"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 you can use: For BC548B = NTE123AP, 2N4401, 2N3904, PN100, 2N2222A, or TUN. For BC558B = NTE159, 2N4402, 2N4403, 2N3906, PN200, or TUP. 220nF = 0.22uF, 25V minimum. Any type or combination will do. 1000uF = 1000uF/25V, electrolytic. Any working voltage over 25V works.
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.
I get regular emails with complaints that there is no audio output when the button is pressed. As it turns out, the problem is always that a low-impedance loud speaker is used and although the circuit is working normally there is no audio or very little. Indeed it maybe difficult sometimes to obtain high-impedance loudspeakers, which probably is that by modern radio's the final audio stage is a transistor amplifier and they can provide a lot more current than a tube.
To accommodate those in that situation, below is a solution which will solve the problem by using a audio transformer
with a ration of 1:2 or 1:4 to 'up' the impedance. What that means is that the ohm's value at the primary side of this
transformer is about 82 ohms. That will accomodate about 100mA max and because the collector voltage is pretty 'nil',
it is impossible for the transistor to get hot. If it does, you have the incorrect transformer!