I have a battery powered 16F84 based project; a universal remote control.
There are 3 core components; the 16F84, a 24C65 EEPROM and a 44780 LCD
The problem is how do I power this lot from batteries...
The EEPROM, LCD and all associated pull-ups/LCD contrast voltage are powered
from a PIC output pin. This works fine and drops the IPD to a uA when its
all turned off.
But when it comes to running it from batteries (4 Alkaline AAA's) I am not
sure what to do.
The LCD needs 5v (+/-5% or there abouts), the PIC and EEPROM etc are more
flexible. So here is my idea;
The PIC and IR LED run directly from the batteries, EEPROM runs directly
from the PIC. The LCD is powered through a low dropout regulator with a
standby input controlled by the PIC (same pin that powers the EEPROM) to
switch it on and off. This prevents the regulators quiescent current (which
is _relatively_ large) draining the batteries.
I think this is a pretty good solution, but what do you think and what have
you one?? Minimal component count is important as the case is very small. Is
it wise to run uC/EPROMs directly fom batteries?
Thanks for any help/advise you can give,
|Your 4 cells would start off at about 6 volts but have an end point voltage
of under 4 volts (possibly as low as 3.2v if you want to wring every last
mAH out of the cells. Chances are your LCD will not tolerate either extreme
of this range. A low power PIC will run at 3v but the 16F84 will
theoretically not do so at full clock speed.
As LCD current is liable to be minimal you could possibly use the PIC to
provide a simple voltage multiplier (very compact) to allow operation off v
low battery voltages. (1 PIC pin, 2 diodes, 2 capacitors, 1 x 5v, TO92
regulator (eg LM2936 which has very low quiesecent current & dropout
If size is more important than price you could consider using one of the
many 2 cells to "X" volts converters available. Linear Technology and Maxim
both make a range of these. Nat Semis 'simple switchers" could also be
useful. You can even get 1 cell to 5 volt converters but will get better
efficiency off 2 cells. 2 (0r 3?) x AAA plus converter could be no larger
than 4 x AAA and give superior results.
Batteries could perhaps be 2 x AA depending on the shape of your housing.
This solution has the advantage of giving you a wide usable battery voltage
An AA cell has slightly more than twice the volume of an AAA cell (216% for
one brand) so the battery volume and energy is similar. for 2 AA or 4 AAA
cells. A single AA and converter would have half the nominal energy content
available but the increased usable battery voltage range MAY be enough to
offset this depending on the voltage range you LCD will tolerate.
((Only real reason to use AA over AAA is probably the much better price and
availability of AA (at least tis so in NZ)))
A 9 volt "PP3" transistor battery may be a usable solution - especially
with an inverter.
No problem with low voltage at end of battery life :-).
> from a PIC output pin. This works fine and drops the IPD to a uA when its
> all turned off.
> But when it comes to running it from batteries (4 Alkaline AAA's) I am
> sure what to do.
> The LCD needs 5v (+/-5% or there abouts), the PIC and EEPROM etc are more
> flexible. So here is my idea;
> The PIC and IR LED run directly from the batteries, EEPROM runs directly
> from the PIC. The LCD is powered through a low dropout regulator with a
> standby input controlled by the PIC (same pin that powers the EEPROM) to
> switch it on and off. This prevents the regulators quiescent current
> is _relatively_ large) draining the batteries.
> I think this is a pretty good solution, but what do you think and what
> you one?? Minimal component count is important as the case is very small.
> it wise to run uC/EPROMs directly fom batteries?
> Thanks for any help/advise you can give,
Thank for the reply.
This is what I have decided on:
2 x AA, a LT1301 step-up regulator which will output 5v @ 200mA, the EEPROM,
LCD and PIC runs from this 5v.
The LT1301 has a SHUTDOWN input. I'll pull this up to Vbatt through a
resistor. (It's within spec). This can then be pulled to ground through a
NPN when any button (active high) is pressed thus starting the regulator and
the PIC, LCD, EEPROM.
The PIC will then pull the SHUTDOWN input low through the same NPN,
effectively latching it. So instead of SLEEPing, the PIC will just set its
SHUTDOWN output to low, the NPN stops conducting, SHUTDOWN goes high, and
its powered off.
In this configurating I will have a standby current of 17uA. Thats not bad
So the question is, any problem with this? Any problems with the transient
response of the LT1301?
More... (looser matching)
- Last day of these posts
- In 1998
, 1999 only
- New search...