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Author Topic: Quad Bilateral Analog Switch Question  (Read 1236 times)
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 I am new, so bear with me.  I need someones expert opinion if what I'm trying to do will work with a bilateral analog switch.  

I am trying to switch the power on a cell phone using the lilypad as the controller.  The only way to do it, is to actually access the contacts on the momentary pushbotton on/off switch on the phone.  The button is actually a curved disc, that when pressed, bows in and makes contact.  The two points that it makes contact with consist of a round dot with a seperate outer ring.  The center dot is +2.65V @ .065ma and the outer ring is ground.  I can connect the center dot directly to the ground terminal on the battery and the system will operate normally.  I am trying to replace that momentary push button switch with an electronic version. I was told I should try to go with an optocoupler or opto-isolator, but I'm having a hard time finding something that will work with this low voltage / current. Then I found out about quad bilateral analog switches.  From what I've read, it sounds as if this might just do the trick.  Am I on the right path??????????  I'm willing to listen to suggested alternatives and / or tips and tricks to working with these switches.

Thanks a million in advance.

Steve
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hi

I have done this before lots of times. You should use the optoisolator- something like TIL113 will work. When turned on they have a forward voltage drop on the transistor of about 1V.

They way I figure out the connections is to take a small diode (1N914/1N4148 type) and put it across the appropriate cell phone connections. It will work in one direction and not work in the other. Match the diode polarity to the transistor polarity in the opto and you are almost done.

Opts are the way to go here as you have optical isolation between the power supplies of the Arduino and the phone. CMOS switches may work but they wil not isolate as ell as the optos.

D
« Last Edit: January 30, 2008, 03:30:52 am by Daniel2 » Logged

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If you cant find an optocoupler, then you can use a 4016 or 4066 IC to do the switching.

The 4066 can run from a supply voltage of 3V - 15V

Just be aware that these IC's can not switch a voltage higher than their own supply voltage.  So if you run it from a 3 V supply you can not switch anything higher than 3V


Datasheet here: http://pdf1.alldatasheet.com/datasheet-pdf/view/8175/NSC/4066.html
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Daniel is right about optocoupler being the prefered solution
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Thank you both for your insight.  I think I'm going to order both, the TIL113 opto and the 4066 and play with them to see which I can figure out easiest on the bench.  Schematics and theory are one thing, but my actually getting it to work always seems to be another.  

On a quick side note, with respect to "isolation", would it matter -or would your opinion change- that I'm using one power supply (3.7V LiPo 13500mah) battery for the whole package.  The phone operates on 3.7V but is only putting 2.65V through the on/off switch.  The Lilypad runs fine on the 3.7V as well as the ADXL320 accelerometer which fits into the whole mix.  When you speak of isolation, is it becuase the V is different (2.65V @ on/off switch -vs- 3.7V LiPo) or were you assuming I was using multiple different power supplies (ie..5V & 3.7V)?

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I think that what Daniel means with isolation is that when you use an optocoupler there is no galvanic connection through the optocoupler. No electrons can go from one side of the circuit to the other.

The optocoupler is basically a LED and  a photo resistor packaged close together. So only light passes from one side to the other, not any current.
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