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Resistor & ground query

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NHN

New Member
Hi

I'm a novice compared to many, but got a fair head on my shoulders IMHO anyway, but I do have a question if you could help please.

I'm working on a mod for my car which involved some circuitry, which works & designed etc, but I was wondering this.

Ok I notice they use a ground to signal some circuits, but also I notice the same circuit also uses a ground routed through a resistor obviously for a 2nd signal on same wire which cuts down on multiple wires, can someone advise what the ground through resistor achieves please, as in surely a ground is a ground???

Thanks in advance.
 

ke5frf

New Member
Without knowing the exact modification you are referring or the circuit to which you speak, and working on some of your language, I think you may be talking about transistor biasing i.e. a pulldown resistor or current limiting on the grounded connection of a transistor.

Some logic circuits that switch (or signal as you put it) a transistor will use a ground to turn a transistor on or off. With bipolar transistors, since the base current controls the E-C current, the base has to be current limited per the input voltage to properly switch the transistor. With a PNP transistor that is being turned ON by applying ground to the base, you are current limiting the voltage applied to the emitter as it is drawn to ground through the base.

In the case of pull up, pull down transistors, sometimes the output of an IC has to be biased as it couples to the base of the transistor, i.e. depending on the IC one might use a pull-up or pull-down resistor.

Some transistors need to be current limited at their outputs as well depending on the load, so some transistors will have a resistor between an output terminal and ground.

I am not sure if this answers your question because I'm confused as to what this meant:
"I notice the same circuit also uses a ground routed through a resistor obviously for a 2nd signal on same wire which cuts down on multiple wires"

It sounds like you are talking about multiplexing here but in what terms I'm not certain.
 
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NHN

New Member
Thanks for the prompt reply, I'll explain little more, this mod is to tap into the drivers door lock, its uses a ground to send a signal/switch to the control module which then locks or unlocks, now I took the lock apart & studied the tracks & found the ground routes through a microswitch which when triggered switches the ground onto a wire to send a signal/switch (lol) to lock, now when it needs to unlock, it has another microswitch that then switches that same ground through another circuit that goes through the resistor but back up that same wire, what I want to understand is, routing a ground wire through a resistor does what to the ground signal/switch exactly, as I said I'm a novice so excuse my understandings here, surely a resistor would only help in a live wire to lower voltage?
 
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ke5frf

New Member
Can you draw a picture and upload it? I'm sure your specific question can be answered here if we see a picture or schematic.

Are you familiar with Ohm's law?

And FYI, when we talk about transistors sending a signal, we refer to it as "switching" in digital circuits especially. Without knowing your circuit, I was working off an assumption.
 

Diver300

Well-Known Member
Most Helpful Member
On a car fuel gauge, it's a common design to have the sender (the bit in the tank) as a variable resistor to ground. The more fuel, the lower the resistance, the more current and the needle moves higher.

That can signal a very large, maybe infinite, number of levels of fuel, all on one wire.

The door lock you describe is similar, but with only 3 levels:-
No current (do nothing)
Some current (unlock)
max current (lock)

You are correct that it saves wires. To get this to work, there are a few requirements that have to be met, but I'm sure the designers realise this.

1)The resistance in the control unit has to be similar to the resistor connected by the unlock microswitch. For example, if both resistors are 1kΩ, then the voltage on the wire is 12 V when nothing is happening, 6 V when unlock is called for and 0V when lock is called for. There is a clear difference between each state. If the resistors were very different, the unlock voltage would be very close to 12 or 0V

2)The control unit has to recognise each state as different, so that it understands 12 V as do nothing, 6 V as unlock, and 0 V as lock.

3)There has to be a short time delay. As the vehicle locks, the voltage will go from 12 V to 0 V. This will take time, maybe 1 microsecond. The unlock circuitry may trigger for that long. It is important that a short trigger like that doesn't actually do anything.

FYI, the driver's door window switch assembly on my car (2001, standard production model) has to signal 13 separate conditions. (Up, down and one-touch for each window, plus rear window isolation). It does that on one wire, by sending the signals in predetermined order, one after the other. That saves a lot of wires.
 

NHN

New Member
I do appreciate the help, so you will have to excuse my ignorance, but if I may ask how is a schematic going to assist this query, surely adding a ground to any resistor circuit would have the same effect on that wire, all I would like to know please is what does the resistor do to that ground wire, how does it affect the ground switch/signal, its a resistance circuit so what does it create resistance on, surely a ground is a ground, now if it was a live wire we could figure its job out.

To draw a complete schematic, I would need to take the door module apart aswell, I know what I want to do works, so I could just install it & not worry about understanding why, but tbh thats the easy & copout way, I like to learn, so I would like to know/understand why it works & what a resistor does to a ground switch wire before it gets to the end means whether it be a transistor or another component.

