Automotive current detector switch - Page 1
Forum section: electronics / beginners

author: tblough
author_rank: Newbie
timestamp: May 02, 2020, 12:48:14 am
content: Hi all - first post with orders from She Who Must Be Obeyed. In brief, the wife's horse trailer and the truck both have backup cameras. I need somehow to detect when the trailer camera is connected and use that to turn on a relay so I can switch a 12V signal to the display that tells it to display a different camera.The camera monitor has three input wires - connecting any of the three causes the monitor to display the input from that camera. I.e. connecting wire #1 to 12V causes camera #1 to display. Camera #1 is the normal backup camera on the tailgate of the truck. I normally have input #1 connected to the reverse lights - shift into reverse and camera #1 is displayed. The problem is when the trailer is connected I would like the monitor to select camera #3 on the back of the trailer.I was thinking that if I monitored the 12V power to the #3 trailer camera, I could sense when it was connected and use a relay to switch the backup light wire from sense input #1 to sense input #3. The camera draws 50ma in daylight and 100ma+ at night when connected (it has IR LEDs which automatically come on in low light). I found this devicehttps://www.ebay.com/itm/DC-Current-Detection-Module-Current-Sensing-Detecting-Delay-Relay-Control-H7Q0/143557214183that sounded perfect, but (no surprise) doesn't really work.I can make simple PC boards and create some simple digital circuits, but I really have no idea how to proceed with this. Anyone have some spare time and willing to help a newbee?

author: David Hess
author_rank: Super Contributor
timestamp: May 02, 2020, 07:55:26 am
content: An easier way is to use a connector between the trailer and vehicle with a couple of extra pins and jumper the trailer side of the connector as a switch.

author: johnkenyon
author_rank: Regular Contributor
timestamp: May 03, 2020, 03:51:23 pm
content: Quote from: tblough on May 02, 2020, 12:48:14 amI take one step back - you are actually trying to confirm whether a trailer is present or not.One way of doing this is to detect whether there is one or more bulbs connected to the trailer socket on the truck.With a filament bulb this is easier than you think - if you feed the bulb with 12v at low current, you can detect whether the bulb is there, without actually lighting it up - a cold bulb has a low resistance.So the trick is to1) Select a suitable bulb (or bulbs) to monitor on the trailer.2) Insert a diode between the truck and the trailer connector, to stop the resistor backfeeding +12v into whatever electronics supplies the trailer bulbs.3) Connect 12V via a resistor (in the order of kiloohms) to the trailer connector4) Run a wire from the trailer connector to your "box of tricks" - this is your "missing bulb" wire.If the normal feed to the bulb is turned off and the "missing bulb" wire is at 12v, then either the trailer isn't hooked up, or all the bulbs have blown.You will almost certainly have to combine the "missing bulb" signal with other information, and latch it for later use to be 100% certain that the trailer is attached.My box of tricks would probably detect the ignition being turned on, check the "missing bulb" status, and then use it to set or reset a flipflop.The output of the flipflop is then combined with a feed from the reverse lights to drive the relay as per your original plan.I wouldn't make "the trailer/no trailer" decision whenever reverse is engaged - as a simplified circuit could be easily confused into thinking the trailer wasn't connected if the bulb is turned on (e.g. if you engage reverse with the tail lights on at night)Another alternative - does the truck's electronics suppress the backing up (uk:reversing) lights when a trailer is connected?In Europe, all cars have at least one rear foglight. Many towing electronic kits tell the car's ECU not to turn on the rear fog lights when a trailer is connected.Maybe your truck does a similar thing - do a walkaround check!If it does, then it becomes a question of logic - IF transmission is in reverse AND setting back lights are off THEN trailer is attached.In this case a feed from the reverse switch on the transmission, and the setting back lights are all your box of tricks need...John