i have some electrical background but i am not familiar with relays...how do they work? i am interested in wiring a fuel pump kill switch but i want to understand exactly what a relay does first.

 

Ok a relay is basically a switch. It will allow current to pass one way while blocking off excess current from backfeeding. Also a relay can be wired to change the polarity of the wires as well. If your objective is to stop the car from being started or stolen, why dont you get a basic alarm and use the starter kill feature. Its much easier that way and you wont need a relay.

Dave

 

Uhmm...wrong, I haven't seen any diodes in a relay. A relay allows current to pass either way on its internal switch.

That's what starter kill is, the use of a relay. The alarm might have one built into it, but you are still using a realy.

Dave is right, a relay is a switch. Its purpose is to allow the use of low current to switch high current. The outputs (channels) on an alarm are low current and would not be able to control things like starter kill and door locks without the use of relays. The wires would burn up or there wouldn't be enough current to operate the deivce.

So, lets say you are installing some fog lights that need 10amps of current. The dash mounted control switch you bought can handle 3amps of current as indicated on the side of the switch. If you were to run the power wire through the switch and use the switch to turn the lights on and off, you would eventually (or instantly) burn the switch up because it is not designed to handle the amount of current the lights require.

This is where a relay comes in. Common relays handle 30 amps. It only take a few milliamps to activate the switch inside of a relay. This way you can use a small, nice looking switch to control your fogs, instead of a BIG trucker 18 wheeler switch that could handle the amp load.

In short, a relay is just a heavy duty switch that can be controlled by a light duty switch. It adds safety, and reliabilty, not to mention better looks to your installation.

Jon Black



[Modified by Jon Black, 7:08 AM 2/27/2003]

 

and you can do logic and other circuits with relays (although it's much more confusing than using transistors). making a relay latch is simple and works well in the case of the fog lights if you want a nifty lookin install with non-latching push buttons

 

thanks for the in-depth explanation, guys. lemme try to take what you've explained to me and translate into theory:

i want a fuel pump kill switch. therefore, the circuit needs to be broken when the car is off, and it needs to always remain on while the car is in motion. do i just wire the relay in on the ground side of the fuel pump circuit, and attach it to a switch? i know by searching the HT forums i can find the information as to where these circuits are located, as i've already done the appropriate searches before asking my initial question.

now if this much is correct, i'll try to extend it a bit further: does the switch i use HAVE to be a simple on/off toggle switch? a toggle presents 2 main difficulties:

(1) the need to remember to switch it OFF when exiting the vehicle in order to activate the kill
(2) the possibility of accidentally hitting the switch while driving, therefore cutting off the fuel pump

when jon speaks of "activating" the switch inside a relay, i will presume that we are talking about the smaller/low current switch not being included in the circuit itself due to the high currents. if this is the case, can a momentary switch be used? in other words, does a continuous current need be applied to keep the relay open, or can the press of a button (or a cigarette lighter, or a horn, or any of the various momentary switches already within reach in any car's interior) be used to "activate" the relay, causing the relay to remain open until the car is shut off? this is basically my goal, as the system makes itself idiot-proof.

again, thanks for all the help. we are discussing a matter that was not dealt with in any way in the HT archives (i read every single topic in this forum that came up when searching for "kill switch"), so hopefully this thread will help with future questions as well.

 

oh, and i just noticed what rjr posted...correct me if i'm wrong, but what you describe as a "relay latch" is very similar to the goal i am looking to achieve? ie a pushbutton "momentary" switch that activates the relay and stays on until i shut the car off as opposed to the problematic on/off toggle setup?

 

Personally I would go for the positive side. Hondas are negative ground and in some cases even when you cut the ground wire the device in question may still be able to complete the circuit due to its touching the frame or other metal. This most likely is not the case with the fuel pump, but I would still use the + side over the - side.

No...you can use the "armed output" of the alarm system to "disconnect" the fuel pump, just like you use it to disconnect the starter. When you alarm is armed then you get no fuel, even if the car is hotwired to start. But if they ****** the alarm out it would reconnect the fuel pump.

