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pressing cell phone button?

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jayc005

New Member
Hi all, im a total newb to electronics and making my own circuits, however ive recently made a 555 monostable circuit to basically 'Press' a button and hold it down, on a cell phone.

The action of pressing and holding activates speed dial assigned to that button.in turn calling the number set.

The idea is to have an alarm system trigger the circuit, which makes the 555 timer do the work.

Now, i have tried a 4 pin optoisolator.. it didnt work, i tried a relay and this didnt work because i cant drive the relay properly, i really dont know what im doing..

can anyone suggest a way of 'bridging' the 2 contacts on the phones motherboard via the 555 timers output signal? ive lost alot of bits but have some bc547 transistors left over, i can buy the bits of salvage them if they are readily available..

i would really like to finnish this circuit so that i can fit it to my car which has recently been broken into, i will use the system as a pager.

thanks, and great forum, any help would be greatly appriciated :)

Jay
 

Sceadwian

Banned
Buttons are simple. One side is +volts other is GND, control electronics on the GND side can tell when the +volts side touches the GND side. So all you do is apply the same voltage as +side has to the GND side from your 555. The circuits have to share a common ground though or you can't predict it's behavior. Check on a meter and find out what the voltage the phone's buttons are given and go from there. Keep in mind the phone and your circuit have to share a common ground! Which means you have to find a gnd point in the phone or use the phone's batteries -lead to link it to the GND point on your 555 circuit. Just be careful, buttons usually have resistors involved which prevent over curernt but sell phones are expecting about 3 volts, feed it too much and you could burn something out.
 

jayc005

New Member
i could use a 317 voltage regulator? and use the power from the output of the 555 timer toughtly 8v to power the 317, and the output power (stabilised) from the 317 will provide the voltage to the center pin, the ground can be shared with the rest of the circuit?
 
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jayc005

New Member
ok , i have connected up a 317 voltage regulator and set the voltage correctly, now, using a shared ground, if i connect the +v pin of the keypad to the output of the voltage regulator, it keeps the button 'held'.

if i press the timer on the 555 it stops for the amount of time set, then resets and the button goes back to 'held'

its working in reverse of how i want it too lol.. any ideas, i can post a little schematic if you like
 

jayc005

New Member
my tester also says when the timer is active the voltage is there and when its not the voltage drops :S im confused, there seems to be continuety between ground and the output pin of the voltage regulator when it is not powered up, so this is essentialy bridging the circuit just as the foil button would have originaly.

im so confused lol.. what i thought would be a simple excersize has already made me build 3 circuits none of them working, still its good practice.. only shame is im not learning why this isnt working. nor why it always works but not in the way i want it to.
 

Sceadwian

Banned
So put an inverter in between, you may be able to do this by altering the 555 circuit? I'm not that great with analog circuits so I can't help too much.
 

colin55

Well-Known Member
Is one side of the button at 0v? When you push the button, is this side of the button still at 0v?
 

jayc005

New Member
hi colin, yes the gnd part always stays gnd even if you bridge the contacts, ive tried an optocoupler but it didnt work?

so what is the purpose of the invertor?
ill look it up, but if anyone can give me an explenation here that would be good :)

thanks for all your help so far.
 

jayc005

New Member
A logic gate that converts the input to the opposite state for output. If the input is true, the output is false, and vice versa. An inverter performs the Boolean logic NOT operation.

i guess this is what you mean by invertor?
 

colin55

Well-Known Member
Take two wires out from the switch and short them together to see if they work.
Next, put a resistor between the wires and see if it works. Put different values of resistance and let us know the HIGHEST value that works.
 

jayc005

New Member
it works if i bridge the wires.

i dont have many resistors, but ive just tried a few, 4.7k works but 10k doesent.

dont really have anything in between

i will use a trim pot and see if i can find the point where it doesent make the connection
 

Sceadwian

Banned
Don't forget, putting two resistors in parallel will half their resistance, and putting two in series will double it.
 

colin55

Well-Known Member
Ok. So we have worked out that a reasonable resistance between the two terminals will make the switch work.
Now take the opto coupler mentioned above and put it across the two terminals. Remember, the opto coupler must be connected around the correct way. This means the "output" of the coupler must be connected correctly. This simply means to try both "ways."

Put a 6v supply on the "input" side of the opto coupler and make sure a 470R resistor is placed on one of the leads.
Again, the battery must be connected correctly and the easiest way to detect this is to measure the voltage across the two (input) terminals of the opto-coupler. It should be about 2v.
Now see if connecting the battery makes the switch work.
 
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jayc005

New Member
ok, ill have to order some more OC's... will the 4N35s work?

and then i power the diode side from the 317? im pretty sure i have tried this in all different combinations and it didnt work, but i was using SFH618A'S instead of the 4N35 OC..
if im using the 317 as the voltage output for the OC, do i really need a resistor? as i can just set the voltage needed.

you guys have been stars so far, thanks again for your patience :) im slowly getting there, i love electronics but suck at it lol

cheers
jay
 

Sceadwian

Banned
It's not usually a good idea to drive a diode from a fixed voltage, temperature and time will change the current going through it, and surges even small ones will completely destroy it. Ideally you run it from a constant current source, but if you don't want to go to all that trouble just use a higher voltage than the diode is rated for and use a resistor to limit the current, the higher the voltage you use the more you waste in the resistor but the closer it behaves to a constant current source, so if losing a little power in the resistor isn't a problem use the main supply voltage and an appropriatly valued resistor.
 
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jayc005

New Member
i have just found a 2561A photocoupler in a satnav psu so i will skank this for now and give it a go.. ill report back soon :)
 

jayc005

New Member
hmm, i set it up with a new OC and it works, but not in the way i would like it to.

basicaly when the timer powers the diode side, it does nothing, when the power drops it presses and a 2 comes up on the screen.. but its not holding, so when i apply power to the whole circuit i dont think a 2 comes up.. only when you activate the timer then it stops..

im sorta lost now anyway, and i think ive killed the phone now too.. sorta might just give up lol it was a nice idea that seemed easy at first but has turnt into a slight annoyance because im close to getting what i want but keep running into problems. thanks for your help chaps, if anyone would like to build me one and sell it to me im more than willing to pay for the circuit :)
 
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