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This is a drawing fragment of a unit that I have to extend a slave display from.

schematic

simulate this circuit – Schematic created using CircuitLab

Rough drawing of what I would like to add to expand this display to make it visable from 20 meters. With 10 3LED modules with their own "Device" switch element and not just the 2 that I have shown in the schematic.

schematic

simulate this circuit

What "Device" could you suggest I look at to complete this project. The old ciruit is switching an GND through the Darlington when we want the LED to light. The Darlington pair is in a buffer chip package that does not appaer to tie the Common to 5v. But I can tie the output of the Darlington to a pull up resistor with 5v or 12v. I have looked at many flavours of FET, MOSFET, BJT, depleted, enhanced, isolated gate, TTL schmitt invertors, op-amps, optocoupler devices.

To recap. Device must conduct when GND on Gate/Base and insulate when pullup voltage in effect and not require a large drive current as we have 500mA to share with 10 LED module switches.

kingchris
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  • We need to know more about the **"Device"** switch element inside the modules you intend to drive. Can you give us a data sheet for a module? – glen_geek Oct 25 '22 at 14:27
  • I am asking help on choosing a device from all the modern FET, BJT etc. When I learn't my electrics. Pentodes and Triodes were the in thing. Transistors were around but thought of as low power. So the device must be able to switch 120mA minimum. Conduct on a GND input and insulate on pullup voltage of 5 or 12v – kingchris Oct 25 '22 at 14:31
  • @kingchris Just about any small transistor, BJT or MOSFET, would be able to handle that. Since you want it to be on with a grounded input, the easiest thing to do is to use two of them, one as an inverter to drive the second. (unless you happen to have a spare inverting gate handy in your "TTL" block) – Hearth Oct 25 '22 at 15:45
  • Though personally, I'd put the additional devices in parallel with the Darlington transistor that's already there. If you replace the Darlington pair with a modern MOSFET, you might be able to get around any output current limitations of the "TTL" block. – Hearth Oct 25 '22 at 15:48
  • OK @Hearth. I can't change the Darlington as I get an extention cable with GND, 5v and three leds Darlington feeds that switch to GND various status lights from a wall mount breathalyser. But two small Mosfets should work. I modeled a N type and P type mosfet in LTSPICE but the one device switched at 1v the other at around 10vs which I can't easly generate as the Darlington feed floats when not grounded. I could try a 12v pullup. – kingchris Oct 25 '22 at 16:18

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