Also completely off topic, where's the thanks icon, lol, as wanna thank people for there help.
 
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NHN

New Member
Thanks for your help aswell btw, it is appreciated from all.

Ok I get it changes the voltage on the wire with say resistor/relay/microswitch circuitry to switch the paths as such, but you're saying the ground can also control the voltage levels aswell, hows this possible.

Talk abotu de ja vour, you mention about your drivers door switch pack, lol, same here, I discovered the same states with my mirror folding/heated circuits as you have with yours, 4 wires & 1 was live feed, another ground & 2 to switch the god knows how many different states with resistors also in line & guess what, that also used ground switching for them, the live was just for illumination iirc, so this could answer a few queries please as to how the resistor affects a ground switching wire state.
 

ke5frf

New Member
I do appreciate the help, so you will have to excuse my ignorance, but if I may ask how is a schematic going to assist this query, surely adding a ground to any resistor circuit would have the same effect on that wire, all I would like to know please is what does the resistor do to that ground wire, how does it affect the ground switch/signal, its a resistance circuit so what does it create resistance on, surely a ground is a ground, now if it was a live wire we could figure its job out.

To draw a complete schematic, I would need to take the door module apart aswell, I know what I want to do works, so I could just install it & not worry about understanding why, but tbh thats the easy & copout way, I like to learn, so I would like to know/understand why it works & what a resistor does to a ground switch wire before it gets to the end means whether it be a transistor or another component.

Also completely off topic, where's the thanks icon, lol, as wanna thank people for there help.

Hi.

Well the other fellow who posted gave us a knowledgeable explanation without the need for a schematic because his intuition is that this circuit works like a SENDING UNIT in a gas tank. I'll except that explanation without hesitation.

Now, as to your frustrations. You implied that you want to understand more than what you currently understand, as in just how to install it. I mentioned drawing a schematic and you didn't like that idea.

Well, I have a little experience with this and TRUST ME, one of the BEST ways to learn how things work is to draw it out and trace it all from point A to point B and you'll be surprised at how much you learn.

I asked you to draw this out because I was having a hard time putting the words into a picture in my mind. Sometimes the person who is helping you needs more than a word description.

And lastly, you really need to start out with learning Ohm's law.
This is no insult BTW, because you have to start somewhere.

This site can help you, but you have to start out by "erasing" some of your incorrect preconceptions. For instance, the part about resistors connected to ground wires. It sounds like your understanding of electronics is (basically) limited to mechanical switches and wiring, the things going on outside of the "black box". Yes, in most cases on the outside of an automotive electronic device you will only have wires, perhaps a red wire for positive and a black wire for negative or ground. Usually there isn't a visible resistor on those wires, but those wires are connected to components inside "the box" and I can think of hundreds of circuits where a resistor will be connected to ground.

Understanding Ohm's law helps.

I'm just trying to explain this in the most basic terms.
I suggest you start by getting a book on basic electronics.
 

ke5frf

New Member
Thanks for your help aswell btw, it is appreciated from all.

Ok I get it changes the voltage on the wire with say resistor/relay/microswitch circuitry to switch the paths as such, but you're saying the ground can also control the voltage levels aswell, hows this possible.
.

This is where you need to start erasing some things, no insult meant by that at all.

A circuit in electronics is a complete path from positive to negative, or more correctly from one potential voltage to another.

There is nothing magical or special about the ground wire, and it is part of the circuit. YES A RESISTOR CAN BE CONNECTED TO GROUND AND SERVE A PURPOSE.

The resistor just becomes part of the circuit, and the ground wire continues on. The wire isn't ground. Ground is ground. I could have 100 resistors all in series and terminating at a ground point and it doesn't become ground until it makes the ground connection.

So erase the idea that the wire is ground or that there is something wrong with a resistor being connected to a ground wire.
 

NHN

New Member
No worries I dont take it as an insult, not at all, take it as a learning curve, so all the info supplied helps me grasp things better.
 

NHN

New Member
This is where you need to start erasing some things, no insult meant by that at all.

A circuit in electronics is a complete path from positive to negative, or more correctly from one potential voltage to another.

There is nothing magical or special about the ground wire, and it is part of the circuit. YES A RESISTOR CAN BE CONNECTED TO GROUND AND SERVE A PURPOSE.

The resistor just becomes part of the circuit, and the ground wire continues on. The wire isn't ground. Ground is ground. I could have 100 resistors all in series and terminating at a ground point and it doesn't become ground until it makes the ground connection.

