Circuit wiring issue

[idi]

Hi all,

I need some help on a little project i am doing. I have attached a diagram of the circuit to give a clearer understanding of what i need.


The circuit at the bottom is for a Nexus tablet. I have extended the power switch so points A and B on the diagram are the switch. e.g. which i touch them together for a quick second, the screen turns on, just like the power button on a phone. I want the screen to turn off and on based on a 12v feed. So when the feed is first turned on, i want the button to be pressed just once (points A and B joined and then 'released'), and when the feed is turned off, i want the button to be pressed just once again.

I obviously can't connect the 12v feed to the circuit since all i need to do is join points A and B.

Is this a simple thing to do? How would i approach it? is there anything i can buy made ready for this?

Thanks

 

[Spoon]

Hi mate. It must have been a long week cause I dont quite understand what you are after. Or maybe im just thick......

Am I right in that:
You have a Nexus tablet that you modified the power button on. (you have extended it). You would like to operate the switch (points a & b) using a 12V signal?

Why dont you use a 12V relay with a Normaly Open contact. The contact can be your points a & b and is operated by a 12v signal.

 

[idi]

Am I right in that:
You have a Nexus tablet that you modified the power button on. (you have extended it). You would like to operate the switch (points a & b) using a 12V signal?

Yeah that sounds right. However I dont wan't points A and B to continuously joint whilst the 12v feed is powered on. I just want it to press once, like a normal push button. and the same when the 12v feed loses power.

 

[Spoon]

You would use a spring return pushbutton to operate the relay, so points a and b will connect as long as you are pressing the button.

 

[idi]

You would use a spring return pushbutton to operate the relay, so points a and b will connect as long as you are pressing the button.

There isn't gnna be a button involved.

I think i've done a rubbish job of explaining this.
When the 12v feed first turns on, i automatically want A and B to touch for a second then release.
When the 12v feed is turned off, i automatically want A and B to touch for a second then release.
I dont want 12v going between points A and B because that would ruin the circuit board of the tablet. I just want them to touch, with no power.

There is no manual switch involved. I need it to be all automated.

Does that help clear it up?

 

[Spoon]

There isn't gnna be a button involved.

I think i've done a rubbish job of explaining this.
When the 12v feed first turns on, i automatically want A and B to touch for a second then release.
When the 12v feed is turned off, i automatically want A and B to touch for a second then release.
I dont want 12v going between points A and B because that would ruin the circuit board of the tablet. I just want them to touch, with no power.

There is no manual switch involved. I need it to be all automated.

Does that help clear it up?

Yes, this makes it a lot more clear.

You are using points A & B touching instead of the power button and you would like to control this using a 12V signal.
You can still use a relay. What you have to look out for is the duration of how long A & B touch. With the tablets I have used you have to hold the power switch for a few seconds before it turns off. You may have to do the same with this application.

 

[uksparks]

What is the object of this? Whats it actually going to be used for as that will help people explain whats needed etc.

To hold the contact with no power... Not exactly sure how you are going to do that without some form of power?

This isn't something I'm too clued up on. Sorry.

 

[idi]

Thanks for the help

So all i need is a relay which will just give a very quick output, and connect it to points A and B?

 

[Flanders]

As spoon says relays are the answer. You can get all sorts of time delay relays that hold on or off for various amounts of time with all combinations including short and long pulses have a look a RS web site

 

[idi]

What is the object of this? Whats it actually going to be used for as that will help people explain whats needed etc.

To hold the contact with no power... Not exactly sure how you are going to do that without some form of power?

This isn't something I'm too clued up on. Sorry.

The whole reason why it is needed is VERY confusing so i tried to simplify it as much as i could.

The power button on the tablet just works when the 2 wires are connected together. I could just use a metal screwdriver and join the wires to press the switch. So i need something to that affect, cos im pretty sure if i put power to it it will blow the board. So when the 12v feed turns on, just join points A and B together for a quick second. This will then turn the screen of the tablet on and leave it on. Then when the 12v is turned off, it will again press the power button which will turn the screen off.

Does that help?

 

[Flanders]

Have a look at some of these they may be what you are looking for
Buy Time Delay Relays | RS Components

 

[idi]

Have a look at some of these they may be what you are looking for
Buy Time Delay Relays | RS Components

Thank you for this. I'll take a look now.

Will a relay work for when it is turned off aswell? e.g. when the 12v feed is turned off, will it send a short output signal?

 

[Flanders]

I think this might do it but are quite expensive if you ring tech help they may be able to help you but you may need an account or you could try Maplins
Buy Time Delay Relays Time Relay Multifunction 2C/O ABB 1SVR730030R3300 online from RS for next day delivery.

 

[Spoon]

Thank you for this. I'll take a look now.

Will a relay work for when it is turned off aswell? e.g. when the 12v feed is turned off, will it send a short output signal?

There are components that detect power loss and send out a signal, so im sure it is possible. Just the ones we work with wont be suitable for you (3 phase). You may have to dig around on the net to find something. Im sure someone on here will suggest something.
Again, you have to check out how long you have to keep points A & B connected for the tablet to switch on and off.

 

[tazz]

it can be done with electronics, using a dual 556 timer chip, but the problem is power will be needed to switch the tablet off again, which means using the 12v constantly or a battery....if your ok with soldering and components. i can sketch you a drawing.