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Trigger 12v relay when the circuit opens

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throttleman

New Member
I am trying to trigger +12v going to a multi-view backup camera to signal it to change views when I press a toggle switch. The problem I face is that the toggle switch is NC and the 12v drops OFF when the button is pressed. I remember using 2 relays in the past to get around this situation but I forget how to make this work and how to wire it. I would like to use as small of a setup as possible and the small backup camera is rated at 12v <70mA so I would like to use as small of a setup as possible. Any help would be greatly appreciated.
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
Can you confirm the switch is a momentary action type?
Does the camera need just a brief positive-going 12V pulse to make it switch views, or does the 12V have to persist until you press the switch again?
What provides the 12V?
 

spec

Well-Known Member
Most Helpful Member
ISSUE 3 of 2016_06_09

Hy throttleman,

Here is a circuit that will do your job.

The output will normally be 0V. When switch 2 is pressed the output to the camera will be 12V and when the switch is released the output will be 0V again.

(the * after the component names have no significance and should be ignored)


2016_06_09_Iss2_ETO_CAMERA_PULSE_CCT_Ver1.png


ERRATA
 
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throttleman

New Member
Alec,

The 12v source is an ignition switched power source. I am using a cigarette lighter power source. The trigger only needs to be a momentary 12v to switch to the next view. Thanks for your help!
 

alec_t

Well-Known Member
Most Helpful Member
Then Spec's circuit should do what you want.
 

throttleman

New Member
Anyone on the forum want to build me one? I am planning on having a crack at it am thinking I should use a 1"x 1" PC board and try to seal it up?
 

spec

Well-Known Member
Most Helpful Member
Anyone on the forum want to build me one? I am planning on having a crack at it am thinking I should use a 1"x 1" PC board and try to seal it up?
Hy throttleman,

As you don't seem to have any electronics experience, I would suggest that you simply get a normally open switch to pulse the camera. That way you would need the minimum of wiring.

spec
 

throttleman

New Member
I currently have the camera operated using a normally open switch. There is another switch that is in a much more convenient location that is normally closed. I am enjoying learning about how to make this circuit do what I want it to. Now I have to figure out how to build it safely so it can operate many years in my vehicle.
 

spec

Well-Known Member
Most Helpful Member
I currently have the camera operated using a normally open switch. There is another switch that is in a much more convenient location that is normally closed. I am enjoying learning about how to make this circuit do what I want it to. Now I have to figure out how to build it safely so it can operate many years in my vehicle.
OK we can help with the construction of the circuit which is quite simple. But first can you answer these questions:

(1) Is the normally closed switch that you wish to use completely free. By that I mean are both terminals not connected to anything.
(2) Do you have a soldering iron, solder, small pliers and small wire cutters.

spec
 

spec

Well-Known Member
Most Helpful Member
That is good; we are all set to go. I will get back with a practical layout.

spec
 

spec

Well-Known Member
Most Helpful Member
Issue 2 of 2016_06_10

Here is a layout for the camera pulse schematic shown in post #3

Please check both the schematic and the layout for errors (there normally are some :))

NOTES
(1) The unit is built on to the inside of the lid of a small plastic box using a twin row tag strip (cut down) screwed to the lid.
(2) The components and wires are soldered to the tag strip as shown.
(3) The input and output to the unit is via three two core cables. Theses should enter the lid via three holes, preferably with grommets in them.
(4) It would be advisable to include a 1A fuse in the 12V supply line.
(5) The transistor can be screwed to the lid for security providing that the lid is an insulating material and a nylon screw is used (the metal tab of the transistor is connected to the collector).

PARTS LIST
(1) Tag strip: http://www.ebay.co.uk/itm/Epoxy-Terminal-Strips-Single-Double-Row-5-20-Solder-Tags-Various-Sizes-UK/121946366282?_trksid=p2047675.c100010.m2109&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=20140221143405&meid=82759e78dfbc43b2975d502f7c29f429&pid=100010&rk=4&rkt=13&sd=131835526552
(2) Resistor: 1K Ohm, 3 off:
(3) Power transistor: TIP42x (where x= any letter) http://www.onsemi.com/pub_link/Collateral/TIP41A-D.PDF
(4) Connecting cable: Mains cable 2 core, 3A or higher
(5) Interconnecting wire: Strip sheath off mains cable and use the core wires
(6) Case: to be defined
(7) Grommets: 3 off


2016_06_10_Iss2_ETO_CAMERA_PULSE_Ver1_LOUT.png
ERRATA
(1) Title lower should read 'PULSE' not 'TRIGGER'

 
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