So erase the idea that the wire is ground or that there is something wrong with a resistor being connected to a ground wire.

Seriously mate, its not a problem, I wont be insulted at all, I'm not one of these people that thinks he knows it all, far from, I'm a person that if I dont understand I will ask & learn from that, I think anyway, lol.

I intend to do alot more reading, but hope my questions dont become annoying as we all started somewhere. :D

So are you saying that there is a live being fed down to the resistor one side & this is whats switching the circuit because it is detecting a resistance level on that wire, thus then that resistance tells the say controller end what state this is in to complete a certain programmed operation?
 

ke5frf

New Member
OK, just a quick google search led me to a page with a very simple circuit that shows a resistor connected to ground:

The Basics - Very Basic Circuits

Click on the link and scroll down to the part about pull up/pull down resistors.

There you will see a picture of an amplifier (triangle symbol) with a switch and a resistor, the resistor is connected to ground. Ground is the three parallel lines at the bottom of the circuit. The amplifier is performing a logic function, when the switch is closed, it draws the supply voltage, usually 5 volts, across the resistor and signals the amplifier high on the input, with the switch open, the resistor draws the input to zero volts or ground. This is on/off...1 or 0, hi or low for a digital circuit. The output of the amp will act accordingly . The resistor current limits the input to prevent burning up the IC in case of overcurrent condition.

The resistor is connected to ground.
 

NHN

New Member
Ok I now see the reason for a schematic in this circuit, as it would show if was pull up or pull down resistor, its taking time but I'm slowly understanding more now.

Fair play you were right to ask for this.

Now I'm still getting my head around this but it would seem its a pull down resistor circuit as the actual part thats being switched (correct me if I'm wrong) is further up the line in the door module.

Ok bare with me, ground is basically just the other end of a path as such to allow current to flow across a circuit so it doesnt build up as such, adding the resistor will change the voltage/current that goes across that path where ever it maybe positioned, not sure that makes sense tbh.

It is 1.15am here and been reading fair bit so again excuse my thickness tonite, usually these things trip a eureka moment the next day, lol
 

ke5frf

New Member
OK, the schematic that I pointed out to you was just an example of a resistor that is directly connected to ground, so please don't assume that your particular circuit works the same way. I was just trying to point out an example where a resistor can be connected to ground in a circuit.

I still can't really comment about how the resistor in your circuit works without seeing the circuit or a schematic of it. That is just how my mind works. The other poster explained that it works like a sending unit in your fuel tank. I accept his explanation, but without seeing the circuit myself I can't really describe it with any precision.

Anyway you said this:
"Ok bare with me, ground is basically just the other end of a path as such to allow current to flow across a circuit so it doesnt build up as such, adding the resistor will change the voltage/current that goes across that path where ever it maybe positioned, not sure that makes sense tbh."

Well, that is "basically" correct. Believe me, a proper tutorial on Ohm's law can't be covered on a message board thread. It really needs to be studied from textbooks with formulas and examples. It also helps to analyze some simple circuits with a volt/ohm/ammeter to see it all in action.

I encourage you to read through some basic tutorials. Google is a great place to start.
 

NHN

New Member
On it already with google :D

So the resistor doesnt need to be directly inline with the circuit as such to effect the voltage, it can also be in parallel aswell as series to affect things overall, this is something I didnt realise, explains some things to me now.

Been reading up quite rightly as you suggested.

I'll get there & again thanks for the help.
 

ke5frf

New Member
On it already with google :D

So the resistor doesnt need to be directly inline with the circuit as such to effect the voltage, it can also be in parallel aswell as series to affect things overall, this is something I didnt realise, explains some things to me now.

Been reading up quite rightly as you suggested.

I'll get there & again thanks for the help.


Very good. But let me comment on "So the resistor doesnt need to be directly inline with the circuit as such to effect the voltage"

I know what you are asking here, you are referring to a parallel circuit, in which case yes, the resistor doesn't have to be in series with other components in a circuit (switches, LEDs, transistors, capacitors etc) to have an effect, but I will correct you one one thing, which you will learn as you read...when a resistor is in parallel with another component in a SIMPLE parallel circuit, it doesn't have any effect on voltage but rather it divides the CURRENT between the different branches of the circuit.

Also, remember this point...All components in an electronic device, be they in series with each other or in parallel, are PART OF the circuit and work together at the same time. Well, at least this is true when all components are switched "on" and actively working in the circuit. While you can analyze a circuit as the sum of its parts and study each part independantly, remember they all act together and as such are ALL part of the circuit. Like players on a soccer team (football for the Brits on board LOL), each player has his job, forward, back, goalie etc. No one player is more important than the other, though some have a more active roll, they all work together to score points for the team.
 
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