This is the next step up from my last circuit on this subject of basic simple 2 way communication. It's a good circuit if even just used for practicing with the Tap Code as mentioned in one of my previoius videos.
The "lamps" shown in the circuit are actually LED's (light emitting diodes). The diode action in the LED's is also used as a circuit element, and is necessisary. You cannot substitute regular "lamps" or a "buzzer" for them. The diodes prevent current in one direction and allow it in the opposite direction. The arrow on the diode symbol indicates the direction of "positive or conventional/standard" current.
I did not build the circuit (UPDATE: I did build a test circuit and it does work, try to use all similar LED's, and when both switches are closed, the "SEND LED signals" S1 and S2 will be brighter, V used is about 4V min) so I am taking suggestions and comments / improvements if it will even work. To me, it seems like it should work. The main improvement is that no "holding or steady" current is required as was in the last / more simple circuit. This saves power.
A potential problem is that when both switches are closed there may be excessive current through some of the "indicators" (light, sounder). Resistors can be used to limit current in a safety sence so that the "indicators" are not ruined, this seems to be the method to try first. Possibly only one resistor is needed in one of the the main lines. I think it would be better to put a limiting resitor in each "communication box/station box" making them more portable, etc. When power is low, maby the resistor can be bypassed with a switch. Zener diodes can also be considered. One battery in the circuit is possible if it is basically placed in series with the communicaion cable/wire.
For a more advanced type of circuit, where very low power is used for the transmission, transistors that (can be controlled by a low signal current) can be utilized in place of the "indicators", for their switching/relay action to control a higher power circuit which will contain an "indicator" within that circuit.
I guess the next step up from this circuit (though basically unrelated to the tap/morse code) is to utilize transistors at each station to amplify weak electric pulses (ac waves of audio) of speech in a microphone/speaker or even recorded things input into a (electric equivalent of) sound input of the system. Overall, this would be like an "intercom" system that businesses often utilize, but on a longer scale. For audible systems, electronic noise/radio waves induced onto the transmission lines might be an issue; perhaps a small high voltage capable capacitor across the lines could remedy this, also I guess it would help alot to use a high (reverse) voltage capable diode in each station box at the input of the transmission line.
If you closely look at the "schematic diagram" or "electronic circuit diagram/drawing", you will notice that for each station that the Send and Receive indicator lights are really in parallel to each other (when the switch is closed), however those lights are not meant to be "on" during this condition though.
Telephone companies use a "twisted pair" of two wires to transmit the telephone converstation back and forth. Twisting the wires helps eliminate noise, but it also introduces "capacitance" into the circuit which will limit high frequencies on the line.
For systems with long runs of communication cable it is possible to do what the utility company does, and this is basically to transmit a high voltage rather than a high current that may or may not be available. They use (voltage) transformers to do this, and the also change the current in the process, but in a equally reverse multiple. I think (possibly, not too sure yet) a "relay sounder" is basically a relay that uses this transformer theory, but the transformer primary is contained withing the relay, and the sounder is probably a secondary connected to a solenoid whose metal "plunger" makes the tap sounds.
Other than "land line"/wire circuits, it is possible to build a simple AM radio transmitter with a narrow band that takes up hardly any space on the dial/bands and use Tap/Morce code and use an existing AM radio as the receiver. Up to you, lots of posibilities to experiment with wireless and/or longer distance communications. Just keep the power low and make sure you don't broadcast on the same channel/frequency as another station due to the laws of the FCC to keep communications regulated for our enjoyment.
I think USB data transfer is a two-wire scheme/method; probably one "bit" at a time for serial data communication. The other 2 pins on the 4-pin USB are for the power. It's much cheaper to have 1 or 2 wire communications rather than try to use 8 or more (communication) lines to transmit a whole byte (8 bits) of data at the same time in a "parallel transmission scheme".
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