Best bet is to use a hidden switch of some sort. Back in the day I used to work for the guys with the 64 Impala lowriders. One of the tricks I used was to wire in a kill switch to the cigarette lighter. They had to push in or pull out the cigarette lighter in order for the car to start. You can be creative and think of some sort of a "switch" that doesnt look like a switch. You could wire in your rear windshield wiper switch or your car radio or one I recently did for a honda owner was to get a cruise control switch and install it in a car with no cruise. He has to push the switch before the car will start. It looks totally stock. Use your imagination.


No. At least not in a simple realy circuit. If you got into custom made logic circuits you could do it but that is beyond the scope of most peoples ability and reliabilty becomes more of an issue here.

Yes.

You can use a momentary switch to allow the starter circuit to activate since it is only used for the 3 seconds until your car starts. Hold down your horn button and hti the starter. This is something a theif most likely would never do because he would think it would honk the horn and attrach attention.

Another thing you could do is use the cruise control module on the column which can be rigged to do all kinds of things. High beam switch...any freaking switch in the car can be utilised.

Im not saying that you cant make a momentary switch to idiot proof your project, Im just saying it will have to be mighty creative and then I would still worry about whether my fuel pump would cut off right in front of a train or something. There are some devices with car audio applications that take a signal and strech it out timewise but they are pretty application specific and may be hard to alter.

Im glad I can be of help, feel free to ask more questions.
Jon Black

 

wow, such a wealth of information here. thanks!

my initial focus on the fuel pump kill is exactly that issue with the car being "hotwired" or an alarm being bypassed...the first thing a car thief will try to do is bypass the ignition system anyway. am i correct in assuming that a starter kill, with or without an alarm system, will not defeat a thief who knows how to hotwire a vehicle?

you bring up a number of very good ideas in regard to placement of the switch; i, for example, don't ever use cruise control or plan on it. this sort of setup seems a lot more feasible than hiding a switch somewhere, not to mention preserving the stock look of the interior. and since turning your ignition key to the furthest position to crank the starter seems to be a kind of momentary switch anyway, it should be easy enough to wire a second momentary switch of some sort in series with that circuit, be it the cigarette lighter (still my personal favorite) or what have you.

anyway, now that i understand how a relay works, it seems my question turns to the construction of a simple, reliable "relay latch", the "always on once activated" function that a relay cannot perform on its own. i'll be waiting for rjr to ring in on this one, since he called it simple ...but if the reliability of such a thing is even in question, maybe i would be best using the circuit we'd already discussed (relay in fuel pump circuit, toggle switch to activate relay) and either hiding a high-quality toggle that won't accidentally trip or using an existing switch in a location where it would be impossible to accidentally bump and unlikely that a passenger would deactivate without knowing its real function.

in addition, say i wire a fuel pump kill to something that MAY accidentally be bumped, like a cruise control switch. if the switch gets cut off while driving, cutting the fuel pump, would i be able to just switch it right back on and continue on my way? my concerns would be mitigated if i knew that the situation would be easily reversed...

and one last question. do relays have a current draw? in other words, would i need to tie the relay into my battery to ensure that i can engage the circuit when the car has not yet been started?

 

The latching relay is *VERY* simple and you could accomplish this with 1 push button and using another positive line for your shut down (IE> your accessory or ignition line). Hopefully this picture will give you some insite. (and yes a latching is what you're looking for!)



The push button on the blue line is a Normally open (like most switchs). the one on the black line is Normally closed...

Now if you don't want to use the 2nd push button (located on the black line.. the two black dots with the upside down T... this button is normally closed), you can use a relay. connect 85 to something that's 12v's when the key is turned let's say (like your ignition wire under the dash), and 86 to ground. run the black wire from the first relay's 86 into this 2nd relay on 30, and connect the 2nd relay's 87 to ground. This way when you turn off the key, the first relay's latch is "broken", so the fuel pump will be shut down until the next time you have the key in the ignition position and push the push button.

Note> You'll want to use the ignition and not the Accessory wire, because a true accessory wire doesn't show 12v's when you crank your car, so every time you crank it to starter your fuel pump's latch will brake. Use the ignition wire as this shows +12 in ignition and starter positions.

