Hi folks,

Trying to post again with IE as Firefox gave me a white screen on last 2 trys.

I’ve attached 2 circuit schematics showing just the wiring that my questions are about.

Basically I'm reading an SD card with a PIC microcontroller and sending that out via a PWM pin to an op amp.
The circuit will remain in sleep mode almost all the time. Once a day it will play a voice message (~40 sec.) when the capacitance switch is activated (CPS).
I do not want to compromise on it being touch activated but I want the batteries (CR2032 lithium cells) to last as long as possible.
I will be reading the SD as quickly as possible and deactivating it via the PIC in between.
The speaker is a 2.25 inch diameter 8 ohm speaker. I’m assuming I need less that a watt for the message to be heard at say 3 feet in a quite room?

Circuit 1 shows the, I don’t know, basic flow of things.
But the op amps I’ve seen will use a lot of micro amps just sitting there.

Circuit 2 shows some consideration regarding power consumption etc.

So here’s my questions and please reply if you can help me with anyone of them.

1) The load on the lower battery supplying the PIC / SD card is pretty low but will there be any balancing out of the remaining charge between the 2 batteries as it operates the op amp etc. or will the lower bat simply go dead first?

2) Does the PNP transistor in Circuit 2 need a weak pull up resistor. My thought is that I will configure the PIC port as high impedance input when I want the transistor off?

3) I’ve shown 2 resistors (Circuit 2) from the PIC to the op amp to shift the signal (the dotted line around the amp is my indication of the additional passive parts that will be associated with the amp). I know very little about analog circuits. I assume that a +/- supply on the amp with a differential output (an output not connected to ground?) is the way to go and that I need to center the input signal? What would be the best op amp to use? And do I really have any idea of what I’m talking about?

4) In general is there a better way to do this?

 

Read up here: http://en.wikipedia.org/wiki/Differential_amplifier
Scroll about 1/3 of the way down, and see the "Long tailed pair" schematic on the right.
Your schematic is missing the resistors, so your circuit won't give any output except smoke.

When the voltages are different between the two input signals, it's multiplied by the gain of the transistors. However, since you don't have any current limiting resistors, you won't get any output.
[eta]
Whoops, I wondered where this reply went..

 

(Apparently the Sarge posted his answer to another post here. Oops)

1) As the lower battery supplies more current it may go first, but that draw is tiny and if you got things correct about zero for 23.9912 hours a day anyway.

2) Yes it should have that resistor B-E. But for lowest current you should use a FET.

3) A simple cap would do the level shift for you to "center" the AC. I've actually got intelligible (means not great quality) audio by just putting a cap and a speaker direct on the PWM pin using a 5V device.

An amp with it's power switched would help lots but I don't know of any offhand. As you are in "cell phone" voltage territory I would expect them to be around, and they may even have a power enable feature built in.

Just checked quick: Nat Semi makes a LM4924 that may be useful, but it is a stereo output. They make 1 mono amp but it needs differential inputs, so it may be better to waste a channel on a stereo amp. Both have shutdowns that drop their current draw to 100nA.

 

Creating a Virtual Power Supply Ground

Bill's Index

 

Thanks ErnieM

You’ve given me some confidence that I’m not completely off base. I sort of understand the cap centering the ac as I guess the op amp side is simply seeing a cap going between charged and discharged or something like that?
All the recommended / typical op amp circuits show a cap on the input and I will be simply copying one of them (the one with the least amount of passive components).
I’ve been searching Mouser and Digikey for suitable amps in the 500mw to 1 watt output range but so far the best I could find still draws 72 ua at 6v in low power mode so that’s why I reluctantly added the transistor.

Thanks again.

Yours truly,
Colin Wilson
<SNIP>