Using a 9V battery to power a 18F microcontroller

[micro9000]

Hello again, I am now trying to power my PIC18F4685 via a 9V battery. I am going to be powering 8 LEDs hooked up to RA0 to RA7. I have measured that the total current draw of the 8 LEDs is 110mA.

From your experience, what resistance values should I be using and what should the external resistive circuit look like? I have tried a few things, but I would like to hear your input on the matter. Thanks in advance.

 

[rajbex]

First of all you cannot power a PIC18F4685 with 9V directly as its operating voltage is from 2 to 5.5V. (Suggestion: You can use a voltage regulator IC (LM7805) to get a stable 5V supply for the PIC). The maximum source/sink current for the PIC ports is 25mA. If we assume that each of your 8 LEDs draw equal amount of current, then each of them will draw about less than 15mA, which is within limit. If I were running my PIC with 5V power supply, I would use 470 Ohms resistances in series with each of the LEDs .
Thanks.

 

[micro9000]

Yes, I realize that the microcontroller's input voltage should be around 5V, which is why I asked what type of resistive circuit you would use for a 9V battery going into the microcontroller.

EDIT: It works! I had a 7805 lying around much appreciated. I was originally looking into a resistive circuit, but this is a much better way to go about it. Thanks!

 

[Birdman Adam]

LM7805 has always been my old standby for regulating power to 5V.

With a 9V battery, you won't be able to power your MCU for long, because a 9V battery quickly goes down to ~7 volts. The 7805 will dropout around that point...

 

[micro9000]

I plan to have two 9V batteries in parallel for the extra mAh, I'll test how long that will last.

 

[AtomSoft]

you can use a LDO 7805 which is a Low Drop Out.... i would also test using a 5v zenor diode to drop voltage...... not sure if a Resistor Divider would be good.... depending on how much current is whole thing

 

[Wilksey]

Just a quick note, I have had a 7805 ACT? (100mA version) connected to a 9V Battery for over 2 years now, the PIC controls a CCTV Camera (Pan, Tilt, Zoom) Pelco D protocol, and although constant use isn't there, it is used a tool for camera engineers on site work visits fairly reguarly, and it is still going strong, it is a Duracell Battery.

I was quite surprised when I switched it on the other day and it still worked perfectly.

So for low current applications, that little regulator is very good in my experience.