Low battery indicator

[quickrik]

Is there a simple way/circuit to detect that the battery powering a PIC is running low and needs to be changed before its too late?

 

[blueroomelectronics]

Use a PIC with LV detect, (built into some modern PICs)

 

[jankop]

Use a PIC with A/D converter, (built into most of the PICs)

 

[quickrik]

Excuse my ignorance but doesn't the A/D converter need a reference which in this case would be the supply voltage, which would be dropping at the same rate as the analogue signal of the battery supply.

I'm using a 16F876a

 

[geko]

If you use a constant voltage source and measure that with the ADC, as the battery voltage falls, the ADC reference falls but the voltage you are measuring doesn't.

Therefore the value you read back from the ADC will increase as the battery voltage drops and you can work out what the battery voltage is.

 

[Leftyretro]

If you use a constant voltage source and measure that with the ADC, as the battery voltage falls, the ADC reference falls but the voltage you are measuring doesn't.

Therefore the value you read back from the ADC will increase as the battery voltage drops and you can work out what the battery voltage is.

Yes battery powers regulator, regulator powers PIC, so measure raw battery voltage before the regulator by using a two resistor divider and measureing 1/2 battery voltage with the PICs A/D function. In your software you can compare A/D value with a constant value you select and do something meaningful when the battery goes below your setpoint value.

Lefty

 

[blueroomelectronics]

The PLVD module requires no I/O. It can be set for different voltage levels and will generate an interrupt.

The PIC16F917 has one. 7 programmable trip points from 1.9V to 4.5V page 125 of the datasheet.

 

[geko]

Yes battery powers regulator, regulator powers PIC,

Actually that's not what I meant.

You power the PIC from the battery and use a micro power voltage reference IC, for example an LM385 to present a constant voltage at the AD input of the PIC.

 

[ericgibbs]

Excuse my ignorance but doesn't the A/D converter need a reference which in this case would be the supply voltage, which would be dropping at the same rate as the analogue signal of the battery supply.

I'm using a 16F876a

hi,
Have a look at the +Vref options in the datasheet, the setting I have in mind is +2.5V. Buy a 2V5 reference ic.[ or a 2.7V zener]

 

[quickrik]

Wow, thanks for all the replies these are all some really good ideas.

I'm going to try leftys voltage divider on the battery to start with because thats simple and I've already got the components

Thank Rik

 

[Nigel Goodwin]

Wow, thanks for all the replies these are all some really good ideas.

I'm going to try leftys voltage divider on the battery to start with because thats simple and I've already got the components

If you're using a voltage regulator it's the method that makes the most sense - and it's cheap!.

 

[quickrik]

Thanks for the help on this. I tried it last night and it worked a treat! I put a pot inseries with the battery to simulate it running down and fine tune my alarm. Came out supprised how little voltage a PIC can run on, don't know how reliably though.