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Thread: Reverse Camera

  1. #11
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    It really is dead simple - Google PNP transistor as a switch, I used a bd136, available from many places such as RS, Mouser, Farnell.

    The simple circuit I used had a small quirk in that using the direction indicator caused the transistor to very briefly try and switch to the camera. Ideally this needs to be developed a little further.

    I'll knock up a diagram of what I used when I am back at my pc.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

  2. #12
    Senior Member C16RKC's Avatar
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    I used a relay as suggested by Eddie in my X358, it was installed for around a year with no issues - worked perfectly every time. It's a method a lot of people have used for their X-types, S-types and X350/X358's on here for quite some time.

    (Until I crashed the car)

    Could 'back EMF' have caused me to steer into a tree? I am very eager to find a way for it to not have been my fault!
    Regards
    Chris Clark
    2001 XJ Daimler Super V8 (Car for 'best')
    2004 BMW 730d (Daily workhorse)
    2007 Land Rover Range Rover Sport 2.7TDi (The Wife's baby hauler)

    Previous cars: 1992 XJ40 Sovereign, 1994 XJ40 Sovereign, 2000 XJ Sovereign, 2000 Daimler V8, 2003 X350 Super V8, 1994 Renault Clio(!), 1999 Mercedes S430L, 2008 XJ X358 Sovereign Diesel, 2003 Mercedes S320CDi, 2007 XJ X358 Sovereign Diesel LWB

    www.autosanity.com

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  4. #13
    Senior Member FastEddie's Avatar
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    Quote Originally Posted by graham-r View Post
    It really is dead simple - Google PNP transistor as a switch, I used a bd136, available from many places such as RS, Mouser, Farnell.

    The simple circuit I used had a small quirk in that using the direction indicator caused the transistor to very briefly try and switch to the camera. Ideally this needs to be developed a little further.

    I'll knock up a diagram of what I used when I am back at my pc.
    No problem with my understanding of electronic components , I have designed 50/60 and 400Hz GPU's for the aviation industry.

    As I stated earlier , I was keeping it simple for those with less knowledge than myself and also doable. No hard feelings.
    2009 X358 XJ LWB 2.7 Tdvi , Liquid Silver , 100k+ and going strong.
    Bumper reflectors drilled and fitted with 3W side marker lights.
    Fog lamps / DRL running with HID colour match of dipped beam.



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  6. #14
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    Then you will understand it really is dead simple, but it wasn't only meant for you Eddie !
    Last edited by graham-r; 12-01-19 at 17:04.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

  7. #15
    Senior Member benwillcox's Avatar
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    I find this discussion quite interesting, as I've previously read lots of posts (not just on here) about fitting a relay with the coil in parallel with the reversing lamp to invert the signal for this purpose, but had never seen anyone mention the back-EMF issue, which was always a little concerning to me.
    My electronics background has taught me that when switching an inductive load such as a relay with a semiconductor then fitting a diode across the relay coil (or a relay with integral diode) to stop any back-EMF is essential to prevent destroying the switching electronics. I suspect that the automotive switching modules have been designed to be very robust and to survive overloads, short circuits, and back-emf etc, but I would always personally use a relay with integral diode to be on the safe side.

    When I was searching for automotive relays for this purpose I found the vast majority did not have an integral diode, but eventually found one like this: https://www.ebay.co.uk/itm/Micro-Rel...item5215e04c8f which does, and it is shown on the screen print on the side of the device.

    I'm not convinced that a bulb in parallel with the coil will offer any safety, basic experiments demonstrating the side effects of back-emf demonstrate that a lamp in parallel with a relay coil will light up from the back-emf when the relay switches off, although I guess that this is dumping the majority of the reverse current so would reduce the impact on the switching electronics. However, if the bulb blows then this resistance across the coil goes open circuit and there is no protection at all.

    This is the diagram I use on my JagDroid website as recommendation on the way to wire such a relay:



    Or using a transistor like graham-r avoids any issue with back-emf anyway but could be a little trickier for a non-electronics person to deal with, due to the small physical package and requirement for a base resistor.

    And for those that have no clue what on earth we are talking about, in simple terms when you have a relay coil energised and you switch off the power to it, the magnetic field collapses and generates a very high voltage in the opposite polarity which can cause sensitive electronics to be damaged. A diode only conducts in one direction, so when fitted in reverse polarity across the coil it has no effect when the coil is powered normally, but turns into a conductor when the reverse current back-emf is generated, shorting it out so that it doesn't cause any damage.


    Ben
    2004 XJ X350 Super V8 LWB - Custom 19" alloys, 4 zone climate control, rear business trays


    JagDroid - Android upgrade to Jaguar Touchscreen for XJ X350/X358, S-Type and X-Type - forum thread HOWTO and website: http://www.jagdroid.org

  8. #16
    Senior Member FastEddie's Avatar
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    The back emf of a relay coil is only milliamps although potentially high voltage with no load ,suppression resistor or blocking diode fitted.

    If the lamps were not there , the discussion would be valid. Each lamp however would require around 2 amps to light at normal voltage.

    Do you honestly believe that a relay using 500mA to energise could then produce 2000mA of back emf ? You would be rich if this were so.
    2009 X358 XJ LWB 2.7 Tdvi , Liquid Silver , 100k+ and going strong.
    Bumper reflectors drilled and fitted with 3W side marker lights.
    Fog lamps / DRL running with HID colour match of dipped beam.



