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Can you build a key charger?
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Can you build a key charger?
The EWS function of the key is effectively a passive RFID system. It operates even with a flat battery in the key because it is powered by the steering column EWS loop. When an electromagnetic field of the right frequency is presented to the key it modulates this carrier signal so that the EWS system can read the data back from the key. The key doesn't actually transmit anything but rather "hacks" the signal being transmitted by the EWS loop antenna.
This same carrier signal is also used to provide charging for the battery which powers the remote central locking.
So in order to charge the key using the EWS loop from a car (and without an EWS module connected to it) you would need some kind of oscillator to generate a carrier frequency similar to that created by the EWS module. It's probably quicker and easier to just try some cheap electric toothbrush chargers!
This same carrier signal is also used to provide charging for the battery which powers the remote central locking.
So in order to charge the key using the EWS loop from a car (and without an EWS module connected to it) you would need some kind of oscillator to generate a carrier frequency similar to that created by the EWS module. It's probably quicker and easier to just try some cheap electric toothbrush chargers!
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Previous: E85 3.0si Silbergrau, 03 E85 2.5i Maldives Blue.
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- ph001
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Can you build a key charger?
I'm going to have a look at this with an rf sniffer probe and portable scope. Will report back within a day or two
- RedUn
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Can you build a key charger?
One month later... Did you go back to the future or something?
IMOLA Z4///MR
- ph001
- Lifer
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Can you build a key charger?
Yes, unfortunately stuck in 1965 at the mo, but as soon as I get back I'm onto this.
Actually our sniffer probe is duff so we are waiting for a new one to arrive
Actually our sniffer probe is duff so we are waiting for a new one to arrive
- Ducklakeview
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Can you build a key charger?
This is the correct replacement, same as the original, a rechargeable Panasonic.
Mike
- ph001
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Can you build a key charger?
3V lithium as suspected. Thanks Mike.
- Ducklakeview
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- ph001
- Lifer
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Can you build a key charger?
OK, new sniffer probe arrived so took a look at the waveform the car emits to charge the key. Strangely it doesn't produce this continuously, it is present for around 2 seconds and then disappears for about 4 seconds - so a 25% duty cycle.
You can see the frequency is around 125Khz and is fairly sinusoidal apart from the small 'tit' at the peaks.
You can see the frequency is around 125Khz and is fairly sinusoidal apart from the small 'tit' at the peaks.
Last edited by ph001 on Fri Sep 29, 2017 3:14 pm, edited 1 time in total.
- GuidoK
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Can you build a key charger?
This is in the same frequency range as for example the Qi wireless charging (the low amp setting, 100-200khz).
Although I dont know if it'll be compatible. I dont know if Qi has some handshake or proximity detection, but I suspect it does.
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- Ducklakeview
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Can you build a key charger?
Yes it has to do the handshake to identify the device before the charging coils are energised.
Mike
- GuidoK
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Can you build a key charger?
Maybe you can put the key next to the phone while it charges and the key can steal some of that induction fields away
Z4 3.0i | ESS TS2+ | Quaife ATB LSD | Brembo/BMW performance BBK front/rear | Schrick FI cams | Schmiedmann headers+cats | fully polybushed | Vibra-technics engine mounts | H&R anti rollbars | KW V3 coilovers | Sachs Race Engineering clutch
- ph001
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Can you build a key charger?
What I need now is for some kind soul to send me the innards of a key (pickup coil and battery are the only real important bits). I'll then wind a small air-cored choke and run it with a 125KHz oscillator and monitor the charge into the 3V lithium cell.