Circuit Question

[russosv]

Hi all!

I'm new to this forum (and to electronics), so I apologize if this question has been asked before. I'm not sure what you would call this type of circuit, so I wasn't sure what to search for.

I've built a very simple alarm clock using a PIC18F4550, with a 4-digit LED display using multiplexing, and a piezo buzzer for the alarm. I'm using an 7805 voltage regulator to bring 9V down to 5V (though I'm sure there are much better options).

What I want to do is add a 9V battery back-up so that I can run the clock off of either a wall-wart transformer or the battery, in case the power goes out. I'd like to connect part of the circuit to a pin on the 4550, so that I can "sense" whether the battery is being used or not (if it is, I'd like to reduce the duty cycle of the LEDs via programming).

Can someone suggest a circuit to do this? I was thinking of using a MOSFET but I couldn't get the circuit exactly right.


Thanks!
Steve

[bodgy]

See attached file!

Colin

[sghioto]

Steve,

This circuit will do the job. The voltage from the adaptor ( wall wart ) should be at least a volt higher than the battery for proper operation. The input pin on the 4550 will be high when running on battery power.



Steve G

[bodgy]

I thought I still had the schematic for the attached circuit including when I had power sense to it.

There are a number of ways you can sense the voltage, the simplest and least amount of ocde reuired would be to have a resistive divider across whichever dc supply you are measuring. Dimension the resistors such that the Pic will see a 'low' once the supply voltage has gone below the value you require. The code then just has to poll that pin and you can check from a high to low and whatever you need.

If you want a more intelligent or want to track the voltage then using the divider just to keep the voltage in range of the chips ADC inputs, you can get a real reading of the voltage at all times.

Colin

[russosv]

Thanks Colin and Steve G!

Steve G, I used your circuit because I had all of the parts lying around... and it works like a charm! I currently have an LED being turned on by the 4550 when the clock is plugged in, and off when the battery is running. Thanks very much for your help!

Regards,
Steve R

[dyarker]

And if you put a momentary contact switch from the 4550 pin to common, you can check the time when running on battery.

[russosv]

Ahh, it took me a moment to get what you were saying. To clarify, I just added a single "status indicator" LED to show whether the clock was running on battery or outlet power.

I had planned to just lower the duty cycle on the clock display when running on battery, however, your suggestion makes a lot of sense since the LED display is probably sucking the most current out of the circuit: I could completely turn off the display when on battery power, and use momentary PB to show the time when needed. I want to make this a portable clock, but I still may add this as an option. Great idea-- thanks!

Steve