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I am new to electronics, and am playing with some ICs (555, 74393, 74595 and logics like 7400) on the breadboard.

To see what happens, I would like to put LEDs at various places in the circuit. But I don't want to drain the circuit so that the logics are altered. I believe I could use MOSFETs for the LEDs, but that would use very much space, so I was hoping for a more compact solution.

This would be one application, but what I really wonder is how to pick the logical state (high/low) voltage of several nodes, and in a compact way, draw more current from these than these nodes than they can provide.

After some googling I found the 74HC541 (Octal Buffers and Line Drivers With 3-State Outputs) and the 74HC9015 (nine wide Schmitt trigger buffer/line driver), that look somewhat promising to me, to put between the circuit nodes and the current consumers.

Are these suitable for that purpose, or are there other ICs that would be better?

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    \$\begingroup\$ If you can see a good LED On at 3 mA then make a logic probe with Red LED and 1k series or where needed. THis will have minimal effect on the output voltage with a near 50 Ohm like driver impedance for any 5V logic output \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 2 '18 at 17:09
  • \$\begingroup\$ Possible duplicate of Can I use blue-green LEDs as MCU state indicators on 3.3 V power? \$\endgroup\$ – Ale..chenski Aug 2 '18 at 19:15
  • \$\begingroup\$ You might want to consult with electronics.stackexchange.com/users/124009/misunderstood regarding which high-efficient LED might be most suitable for the task. \$\endgroup\$ – Ale..chenski Aug 2 '18 at 19:17
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    \$\begingroup\$ Comment because it's not a real answer: You have one problem, your solution of adding LEDs, now you have two problems: you gotta debug your LED driving elements in addition to your circuits. Instead, if you want to see what is happening, get an oscilloscope, an instrument designed to provide that visibility. \$\endgroup\$ – whatsisname Aug 2 '18 at 20:23
  • \$\begingroup\$ I seem to be able to see my LEDs just fine at 0.25mA or less (5V/20k). Of course they're not very useful for lighting things up but you can still tell they're on. Just use a fairly big resistor (like 20k) and it won't draw much current. \$\endgroup\$ – user253751 Aug 2 '18 at 21:41
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If your worried about altering the state of a circuit, then use a comparator or op amp to drive the LED. This is called impedance buffering. The input impedance is usually in the uA's or less even down to the pA's (you can find out by looking at the input bias current in the datasheet, this is how much current leaks into the input ports of the amplifier). So that is like adding a resistor to the output of your gate with a resistance of 1MΩ or higher. For a gate that is sourcing 10's of mA it won't even notice.

For digital circuits even using gates to buffer can be quite common, make sure the gate can source enough current for the LED (check the datasheets for both) and make sure you add series resistance between the LED and whatever your using to drive it with.

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  • \$\begingroup\$ This is a good solution. For breadboard work especially, you can make a small separate board with wire lead probes or screw terminals. If you want you can also give it a separate power supply like a 5v usb charger and put on a potentiometer to control the threshold voltage(s) on the comparator. It would be an excellent and cheap tool for monitoring state engine type logic circuits. I also highly recommend getting a cheap digital oscilloscope. I got pretty much the cheapest one I could find and it's extremely useful. \$\endgroup\$ – K H Aug 3 '18 at 6:25
  • \$\begingroup\$ Thanks. I think I will get some buffers. If I can read the datasheets correctly, the 74ac541 can draw 100mA total, but slower than the hc equivalent. I will also get myself a cheap oscilloscope. \$\endgroup\$ – harelabb Aug 3 '18 at 9:32

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