 

oh, the current draw on a relay is under 200 milliamps. You have to use them with alot of alarm brains that use transistors to control the output wires, and are limited to 200 mA. So the draw shouldn't be anything to worry about. Latching relays were used (and still are) quite often. Have you ever used a machine that uses 220-460 volts? Like a big trash compactor, or a Bailer (which crushes empty cardboard boxes and then you can tie them into one big lump). Even my old high school used them to control power to the shop room. A grey box with a Green "GO" button and a red "STOP" button. This was nothing more than a huge latching relay. Hit go, relay latchs, allows the machine to work or the room to have power until you hit "STOP". So reliability shouldn't be much of a concern

ok one last thing. It's pretty easy to see how they work. the wire going to 30 has 12v's and when 30's activated it sends 12's into pin 87. 87 loops back to the positive side of the relay's coil (85), and 86 provides the ground to complete the coil's loop. 85 won't be energized though until the push button on that line is completed. Once this happens, the relay is ative, the circuit to pin 87 is completed. When you let go of the push button, some of the power going in 30 and then out 87 is used to keep the coil energized and keeps the relay active. The push button on the black line out of 86 is normally closed, so when you *do* push it, it brakes the ground path which shuts down the coil, braking the connection between 30 and 87, which in turn shuts down the whole circuit until the next time you push the other push button


[Modified by rjr162, 4:02 PM 2/28/2003]

 

absolute genius. i'll reply in greater detail, but 2 quick questions:

(1) the pin setup you describe, is this standard for all relays? in addition, will i be able to figure out which pins are which when i purchase a relay? in the past, when working with op amps i know i had to go on the manufacturer's website to figure out which pin was which.

(2) how reliable is this circuit? since the circuit relies on the 87 --> 85 loop to keep the coil energized as opposed to a physical connection between 85 and the 12V power source, is there any chance that during operation the current could drop low enough to cut off the output through 87? i understand that if this were the case, i could probably hold the pushbutton in to get myself to safety, but i'd prefer it never happen.

 

well...im not sure if this is what you/he are/is looking for. I think he is looking for a design that allows him to push a momentary button once, complete the circuit, let off of the button and the circuit continue to work. I dont think you will be able to acheive this with your design or conventional switches.

What you have here will break/open/kill the circuit when you let off of the blue switch.

The is the same design as a starter kill circuit, except the blue switch is actually replaced by the switch inside your igintion that sends out +12v when you turn your key all the way foward to activate your starter. The reason why it doesnt affect the starter kill is the starter is only used for a few seconds unitl the car starts and then the starter circuit is broken when you let off of the key.

(Im not trying to argue here I just think the guy's application is the fuel pump, not the starter, and I dont see you blue switch keeping the circuit closed, unless it is NOT momentary, which I think our guy wants so it makes it simple to use)

OK, yes a theif that knows what he is doing can get around a starter kill relay in no time. In fact I think it ACTUALLY ASSISTS a theif becuase most of the wires he would have normally needed to cut to short are located right on the starter kill relay!!! The best thing to do is hide it really good if you are going to use a relay.

OK, reallys do draw current but not much. But it I would recommend that you not wire a relay in a fashion that it is continuosly drawing current when your key is off or car not running.

There are ways you can activate the relay unsuspectingly. Since a fuel pump builds up pressure and it will probably work for a few seconds *after* it has been disconneted you will want a switch that disconnect it for a few seconds (at least) before it will actually kill the car (because it ran out of gas).

It is my conclusion that if you unsuspectingly cut the fuel pump off while driving yourself that it would take a few seconds for the car to die (and it will stumble and hesitate due to lack of fuel, which you will immediately notice) and then all you would have to do is close (flip the switch) the circuit, and it should take a few seconds for the car to get enough fuel to run properly again. Have you ever riden a motorcyle and it started to run out of gas and you had to flip the Reserve on it? This is my guesstimation as to what will happen. But, I am no mechanic and I dont know the full ins and outs of Honda Fuel Systems.

OK, other options, kill the spark, or other needed systems to run the car. Im sure you could hit some wire on the ECU that would kill the car, and would probably be easier that the fuel pump and just as effective.

Yes, relays are clearly marked as to pin numbers. Buy a good one from a stereo shop for about $3 or $4 not one from Pep Boys for $9. Bosch is a good brand, as well as the German one but I forget the name.

So my recommendation is that you pick out a good located for a toggle switch or (other on off/switch like the cruise control switch) and use that to break your circuit. The cruise has an LED that can be wired in to let you know that your fuel pump is on. You can possibly replace the LED with a dual colored red/green one that will flash red when the fuel pump is off and hold a steady green when it is on. But this may alert the thief to the switch since he will have power off when he jams a screwdriver into your ignition and turns it like a key. You could make your trunk release light in the cluster flash or something like that to alert you that your fuel pump has been shut off accidentally while driving.

