Super-Simple Intercom

I designed this, surely the world's simplest intercom. The circuit was published worldwide -- and eventually the copyright reverted to me. One could use it to connect a home to a workshop, say, or a baby room with a kitchen. This offers excellent performance, though with a trebly tone. The intercom works on the master-slave (base-remote) principle, where the person with the master unit has total control. The person with the slave unit needs to speak and to listen according to whether the master wishes to speak or listen! The circuit is so designed that the remote unit can be wired up by means of a simple twin-flex cable. The sensitivity of the circuit may be significantly reduced by omitting C1. Note that with C1, there will be feedback (squeal) if the piezo cone tweeters are too close together. IC1's current consumption is about 10mA, so that this circuit would ideally run off a 12V DC plug-pack power supply. OBSERVATION: This can also be used as a window or door alarm. Mount each piezo cone tweeter on opposite sides of a closed window or door, and as soon as you open, there is the shriek of feedback.

POSTSCRIPT: One can obtain a pair of simple piezo cone tweeters* from Amazon -- sometimes difficult to locate. Amazon also have a pack of five LM380N IC's*, and DPDT switches* sold in twos -- one of them can be used for S1, the other for S2. You could look at this twin conductor wire* from Amazon -- 100 feet. Amazon also has the capacitors -- in big packs! Pull these from some junk electronics if you can.

*This is an Amazon affiliate link. I earn a commission, at no cost to the buyer.