing to be done, and I happen to have a spare central locking kit in my draw ( Who doesn’t??? :nerd_face: ) So I’ll connect that up to this system as I progress.
Not quite as quick an update as I thought.
If anybody is still reading, I am curious to hear what anybody in the know thinks about sharing the Ground ⏚ with the 12V and 3V circuit!?!

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Hey @Pilgrimsmaster

I’d be careful directly connecting the 12V and 3V grounds together if there are actuators or mechanical switches on the 12V side. These type of devices create a lot of electrical noise in the circuit, and the 3V system is less capable of handling the fluctuations.

If you don’t have access to two separate power sources (which is usually the case) you can just try to situate the ground wires for each circuit so that they’re connected to the main ground at different points. This can prevent switching noise and back EMF produced in the higher voltage circuit from interfering with the digital side. You could also consider using thinner gauge wires to connect the digital side to ground, since thinner wires present a higher impedance pathway for high frequency voltage spikes.

If you’re getting fancy, you could put an inductor between the digital side and ground, to completely absorb voltage spikes. This can present a bunch of other potential issues if there are high speed data signals, because the impedance of inductors changes with frequency. You might virtually remove ground for certain signals if you don’t select an appropriate sized inductor. Here’s a good resource on grounding if you’re interested:


Really appreciate the input, and an informative link. Some MORE testing to be done
What about the addition of a ferrite bead for noise also / instead of inductor?
At least I gave my topic Title of “Cheap” not “Easy”

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Yup, ferrite bead would work. That’s just a small (single turn) inductor. The resistance of an inductor is frequency dependant. They call it “inductive reactance”.


You might want to test it. If the 3V side has higher frequency signals for communication, it might present an issue. Starting with a tiny inductor (L) like the ferrite bead will mitigate most potential issues.

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I’d really like to help you but either you are really confused, or I’m really confused.

Swap out the ring for the xEM reader antenna and/or install the antenna into the wing mirror. or the @TomHarkness antenna when that is available.

You can’t just swap antenna between systems. Antennas are designed and then circuitry is selected and tuned for that specific antenna design. If you change the antenna, you also have to redesign the associated circuit.

There is a voltage difference from ~2.3V (Idle) - ~4.4V ( fob within approx. 100mm )

Where are you reading this? You can’t measure the voltage on the antenna itself. If you are, that’s the 125khz 'carrier" you are reading and will be there any time any tag is presented.

I then dropped the voltage it would cut out at 3.5 Volts,…

I’m failing to see the point of your low dropout test. The relay is meant to be powered by a constant power source, not a changing one.

You don’t want a 3.3v relay. You want a 12v relay.

The trigger on this particular module is on the ground ⏚ , I am pretty sure the antenna coil is +ve, so I need to confirm this.

Again, you can’t use the actual antenna wire to control anything.

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More Great learnings, thank you very much.

sounds like “I’m out of my depth”
maybe back to the drawing board.

The low voltage cutout, in my mind at least; was to see the operating range of the 5V Relay to get and idea of what to expect from the 3V relay.
knowing my voltages I tested earlier (now known to be the 125kHz carrier) were either side of 3V. and the module I had would trigger when a voltage change occurred. therefore I thought that if I had a 3V module it would still operate at around my tested 2.3 idle and when a tag was presented and the voltage increased to 4.5V would it would trigger the 12V relay. it says 6V in the picture but it is actually 5V.

I didn’t actually consider the fact that any 125k HZ tags that were presented would unlock my car, not that I actually lock my car, but I understand most people wouldn’t like that security vulnerability and like you said in an earlier post, I could maybe look to pick up a voltage on the board operated post authentication.

Luckily I have an electronics engineer mate who is currently overseas until October. So I might need to wait until then for an assist…
Thanks for the clarification and feedback

Maybe I just need to go and buy a Tesla, just so I can use TeslaFlex “Key Card” Implant :lying_face:

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I’m in the process of doing this to my car, but it’s a bit more complex in my case as it has an alarm/immobiliser. I won’t talk much about the immobiliser here, as that could be a post on its own and you’ll get better advice on car forums (they tend to do it for Remote Start kits).

I’ve gone with an xEM access controller, that uses a relay to press the ‘unlock’ button on a modified spare remote that will be embedded within the vehicle. (Easy to operate, hard to find without pulling the dash apart)

This disables my alarm and bypasses all 3 stages of my immobiliser (car immobiliser functions normally when the alarm is on, so car can’t be easily hot wired without pulling car apart fully).

I used a similar push button start as yours, but i used the version without RFID reader and used the engine lockout wire to prevent it from starting my car while the alarm is on, so a broken window would mean the button doesnt work and the immobiliser means even if it did they wouldn’t get very far. I decided RFID on ignition was unnecessary since i already had the alarm and have to scan to get in the vehicle/disable immobiliser anyway.

If anyone is interested, I can take a video of the system in operation once the install is fully completed.


That’s the tricky thing, there are so many configurations, alarms, immobilisers, push to start, rfid proximity .
The Remote fob with the xEM controller is a great solution; does your remote have a single Lock / unlock button. I hope so, It makes things so much easier.
Mine unfortunately does not, but I am making progress on a workaround to the one above and building in some redundancy, I will also update this thread when I am done.

Yep, i’d be keen

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Mine is separate Lock and Unlock buttons, so my solution for locking is in the alarm. It has a ‘Passive Locking’ setting I have turned on, so after I close all the car doors, it arms in 30 seconds - normally this is a bad thing since it allows you to lock your keys in your car easily, but I’m not so concerned by that with an implant.

I have also wired up the lock button to a wire in case i want to put in a soft-latching circuit down the line, and I’ve grabbed the boot release button too (down the line I’ll install another xEM controller in my tailgate).

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Good thinking!!!


What push button are you using? What you’re talking about is similar to what i would like to do, and sounds very secure. I dont know anything about the electronics side of cars. So I’m unsure what is the best thing to get in my case.

I got a cheap system off ebay, a quick search and you’ll see the ones i mean, they are very commonplace over there.

Unfortunately every car and every setup is a bit different, so you’ll have to learn a bit about cars or find a friend to help you - you can also find information on car forums if you look for Remote Start, including how to bypass your cars factory immobiliser and defeat the steering wheel lock.

Bit of a learning curve I’m afraid, but the results are worth it!