Voltage checker for Basic Stamp II robot project

Does the Stamp have an ADC input which accepts 0-5V? Then a simple
resistive divider would do. Two resistors (say 10k, it's not that
critical, but much lower values will draw appreciable current) in
series between the battery +ve and -ve, connect the Stamp's ADC input
between them. The stamp will now read half the battery voltage, which
can go up to 10V or so without blowing up the Stamp. If you're getting
a load of noise add a capacitor between the ADC and one of the battery
terminals (perhaps 100n to 1uF) to smooth the readings out.


Tim
--
Did I really still have that sig?

Unfortunately, the last time I checked, the Stamp doesn't have an actual
ADC input -- instead, you 'll need to use a simple rc circuit and
measure the cap discharge time using pulsein (I think -- it's been a
while since I looked at the stamp). Check the stamp docs for details.

You might consider moving up to a BasicX BX-24 or OOPIC for this
application. The Stamp can't be beaten as a learning tool, but you
outgrow it quite quickly. Both of the former devices can be programmed
in a Visual Basic-like dialect, and feature true ADC (among a lot of
other features). I think you'll find both the BX-24 and the OOPIC as
easy to use as the stamp.

Hope that helps -- tAfkaks

--
(Replies: cleanse my address of the Mark of the Beast!)

Teleoperate a roving mobile robot from the web:
http://www.swampgas.com/robotics/rover.html

Coauthor with Dennis Clark of "Building Robot Drive Trains".
Buy several copies today!

    One simple solution is to use a comparator chip and reference Zener to
pick a point that you deem as "low battery" and treating that as a digital
input.  One chip, one diode.

Cheers!

Sir Charles W. Shults III, K. B. B.
Xenotech Research
321-206-1840

On 26 May 2006 05:33:54 -0700, "Couldbeflying"


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@kcwc.com                                        http://NewsReader.Com/

Thanks Kurt, it works quite well. The logic state changes at 5.4V. This
is good enough for me. Also, thanks to everyone else who responded.

Richard

If the chip requires 1.4v to pick up, then try 6 diodes in series
instead of 8. Does the voltage divider continue to draw down the
batterys after the minimum voltage value is reached?

My multimeter says the circuit is drawing 1.1 - 1.2 mAh when the Stamps
asleep. Considering my solar panels will be producing at least 6W
(500mAh), I think there will be no problems with charging.

Thanks again
Richard

Nit-pick:
mAh describes a capacity; each mAh should provide one mA for one hour
mA is a rate of electrical flow

Thus a circuit drawing 1mA, powered by a 500mAh battery, would run for
three weeks.

- Daniel

On Sun, 28 May 2006 11:44:36 -0500, D Herring


I assume that is an "ideal" battery that maintains full output
voltage until it is fully exhausted.