Lets keep bouncing some ideas and make this work.
Jon Black

 

Relays are pretty damn relaible. You car is full of them, they better be!

I dont think you understand (or maybe I dont) how the pushbutton works in his circuit works. I dont think it is momentary in his curcuit. I think it is on or off, which I am assuming is his definitition of "latching."

Jon Black

 

I wouldnt do that. I would wire the relay to where it only broke the circuit if the car were started or attempted to be started. Like, I would set it up to where the fuel pump was always in working order, until some theif tried to start the car or turn on the key (or screwdriver). At this point that your car gets accessory +12v *then* it would break the fuel pump circuit, until your secret switch were put into correct position.

There is no sense in having your relay draw current when it doesnt have to. Plus, the life expectancy of the relay would be significantly diminished due to it carrying current all the time, as opposed to the situation where it only has to work in the event somebody tries to steal your ride.

PM me with your phone number and I will be glad to call you and discuss your concerns with you.

Jon Black

 

I really have a hard time believing this will work as you propose. I have never tried it but I will. My understanding is that 85/86 are one circuit (a coil that creates a magnetic field that pulls the magnet to closed the 30/87 switch and break the 30/87a switch) and that 30/87 are another.

I will have to build this and break open a relay to inspect it mechanically. If it does work, it will be something new to me, which I will be pleased to learn something new.

Hope you dont think Im being a dick
Jon Black




[Modified by Jon Black, 9:18 AM 2/28/2003]

 

the coil is going to last just as a coil in a transformer will. And I have a power supply from the 60's something my grand father gave me and it still works. Not to mention your car uses a ton of relays in it its self.

The circuit does work. It is a push button. When you push the NO push button (blue wire color thingy in pic) it completes the circuit to pin 85, causing power to flow out pin 87. Now if you see 30 has a constant power source. So when you complete the push button, it kicks the relay over to send power out 87. What happens at that point then is a small current is pulled off of 87 and looped back into 85, so even if you let go of the push button, the power comming in pin 30 and going out pin 87 is then sent to pin 85, thru the coil, and out to ground via 86. This mimiks as if you were sitting there holding hte push button, or just shorted the push button all together. Now the only way to shut down the circuit (well to brake the look the easiest way) is to kill the ground to the coil. This will cause the contact between 30 and 87 to brake and the power to pin 85 will be shut down. This does work, and an example of a recent use was for a neon light. Customer didnt' want a toggle, so we used this setup.

 

Cool deal. I didnt know about this circuit but will definitely try it and implement it into my next project. Thanks for the tech-tip.

Jon Black

 

here's sorta an "animation" of it.

Circuit at rest with no output power



Activating of relay and latch circuit



Circuit resting in latch mode, wire from pin 87 to 85 keeps coil "alive"



Braking ground path, killing latch



Circuit resting in an inactive state once again



Ok i guess my picture for the Normally Closed switch is wrong.. i drew a normally open lol. But just pretend i drew that right

 

ok here is my understanding of this whole setup. this is partly to clarify for jon and partly for rjr to correct me if i am wrong. i'll ignore the black switch (or relay) for the time being. when you try to start your car WITHOUT pushing the pushbutton, the fuel pump will not work because the circuit that activates it is not complete; 86 is attached to ground, but there is no current present at 85. this means the relay does not complete the circuit from the power source to the fuel pump.

when you press the push button (momentary) switch, this completes the "activator" circuit that would normally be completed by a toggle switch of some sort. this completes the circuit to the fuel pump and allows you to start your car. if you were to use say, the cigarette lighter as your momentary switch in this instance, the switch itself stays down for several seconds which would be your window of opportunity for turning the key.

here's the ingenious part of the circuit, and you have to think of electricity as PULLING current from where it is available. once the juice flows into the fuel pump, and the momentary switch (the blue one in the above diagram) closes, 85 is tied directly to 87 and will begin drawing current from the output to the fuel pump. because the current draw at 85 is so small (200 mA or whatever), this won't have any discernable or detrimental effect on the amount of juice making its way to the fuel pump.

