On Wed, Jun 10, 1998 at 08:18:51PM -0400, Weaver wrote: > 1) PICs inputs/outputs are all TTL except for a few shmitt triggers in > weird places, right? They're TTL compatible, but they're not TTL. This just means you can feed them with TTL level signals and expect it to work. > 2) Is there somekind of simple circuit or IC that converts a varying > voltage to a current? I need this to use with a DAC that outputs voltage It's called a resistor :-) > and I want to drive a speaker from it. I am reasonable sure that most > normal speakers run on varying current. They do, but you achieve the varying current by applying a varying voltage, and the voltage divided by the impedance of the speaker equals the current that flows. So don't think of speakers as current operated, you will only get more confused. Your DAC might need an amplifier to be able to supply the current necessary for the speaker, but the amplifier will amplify voltage, not current. > 3) Could someone please explain negative voltage? I am confused. A negative voltage is one that is negative with respect to the reference that you measure it against - put another way, a voltage is always a voltage between two points, and conventionally ground is used as a reference point and other points are measured against it. If the other point has a potential (voltage) that is negative with respect to the reference, it's a negative voltage. But if you choose a different reference point, it might be positive with respect to it. These days, it's common (and convenient) to run most stuff from a single supply voltage which is invariably positive, but analog circuits sometimes use both postitive and negative supplies, referenced to ground. This allows small DC signals that are near zero to be measured and amplified easily, whereas it is more difficult to work with signals that are very close to the lowest or highest supply voltage. -- Clyde Smith-Stubbs | HI-TECH Software Email: clyde@htsoft.com | Phone Fax WWW: http://www.htsoft.com/ | USA: (408) 490 2885 (408) 490 2885 PGP: finger clyde@htsoft.com | AUS: +61 7 3354 2411 +61 7 3354 2422 --------------------------------------------------------------------------- HI-TECH C: compiling the real world.