555 monostable and arduino

hey,

I am trying to use a 555 monostable with my arduino, so when (if) my arduino power fails it will send a pulse. I am using the following scheme:
http://www.falstad.com/circuit/e-555monostable.html

I connected the H input to my 5V pin in my arduino, if I manually move it to the ground it works great and sends a pulse,

now when it is connected and I remove the power from the arduino it does not sends a pulse. the pulse is connected to a led so its very to see whether it sends a pulse or not.

any ideas why it doesnt work?

thanks.

What powers the 555 when the arduino loses power?

external 9v battery, I used a resistor so the chip itself should get 4.5v

the ground of the arduino and the chip is the same.

does anyone has any ideas?

It could be that the power decays too slowly to trigger the 555 through the 10uF capacitor. Try increasing the value to 100uF and see if that does it.

external 9v battery, I used a resistor so the chip itself should get 4.5v

If you mean Vcc comes from the 9v battery through the series-R, probably not a good idea. You should always connect a hard voltage - from a low-impedance source - to Vcc.

I want this chip to be connected with as a "safe proof circuit" so the vdd has to be from a battery,

which one do you recommend for this?

Hi,

If you read the thread I have started elsewhere you can see that I have had trouble getting a 555 to behave nicely when sharing power with other components, its not related to the problem you are having right now, but you should be aware that this chip is extremely noisey and could effect other parts of your circuit.

As I say, I don't think its a problem in your current application - other than as previous posters have suggested a fading battery might not have enough power to deliver the kick that a 555 uses as it changes states.

http://arduino.cc/forum/index.php/topic,115696.0.html

Duane B.

rcarduino.blogspot.com