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Constant Negative when Power Lost


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geepherder 
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Posted: April 29, 2019 at 5:17 PM / IP Logged Link to Post Post Reply Quote geepherder
The easiest thing would be to install one of those "Smart Glow" fuses with an LED that lights when the fuse is blown. If your fuse is in another location, you could possibly cut and isolate the original feed and run your own from a source closer to the brake light switch (ignition harness, etc.). Then you could install a "Smart Glow" fuse in view of the brake light switch.
My ex once told me I have a perfect face for radio.
ewingr 
Copper - Posts: 53
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Joined: February 21, 2007
Location: United States
Posted: April 29, 2019 at 7:42 PM / IP Logged Link to Post Post Reply Quote ewingr
First an update: I did some piddling around, and it is working. I am concerned that I have a poor connection in the setup somewhere, making it intermittent. I'll have to do more testing. But I know that it works if hooked up like discussed.
I actually am using smart glow fuses throughout the car. But none are in view while starting and driving the car...and I don't think it too practical to check the fuse all the time...even if only a bend down to do so.
Maybe hooking up the brake warning light is not such a good idea. From what I understand from your posts, there is no way to accomplish this w/o a constantly power draining relay.
Thanks
Roger
geepherder 
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Posted: April 29, 2019 at 8:40 PM / IP Logged Link to Post Post Reply Quote geepherder
There's a way to do this without near as much power consumption as a relay. You can use an op-amp to do what you want. Options are endless, so it's probably best to read up on them first then see what's available near you locally (or online if you prefer).
I'm curious why you need this circuit. Have you been blowing a lot of fuses?
My ex once told me I have a perfect face for radio.
ewingr 
Copper - Posts: 53
Copper spacespace
Joined: February 21, 2007
Location: United States
Posted: April 29, 2019 at 9:01 PM / IP Logged Link to Post Post Reply Quote ewingr
I'll check it out.
Actually, yes, I was blowing fuses. I couldn't find any shorts. Altlhough I did find what looked like a hot spot (brown 'burn' in the casing) on a wire from the switch. But when I cut that out, and pulled the casing...the wire itself looked fine. Not sure if that was the problem.
But while inspecting, I realized that with all the 'features' I added to the car, the brake light switch is heavily used. I'm not sure this is all:
  • Brake Lights, (of course)
  • Cruise Control
  • Actually, a second wire for the cruise control...don't know why
  • VCC signal -> LS Engine Harness
  • Push Button Start
  • Remote Keyless Entry System (Not sure why on this one either)
So, I have introduced a relay to send the brake pushed signal to all those things. Of course, at this point, I'm not sure I have 'fixed' anything. So...I want to have a warning light come on if the power to the brake pedal switch goes out. And I have a brake indicator in my Dakota Digital gauges that was not being used...requires negative to engage. It was originally intended for a switch from emergency brake, which I have no switch there. So, it seemed like a good way to know if my fuse blows.
Thanks
Roger
ewingr 
Copper - Posts: 53
Copper spacespace
Joined: February 21, 2007
Location: United States
Posted: April 29, 2019 at 9:07 PM / IP Logged Link to Post Post Reply Quote ewingr
Op Amp I presume is what you are talking about.
Looks like I need to go to school for a few weeks to get a grasp on that Constant Negative when Power Lost - Page 2 - Last Post -- posted image.
I'll do some more research though...
Thanks
Roger
geepherder 
Platinum - Posts: 3,585
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Posted: April 30, 2019 at 3:11 AM / IP Logged Link to Post Post Reply Quote geepherder
This site is pretty good at explaining stuff like that at a level that's easy to understand:
http://www.talkingelectronics.com/te_interactive_index.html
http://www.talkingelectronics.com/projects/OP-AMP/OP-AMP-1.html
My ex once told me I have a perfect face for radio.
ewingr 
Copper - Posts: 53
Copper spacespace
Joined: February 21, 2007
Location: United States
Posted: April 30, 2019 at 7:44 AM / IP Logged Link to Post Post Reply Quote ewingr
Thanks!
Thanks
Roger
lurch228 
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Posted: May 06, 2019 at 1:44 AM / IP Logged Link to Post Post Reply Quote lurch228
Use a diode on the 12v feed wire after the fuse band towards the feed and connect the non banded side to your negative input for the dash brake light input. As long as you have power ie fuse not blown the diode will be positive biased and negative brake input won't have a path, when fuse blows and you press the brake you will have a ground source through the lights triggering the dash brake light. In theory this should work with out any issue as resistance through the switch wires and bulbs shouldn't allow the input to go to ground. . but if dash brake light lights when brake is pressed and fuse is not blown,then you will need to add a resistor between the diode and negative trigger value will be trial and error until you find what works.
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