  9. #17
    Senior Member benwillcox's Avatar
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    Quote Originally Posted by FastEddie View Post
    Do you honestly believe that a relay using 500mA to energise could then produce 2000mA of back emf ? You would be rich if this were so.
    You're absolutely right of course, I was thinking of large inductors rather than small relay coils generating enough back-emf to actually light an incandescent lamp up, my mistake. As we know though, even a small relay coil can generate many thousands of volts of back-emf albeit at a low current.
    It's certainly true that I've never heard of anyone actually frying their lighting control module adding a relay like this so I'm sure you are correct that in practice the bulbs are enough of a load to reduce the back-emf. As you rightly say, it's actually 2 bulbs connected in parallel so the risk due to blown bulbs is reduced (I had previously thought it was a separate output per side).
    Having said that, I still personally would prefer to fit a relay with diode (or add an external diode) just to be absolutely safe, but that's just my preference.

    Cheers,
    Ben
    2004 XJ X350 Super V8 LWB - Custom 19" alloys, 4 zone climate control, rear business trays


    JagDroid - Android upgrade to Jaguar Touchscreen for XJ X350/X358, S-Type and X-Type - forum thread HOWTO and website: http://www.jagdroid.org

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  11. #18
    Senior Member FastEddie's Avatar
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    Quote Originally Posted by benwillcox View Post
    You're absolutely right of course, I was thinking of large inductors rather than small relay coils generating enough back-emf to actually light an incandescent lamp up, my mistake. As we know though, even a small relay coil can generate many thousands of volts of back-emf albeit at a low current.
    It's certainly true that I've never heard of anyone actually frying their lighting control module adding a relay like this so I'm sure you are correct that in practice the bulbs are enough of a load to reduce the back-emf. As you rightly say, it's actually 2 bulbs connected in parallel so the risk due to blown bulbs is reduced (I had previously thought it was a separate output per side).
    Having said that, I still personally would prefer to fit a relay with diode (or add an external diode) just to be absolutely safe, but that's just my preference.

    Cheers,
    Ben
    I have just had a tinker with a Durakool standard automotive relay , this only has a suppression resistor built in and no diode.

    In parallel with a 5w wedge bulb and removing the power , I cannot see the lamp intensity increase at all when the power is removed.

    Many outputs of the ECM's drive relays already , would be interesting to know if the outputs already share the same sinking circuitry.

    I would think the ECM's have been designed fairly robustly from the outset and may already have a degree of protection built in.
    2009 X358 XJ LWB 2.7 Tdvi , Liquid Silver , 100k+ and going strong.
    Bumper reflectors drilled and fitted with 3W side marker lights.
    Fog lamps / DRL running with HID colour match of dipped beam.



  12. #19
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    Ben

    You are right about using a transistor and resistor being a bit more fiddly, for that reason I used a medium power transistor with a mounting hole (a TO126 package iirc).

    Being beefier than a signal transistor it also should be more robust which is ideal for use on a car. Having said that my reverse camera has stopped working and I still need to investigate !

    The main problem with a transistor is that it responds very quickly and this results in the glitch caused by the voltage momentarily dropping when switching on the indicators, most relays would not respond quickly enough to show this effect.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

  13. #20
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    Click image for larger version. 

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    This is a link to the BD136 transistor datasheet

    The above is a sketch of what I fitted to my S Type to provide a reverse switching signal to trigger relays to switch between the RGB of a camera/SatNav, this works for the X350 - it is not needed for the X Type.

    Briefly the problem is that the S/X350 have a system where the lights have +12v on both bulb terminals, this results in a voltage (potential difference) of 0v and the bulb is off, they are switched on by removing one of the +12v (pin 4 on the light connector) and providing a zero/ground/negative route to complete the circuit. The latter is provided by the Lighting Control Module, it is done this way to provide bulb fail detection.

    The result is that there is no 'zero -> +12v' transition to use to switch the camera/SatNav relays, the above circuit (and the relay circuit described in other posts above) provide this. When the reverse light is out there is pin 4/+12v at the base (b) - this keeps the transistor off, put the car in reverse and the voltage at b becomes effectively zero and switches the transistor on - this conects the collector (c) to the emitter (e) and +12v appears at e (out).

    The transistor approach has two advantages - it takes very little current (microAmps) when switched on and the cost is minimal (approx 20-30p) - a relay of the type described above costs £3-8

    Some last things:
    The above was a hacked together test device, it worked pretty well for 6+ months,
    ignore the red components
    And I think it might be better to tie e/out to ground via resistor to improve electrical noise rejection, these are aspects that I am trying to find the time to sort out - maybe today.
    Last edited by graham-r; 13-01-19 at 10:52.
    S Type 4.2 SE Plus 2002 original spec: Xenon HID/SatNav/Voice/Cassette Player/CDC/TV/GSM Phone/Sun Roof/Rear Blind/CATS.
    Retro-fitted: CD Player/Bluetooth Phone/Aux Power Socket, USB & Volt Meter in Boot/Reversing Cam/Dash Cam & PSU/Dual USB sockets in Armrest/
    Wireless Phone Charger under Instrument Surround.
    Pictures of technical stuff

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