"Baby-monitors, baby-phones, or simply intercoms are for sale everywhere in a variety of models and colors. Some work on AC, others wireless or just via a little wire. We all have our preferences. Just in case you're looking for a VERY reliable Intercom, the circuit below will suit your needs." Replacements: BC548B = NTE123AP, or TUN. BC558B = NTE159, or TUP. 220nF = 0.22�F 1000�F = 1000�F/25V, electrolytic This Intercom is powered by two 9volt batteries and uses only current when the Intercom is used. Both units are connected via a two-wire little cable or simply two wires (dotted lines). The loud speakers act both as loudspeaker and as a microphone. When you press S1 and speak into the loudspeaker then this signal is amplified by the transistor stage and made audible in the right loudspeaker and vice-versa. An added benefit of this system is that when the switch is pressed it is quiet, not even annoying noise. This circuit has worked for me to my full satisfaction for many years now. If you have questions about this circuit, please direct them to Jan Hamer or visit his website in the Netherlands (if you can read Dutch). Circuito baixado de: www.uoguelph.ca/~antoon