On Mon, 18 Jan 1999 00:24:45 -0800 Gerhard Fiedler writes: >how's an opamp with a diode as a precision rectifier, followed by a RC >low >pass, and then into a schmitt trigger input of the pic? I would try a simple circuit first, then make it complicated only if necessary. First you may need to add an an amplifier to get a signal of 3 or 4 Vpp. Apply the signal through a capacitor and 100K resistor in series to a PIC input pin. Connect a 1 M resistor from the pin to ground. With no signal, the 1 M resistor holds the pin low. When signal is present, the protection diode helps rectify it so peaks reach the high threshold of the pin. Software samples the pin rapidly and if it reads high, starts or resets a timer that indicates voice is present. In effect the software acts as another rectifier diode, low pass filter and shcmitt trigger combination. PIC pins will tolerate "analog" inputs OK. The PIC may draw a couple mA more Vdd current due to the partly biased pin, but only while signal is present. ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]