# Battery Level Monitor

Discussion in 'Electronic Basics' started by Michael, Sep 17, 2006.

1. ### MichaelGuest

Hi,

I'd like to create a 12V battery monitor and interface it to a PICAXE-08M
running at 3.3V.

What would be the best way to do this? I was thinking about using a 3.3V reg
for the picaxe and then use a voltage divider so the changes in voltage can
be picked up using the onboard ADC.

Would I be right in assuming the resistor values should be about 10Kohms and
2.96Kohms?

Cheers,

Michael

2. ### ChrisGuest

Hi, Michael. First, your 3.3V power supply does change some things
(see prior thread, "Interfacing A Pressure Sensor"
http://tinyurl.com/eqaun ). You need a weak pulldown (say, 33K or so)
resistor at the final output of the LM324 to get the output down to 0V
(actually 20mV or so). With the pulldown, you're going to be somewhat
limited as to maximum output voltage -- about 1.5V or so less than the
+ supply. That means you're talking about a maximum output from the
LM324 of 1.8V. And that means you're throwing away 45% of your counts,
which might be a problem -- it certainly isn't optimal. If you want to
stick with the LM324, then set your pairs of gain resistors so that
maximum output is about 1.8V (i.e. if your maximum sensor output is
200mV, make it a gain of 9 using 11K and 100K resistor pairs).

This inexpensive solution should get you better results than the
initial effort of putting both inputs of the Wheatstone bridge directly
at the PICAXE inputs. If you want to do better, you'll have to
consider a rail-to-rail input and output quad op amp like the R-to-R op
amp like the TLV2374ID, which is out of stock at RS.

As far as the voltage divider to read battery voltage, you'll have to
the battery voltage or not, you will have to be concerned with the Vin
at the PICAXE input pin exceeding the voltage at the Vcc pin of the
PICAXE. It's trivially easy to latch up a uC by exceeding that
voltage, and you'll have to give that some serious thought.

If you're working off battery voltage to power your PICAXE (say, with
an LM317), all you have to worry about is powerup/powerdown sequencing.
It might be a good idea to use a smaller divider with diode
protection, and then use the 4th op amp to boost it up, like this (view
in fixed font or M\$ Notepad):
|
| VCC
| +
| |
| .-.
| | |10K
| | |
| '-'
| | ___ |\
| .---o---|___|--|+\
| | | 10K | >--o---->
| | .-. .-|-/ |
| V | |100 | |/ .-.
| - | | | | |11K
| | '-' | | |
| | | | '-'
|=== | | |
|GND === '-------o
| GND |
| .-.
| | |1K
| | |
| '-'
| |
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The 100:1 divider cuts your 13.8V down to about .138V. That's not
enough to cause serious conduction in the diode, but will protect the
op amp input. Then the 11:1 ratio for the feedback will give a gain of
12, which will bring your .138V up to 1.65V, adequate to have a good
handle on the battery voltage. Use a 1N4148 or 1N914 for the diode.
This may look overly complex, but you're making sure that, under any
circumstances, the LM324 input isn't seeing more than 0.6V or so, and
also that the PICAXE input will never see more than Vcc. It's not
elegant, but it should do, especially for a hobbyist with a limited
budget.

I hope this has been of use.

Good luck
Chris

3. ### MichaelGuest

Thanks Chris,

I was thinking that the PICAXE should be run off a regulator, and then use a
voltage divider for detecting the voltage....Or is that what your getting
at?

Michael

4. ### ChrisGuest

Hi, Mike. You didn't describe the environment in which you're powering
the PICAXE. It might be possible you're using this in a car, or
something else.

The point is, you want to try to make things so you don't exceed the
maximum input voltage at any pin. This is particularly important with
microcontrollers, where the reverse current can cause mysterious
glitches which might crash the uC program, even if it doesn't damage
it.

If you're monitoring an external battery, and your regulator is getting
power from that battery, you'll have a problem at turn-on, when the
full 12V is present, but the regulated output voltage is ramping up.
This may only be a millisecond or so, but it's enough to cause
problems.

Let's assume a 12V battery is hooked up to your voltage divider with
resistance might prevent latchup (a destructive condition common to
CMOS where reverse currents caused by a low impedance voltage at a pin
above or below the power rails causes a destructive SCR effect).

However, it might cause unintended effects on the uC chip which prevent
a normal reset or reliable operation. Better to be safe. Especially
if you're cost is only an LM324, a couple of more resistors, and a
protection diode.

Good luck
Chris