my only question here is as to how instantaneous the switchover will be; if there is a delay between the point when the "blue" momentary cuts off and the current is retrieved from pin 87 (as i imagine there would be a miniscule one), it would have to be short enough so that the relay wouldn't deactivate.

in regard to the "black" switch, rjr: when you turn off the car, effectively cutting off the 12V to the first relay's pin 30 and therefore the current that has been holding 85 open, wouldn't this achieve the same effect? once the current from 87 ceases to be present, wouldn't this automatically remove the 200mA required for 85 and 86 to be a complete circuit? it seems to me that, when attempting to restart the car, you would be right back to square one (as desired) where you'd need to push that "blue" momentary switch in order to activate the relay.

i guess i don't understand the necessity of a second switch or relay, so please clarify that for me. i think i get the rest of it though, but correct me if i am making any errors in my dissection of its function.

jon, thanks for the tips on purchasing a relay. i'll figure out the specifics and look for a bosch. if a good quality relay is reliable as long as current is applied to pin 85, i don't think accidental shutoff would be a problem; in fact, i am inclined to presume that even if the "blue" momentary were to be reactivated, it wouldn't have any effect on the continuity of the circuit between pin 85 and ground.

i'd like to keep this entire conversation in the forum as opposed to IM/PM/phone so that it may be helpful to other people currently and in future searches. and so far a lot of my questions have been answered...i may be just about ready to head to the store and crack open my hatchback!

thanks
--jeff

 

the delay is microseconds. You could indeed tie the 12V to like the ignition line its self. The point of the switch in the ground would be to turn it off if you had a constant power supply at the point marked 12V. As you said (and I just said above) you could just use a switched power source and eliminate the need for the 2nd switch. anything to brake the current for a second (well literally a split second) will shut the circuit down.

If you want to see how fast a relay reacts, just feed 12v's into pin 85, connect 86 to 30 and run 87a to ground. The relay will now oscilate at it's top speed. Hear how fast that thing goes.


[Modified by rjr162, 6:46 PM 2/28/2003]

 

I can't believe all the electronic information in this thread, when someone should have just said "buy a toggle switch that will handle 20-30 amps (easily found), and hide it so you don't accidentally hit it!?...".

That's all it takes for this application (KISS).

 

but what's the fun in that?? Of course you couldddddddddd... but toggles are ugly in my opinion (well the aviator covers are cool!)

 

AWESOME, that means i do completely understand the circuit. i was presuming that the power source was the ignition, i must have missed that part...is the ignition a perfect 12V where i can just tie the red wire for the relay into positive side of the ignition, and that way when i turn the key to activate the starter relay and ignition system it also activates my fuel pump relay? i think i will use the cig lighter as the blue momentary and hide the relay behind the center console, i'll have to look into how that is wired.

that way, even if a thief jams a screwdriver into my ignition and turns it, or hotwires the car or whatever, or tries to roll it and pop start it, he wont be able to go anywhere without the fuel pump, and the ONLY way the fuel pump will work is if the cigarette lighter is depressed (and it only stays depressed for, what, 10 seconds before it pops out? i don't think the timing of that would be affected by the 200mA draw from 85)....what thief would know that without reading this thread? if i'm not mistaken, once pin 85 starts drawing from the output of 87 it wouldn't even matter if i were to pull out the cig lighter and, say, charge a cellphone in there, correct? because either way 85 would be stealing an inconsequential amount of juice from one of the circuits?

as long as the delay is microseconds, there's no chance this circuit will cut off while driving then. and if the power source is the ignition there's no chance of the circuit draining battery juice when the car is off.

so i might be about ready to do this, no?

 

read my first post. i didn't want to use a toggle for 2 reasons:

(1) you or a passenger can accidentally hit the toggle while driving and cut off the fuel pump.
(2) you have to remember to turn the toggle back off when you exit the car. i dunno about you, but i'm pretty forgetful...thank god honda makes it so hard to lock one's keys in a car, let alone remembering to set a kill switch every night.

this setup automates the resetting of the fuel pump kill, without being much more complicated than the setup you describe...you still just need a relay and a switch. and if i use my cig lighter i won't even need a switch. in addition, you can't accidentally kill your fuel pump on the highway. i think it's definitely worth the effort.

 

yeap you should be good with that. This is sorta a simplified version of the immob on the 97 up preludes. If it doesn't read the right key code (or any at all) it shuts down the fuel pump