Arduino Pro Mini 3.3v RAW vs VCC

[levardar]

Hello.

1. If I am using this board with an BMP085(minimum 1.8v like atmega328P to maximum 3.6v) pressure sensor and CR2450 battery witch is 3v, should I connect the battery to VCC or RAW going in the voltage regulator. I want the battery to work as much as it can ....
2. How can I use a led on Arduino Pro Mini 3.3v to indicate low battery(so I can change it before it dies).

Thank You,
Razvan

[nickgammon]

Raw goes through the voltage regulator, so you won't want that. If it is less than 5V hook the battery up to Vcc.

How can I use a led on Arduino Pro Mini 3.3v to indicate low battery(so I can change it before it dies).

You can self-detect the power level by using a technique invented by various people, a copy of which is on this page:

http://www.gammon.com.au/power

Scroll down to "Detecting low voltage".

Of course the LED will itself use power, so you probably want to flash it briefly.

[levardar]

Thank you Nick for you advice!

[retrolefty]

Hello.

1. If I am using this board with an BMP085(minimum 1.8v like atmega328P to maximum 3.6v) pressure sensor and CR2450 battery witch is 3v, should I connect the battery to VCC or RAW going in the voltage regulator. I want the battery to work as much as it can ....

You can wire the battery to the boards Vcc pin.

2. How can I use a led on Arduino Pro Mini 3.3v to indicate low battery(so I can change it before it dies).

One way is to first have the ability to measure the battery voltage wired to the Vcc pin. You can use the method Nick suggested in his prior post.

Or a little 'hack' might work as well. There is a on-board red led power light on the Pro Mini board. If you solder a small jumper wire from the junction of R11 and the anode of the red power-on led to an unused analog input pin, you should be able to measure the constant forward voltage drop of the red led, which should be about 1.5 or so (measure it with a DMM so you know what yours actually is). Because the analogRead() command uses Vcc (Avcc actually) as the reference for analog reading, you will find that the 'count value' returned from reading the led Vf will gradually rise as the battery voltage decreases. So you can have a simple function that occasionally reads the analog input pin and when the count value has increased to the 'cut-off' value you want to alert on, you can then turn on the pin 13 led. Simple, no?

Lefty

Thank You,
Razvan

[levardar]

I need to change the battery to a Li-PO One, 3.7v, so in this case it will be a problem to put it straight to VCC, maybe I will recharge it directly through a Li-PO charger, when the battery is still connected to the system. There will be like 4V, which is not good for BMP085 Sensor or for the Ftdi chip with 3V3. Is write in this case?

Thank you,
Razvan