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Looking for a DC circuit to be used for Morse transmission in scouts. Visible elements: one, preferably two, buzzers (or lamps, i.e. LEDs) and a button (station A), connected with no more than two wires to a bus with more similar stations (stations B). They work like this:

  • Pressing button on station A will light the lamp in all stations A and B
  • Not pressing any button will not light any lamp in any station

However, it should allow for an "infinite" number of units to be connected in parallel, i.e. each station has its own power source to amplify the incoming signal.

A possible solution could be as follows.

schematic

simulate this circuit – Schematic created using CircuitLab

Another proposal, by Loganf, was simplified to this. But how does he plan to have the three wires? What is the idea of having a MOSFET on the transmission side?

schematic

simulate this circuit

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    \$\begingroup\$ A minor point: a buzzer is better than a lamp - it's much easier to read morse by listening \$\endgroup\$ – Chu Sep 4 '15 at 9:04
  • \$\begingroup\$ (PeterJ 2 hours ago):[The first possible solution] generally looks OK assuming the voltage is really something higher like 12V. You'll also want something like a 1K current limit resistor at the base of the transistor. \$\endgroup\$ – Jonas Sep 4 '15 at 12:30
  • \$\begingroup\$ The voltage could be around 12 V, maybe rather 3 V though and the current entering the base of the transistor could be very small, if there are many connected stations and since the cable has also some resistance. \$\endgroup\$ – Jonas Sep 4 '15 at 12:30
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    \$\begingroup\$ @Jonas The current "entering the base" can be quite large at times. You never know what kind of voltages will get induced in these wires. Usually because of the ESD but also a lightning strike nearby. It's better to be safe than sorry. \$\endgroup\$ – Alexxx Sep 4 '15 at 12:51
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Hmm, if you are making it simple then I suggest the following circuit for each station. They will all be the same and connected in parallel up to the supply and cable loss limits that you have.

All devices on the bus will be signalled and your own buzzer will not sound.

EDIT:
The LED can draw from 5 to 15mA (visible in the dark, or daytime), the Buzzer from 1 to 20 mA (Piezo or electromechanical) so with the regulator quiescent current (say 1mA) you could hope to keep it under 20mA total per station.

If you use alkaline batteries intermittently you could expect to supply 100 to 500 mA current, allowing a maximum of 5 to 25 stations depending on cabling and battery quality.

With 6 stations each station could transmit for about 5 hours (500mAh capacity) or a total of 30 hours of buzzing (at 100% duty cycle many days in practice), that is a very long field exercise, the scout master would go nuts from the noise before the batteries failed. With 26 stations (using good quality batteries) you would be limited to 1 hour of buzzing by each station, that is still a long field exercise.

The batteries would fail first in the station that transmits the most, if it was HQ then they could keep a spare battery on hand or use a larger one. Other stations might go for a long time with standard small PP3 alkaline type if they make shorter responses.

When a battery fails you only loose the transmit capability and can still follow a recall order or make arrangements to series connect batteries with adjacent stations to double voltage if running low. If the LED was connected before the voltage regulator every station would be able to visually gauge the relative battery capacity and report on this (a small moving coil meter could be used instead) to practice signal strength reports, with distance units or those with depleted battery capacity providing a lower signal. Placing the LED onto the common point of the switch would let each station monitor themselves and their own battery status and shorts on the line. A mute switch for the buzzer would allow for operation in the quiet.

Many automotive grade voltage regulators will be hardened against reverse voltage (bus or battery), over temp, short circuit, over voltage, some static discharge and more likely to handle field conditions better than a transistor as a first line of defence to the outside world.

schematic

simulate this circuit – Schematic created using CircuitLab

Modified schematic with monitoring:

schematic

simulate this circuit

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  • \$\begingroup\$ Kiiti Kalle, I have taken some ideas to edit the question. \$\endgroup\$ – Jonas Sep 5 '15 at 8:21
  • \$\begingroup\$ Main problem is, that your battery has to power all other stations. \$\endgroup\$ – Jonas Sep 5 '15 at 9:29
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It sounds like you need to separate the power and signalling. usually "multiple transmit" lines have a dominant (driven) and recessive (idle) state, like I2C or similar. for example:

schematic

simulate this circuit – Schematic created using CircuitLab

this just shows one direction. you would need 3 wires to reference the signals, ie, Tx, Rx and 0V.

... I haven't sized the resistors or voltages. D1 is your buzzer

edit: of course, there will always be a limit to the number of stations, in the above schematic, its limited by the base current in Q1 vs the drain current in M1. with a big mosfet, the limit will be the voltage drop across your connecting wires

not sure why AC is mentioned in the question?

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  • \$\begingroup\$ Thanks Logan, please see my edit in the question, where I have reproduced your circuit too. \$\endgroup\$ – Jonas Sep 5 '15 at 9:25

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