Constant Negative when Power Lost

I need to send a constant negative signal when power lost, no signal when power on.

Also need to just send positive when / during receive positive.

I suspect I need two relays to do this. The relay diagrams I find are for specific solutions, for which I'm not familiar enough wiht relays to pull out my needs.

Help appreciated.

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Thanks
Roger

Ground terminal 85
power to terminal 86

Terminal 30 is output, power when there is 12 volts across 85 and 86. Ground when there is not 12 volts across 85 and 86
Terminal 87A is to be grounded.
Terminal 87 gets power.

REally appreciate your input.

I need to check with the device mfgr... but don't think I can send 12v to the device. So, for the power source that would kick off the negative, when that is on, I do NOT want to send 12V through the output. I only want to send a ground if I lose power, nothing if I have power.

It sounds like the above will send power out the output. Am I misunderstanding?

I don't know, maybe what I want to do can't be done.

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Thanks
Roger

Remove power from 87

I misread your first line.

What do you mean by you need positive during receive signal?

i am an idiot wrote:

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Remove power from 87

I misread your first line.

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Thanks so much. I wondered if it would be that simple.

i am an idiot wrote:

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What do you mean by you need positive during receive signal?

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Sorry, that really was poorly worded.

Let me explicitly explain what I'm trying to do:

I have several things that rely on when I press the brake. So, I'm wanting the wire on the brake switch that goes hot when I press the brake to feed a relay, which then sends power to a power distribution block for the various things that need that signal. Does that make sense?

FYI...the other one you already answered is to feed the negative to a brake indicator on the dash if the power feed to the brake switch should blow a fuse. That will alert me to the blown fuse.

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Thanks
Roger

i am an idiot wrote:

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Ground terminal 85
power to terminal 86

Terminal 30 is output, power when there is 12 volts across 85 and 86. Ground when there is not 12 volts across 85 and 86
Terminal 87A is to be grounded.
Terminal 87 blank.

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I just want to confirm the above. Two questions.

1. I presume that the power on 86 should be the power from my brake switch, which if a fuse blows, then I get the negative signal on 30

2. If I did put power on 87, then 30 would get power if 86 is powered. Whereas if I remove power from 87, then when there is power on 86, nothing at all will feed through 30.

I apologize for being a pest.

Thanks so much. I really do appreciate your input.

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Thanks
Roger

I want to have a relay that when a particular circuit loses power that it sends a negative to the output wire. I was told to wire it like this:

85 and 87a -> Ground

86 -> Power (NOTE: I am using this terminal as the one that I want to monitor. So if it loses power, the end result is that the relay has no power on any ports)

30 -> Output (I have wired this to the negative side of an indicator. So when I lose power, which is the power on 86, then a gnd gets sent here)

I find it to not be working.

Have I misunderstood the instructions I received?

Any help appreciated very much.

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Thanks
Roger

First verify your relay is a SPDT configuration. If it isn't, it may have two terminals labelled 87 rather than one 87 and one 87a.

Also ensure 87 and 87a are not reversed.

The connections listed will work, but the relay will remain energized unless power is lost (blown fuse), which places a constant drain on the battery.

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My ex once told me I have a perfect face for radio.

I will check your suggestions.

And your comment about constant drain on battery is another topic I was concerned about. The relay is warm...which I figured it shouldn't be. By being a constant drain on the battery, that would make it warm.

Do you have a better solution I could/should use to avoid that? Two relays?


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Thanks
Roger

geepherder wrote:

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First verify your relay is a SPDT configuration. If it isn't, it may have two terminals labelled 87 rather than one 87 and one 87a. Verified...have 87 and 87a

Also ensure 87 and 87a are not reversed. Verified

The connections listed will work, but the relay will remain energized unless power is lost (blown fuse), which places a constant drain on the battery.

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Also, as you pointed out...it is in fact using juice...the relay is warm. I pulled it out for now.

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Thanks
Roger

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.

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My ex once told me I have a perfect face for radio.

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.

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Thanks
Roger

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?

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My ex once told me I have a perfect face for radio.

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.



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Thanks
Roger

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 :oops:

I'll do some more research though...

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Thanks
Roger

This site is pretty good at explaining stuff like that at a level that's easy to understand:

https://www.talkingelectronics.com/te_interactive_index.html

https://www.talkingelectronics.com/projects/OP-AMP/OP-AMP-1.html



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My ex once told me I have a perfect face for radio.

Thanks!

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Thanks
Roger

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.