Vitaliy <spam <at> maksimov.org> writes:

> What is the most straightforward way to implement this?

Low cost quad opamp (LM324 etc) and input bias network with reference
slightly above ground (exacly like line receivers have - e.g. 1.5 to 2.5V 
or so). The  opamps are used as voltage repeaters or slight dc amplifiers 
(not as comparators). 

When there is no input (wire not connected) the opamp input is held at the 
reference voltage and the output becomes 'center' so the bidirectional led 
is off. Most (all) the resistors can be made identical and thus part of a 
DALE etc array.

Alternate way to do it (5 channels per IC):

                               LEDs 2mA

                            +----|<|---+
                     |\     |          |         /|
Signal o---1M----*---+ o----*----|>|---*--*-B---o +---+
                 |   |/     |          |  |      \|   |
                 |          |          |  |           |
                 +----1M----+          |  +----1M-----+
                                       |
                                       v
                                     other channels

The inverters are CMOS4069B or similar. B is a PNP/NPN push-pull current
booster, needed for more than 2 channels. Alternately use several gates in
parallel for the bias source. E.g. 4069 can use 3 gates as bias and 3
channels, gives 3 channels per chip.

Peter P.


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