Hi,
I Have bluetooth headset, I Want to turn it on automaticlly....
the steps for power up the device is:
Click the button for 5 seconds and the device power up!

I Know i should use LM555 in this case...
I Have some tests, and all I need is provide some volts from the 1 switch pad, To the other switch pad(Microcontroller) for 5 seconds


Thanks in advance

 

Bump
Please important

 

You can use the one-shot monostable configuration of a 555 timer. But have you thought of that you now need another button or signal of some sort to trigger the 555? At least you don't have to push the button or provide signal for 5 seconds anymore. http://www.kpsec.freeuk.com/555timer.htm#monostable

Since your microcontroller needs "volt" at the input (you're sure?) you can probably just hook the 555 output straight to the uC input pin from the old switch /whops, remember no higher voltages than uC is running at), might want to put a 1k series resistor on for safety. If you find out the switch was rather pulling the uC pin to ground, you need to invert the signal i.e. with an NPN transistor pulling the pin to ground.

 

the 555 will start count 5 seconds when volts will be input into it.....
can you explain it more ?

 

If you wire up a 555 timer in monostable configuration correctly, then it will output a pulse of the duration you desire when you trigger it by a high level or short pulse. Look at the link I posted.

 

but i connect the 555 to 12v,
And i need the output will be 3.7v (of the 555)
what to do ?

 

You must connect the 555 to 12v? The circuits on that site are only examples, the 555 goes down to 5v I think, then you can just cheat by putting a resistor divider on the output.

 

I Must 12v
Are you sure its get down to 5v?
And how to divide it with resistor?

 

Datasheet: http://www.datasheetcatalog.com/datasheets_pdf/N/E/5/5/NE555.shtml
4.5 - 16v Supply voltage

Voltage divider
http://www.allaboutcircuits.com/worksheets/e_divide.html

 

So can I just divide the voltage into 3.7v and 1.3v and use the 3.7v ?
There is any risk ?

 

I know many people do this with TTL serial communications when interfacing 5v devices to 3.3v devices. But I also know the technique is not approved by everyone on this forum, don't remember why.

edit: by the way, you don't want to use the 3.7 volt, this is at battery maximum charged level. When battery level drops below this, you don't want to put a voltage on the microcontroller pin which is higher than its VCC. If you know which microcontroller it is you can check its datasheet for minimum input low voltage, Vil (min)

 

Im afraid that this voltage divider will set the resistors on fire and its problem.....
What's the risk?

 

Nope, will not happen