Re: Voltage checker for Basic Stamp II robot project

shb*NO*SPAM*@comporium.net (Si Ballenger) wrote:
On 26 May 2006 05:33:54 -0700, "Couldbeflying"
<rryerson0710@xxxxxxxxxx> wrote:

Hello,
I am working on a outdoor all-terrain robot. The robot is pwered by 6V
NiMH batteries that are charged by solar pannels (I guess like the Mars
rovers). Anyway, I need a small 6V... voltage checker circuit to be
momitered by the BS2. My idea was if the battery pack voltage dropped
below a certain level (say 6V) then the stamp would put the robot to
sleep allowing the solar panels to recharge the batteries. The stamp
would periodically wake up and take a voltage check. When the voltage
got above a certain level the robot would resume it's activities.
I would prefer an off the shelf circuit if possible. I could maybe
stumble through making a small circuit myself if I had good explanation
and examples. Can anyone recommend anything for this?

Thanks so much
Richard


You might make a low voltage detector out of ~eight common
diodes in series and a high value resistor to ground. The diodes
will pass voltage above 5.6v, but not for lower voltages. Connect
the diodes between the battery and a high/low pin on the chip.
Below 5.6v no voltage will be applied to the chip pin. Above 5.6v
the diodes wii start passing some voltage to the pin.

But won't the voltage at the pin be 5.6 volts below the supply? Meaning at
a full 6V charge the pin will only be .4 volts? So it will likely register
as a zero value in all cases?

Reading my Boe-Bot manual it says the stamp threshold input voltage is 1.4
volts. Anything over that is registered as a one, and anything below that
is registered as a zero.

Creating a simple voltage divider with two resisters is likely to be your
best answer. Just calculate the values so that the voltage you want to
sense gets divided down to 1.4 volts. So if you want to sense 5.5 Volts,
and the larger resistor is a 10K resistor then the other must be:

R = 1.4 * 10000 / (5.5 - 1.4)

R = 14000 / 4.1 = 3415

Rounding to a common value, we get 3.3K ohms and calculating back to the
voltage with this, 1.4 / 3300 * (10000+3300) = 5.64 V so it's a bit higher
than the 5.5 we were look for. You can adjust by using multiple resistors
in series to get to the value you want or by using a pot that you adjust
for the trigger point you want.

So you use a 10K ohm resistor connected to the positive power battery and
to the input pin, and a 3.3K ohm resistor connected from the input pin to
ground. It should read as a 1 if the supply voltage is over 5.6 and and 0
if it's below 5.6.

However, the above calculations are based on the assumption that the input
pin has a fairly high input impedance and I don't know if that's true or
not. It would just mean the resistor values would have to be a bit
different if it's not true.

If you have extra inputs, you can use multiple circuits with slightly
different resistor values to detect different voltage levels.

Also, it would be good if you could detect the charging current from the
solar panels. If the bot isn't in a bright enough light to be charging the
batteries, you might not want to stop. :)

You should as someone else suggested, be able to use a capacitor and a few
resistors to create an RC delay circuit that you use with the RCTIME basic
command. But I wouldn't know how to design that without experimenting.

--
Curt Welch http://CurtWelch.Com/
curt@xxxxxxxx http://NewsReader.Com/
.

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