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?
Best Answer
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.