Output pin drop too high

[George]

I've built a remote controller using an Linx LR series transmitter, and while works, I have an issue - the controller goes to sleep when not used and I power down the transmitter with the PWRDWN pin, I'm also driving an LED with the output pin of the PIC (16F676) the datasheet says that logic high limit is Vcc - 0.25V however the PIC is dropping 0.62V on it's output pin. I was thinking about powering the Module through a diode to drop the Vcc but am only running it on 3V when the batteries (2 x AA) are full, so thought it may compromise it's battery life too much.

Is there any simple way that anyone can think of to enable me to power the PWRDWN pin at closer to Vcc? It will work at a pinch if I am not powering the LED, but unfortunatly the LED serves a dual purpose as I've run out of pins on the PIC and I just illuminate that LED when I go to transmit.

Thanks in advance

[mackrackit]

George,
I would not worry about the battery life.

I am working on a system just by chance using the same PIC and transmitter. My application is what I call a laser fence. The PIC with the transmitter has a photo diode looking for a 10 Hz pulse, if any thing other than the 10 Hz pulse is detected the transmitter will then transmit. While the pulse is correct there is a LED blinking, when the beam is broken or not the correct pulse the LED will be on continuous.

The whole thing is running at 3 volts or less on batteries. Four AA rechargeable.(series / parallel ) The PIC is running a 4 MHz external, never goes to sleep. The transmitter VCC going to LADJ and PDN pins all of the time.

I too was wanting to know how long the batteries would last as this will be remote and on solar when complete. About three weeks ago I started my test with out the solar with hopes that it would all run for two or three days on a charge. As I write this there is still 2.21 volts in the batteries. At full charge the NMH batteries had 2.6 volts.

I know this does not answer your question, but do not worry about powering the PDN pin from the PIC. I got three weeks plus with four batteries, that should work out to 1.5 weeks plus with two batteries.

The answer to your question would be a low gate voltage P channel mosfet. Fairchild should have one but I do not know the part number. I know their FDB7042L N channel works well at these voltages.

[Darrel Taylor]

Maybe you could use an NPN transistor to drive the LED. 2N3904 or similar, with around 5.6k resistor to the base.

It'll keep the LED from pulling the voltage down.

_

[George]

Thanks, nice and simple - works too!!!