Perhaps you are. Nothing you've said eliminates the need for the Schmidt-triggers. Without them, you have no vehicle for sensing your relay contact closures with any sort of noise immunity. You were concerned about noise. The optoisolators don't help at all with that. They could, in fact, make things worse. Moreover, they are costly, considering that they contribute no benefit. The relay contacts alone already can provide electrical isolation from the sending circuit. All you have to do is to remove the +12-volt connection and source the current from the receiver instead. You won't need very much current, but, as Kai has already pointed out, you do need some current. If you simply provide a +5 volt connection at the receiver, through a 1 k-ohm resistor, that should be adequate, though, if the cables are very long, > 10 meters, then perhaps a lower value might be indicated. The lower the resistance, the more energy has to be in the noise spectrum, in order for it to be a factor. Perhaps the best thing would be to attach your relay contacts to the receiving station and sense it at a resistor to GND. When the voltage is high, and you'll have to determine that based on cable length and cable characteristics, the Schmidt-trigger inverter will output a low level. If you want to be precise, you could use a higher voltage supply as you've already suggested you're doing, and a constant-current source. That way, you can closely control the voltage at the receiver. I doubt that's necessary, however.



Erik has pointed out that you haven't been clear about your operational requirements at the receiver. Do you need only one common acknowledge switch to acknowledge all the inputs or do you need one for each remote station?

If you intend to have one 7-segment display at the receiver for each transmitting station, you still only need 7 segments and one digit select for each display. That means you need 8 lines per display, though 7 of them can be common to all 10 displays. How the display segments are driven will depend on the requirements of the individual LED displays. The MCU will probably not be sufficient to drive the segments, and certainly won't be adequate to drive the digit selects, assuming common cathode displays. If they're common anode, you probably will need even more buffering.

I'd suggest you look at 7-segment decoder/drivers, and digit selectors such as 7445 just to get an idea of how this can be done. If the displays are common-cathode, a 7445 will drive the digit selects nicely, and a 7449 will drive all 10 display segment sets just fine. If your displays are common anode, then perhaps 4511 or 7447 might work for you. It's probably less costly to use one of these devices, intended for this purpose, than to use some discrete buffering approach. The 7445 will easily sink the current from all digits in common cathode displays, and, if the common anode is buffered with a PNP (e.g. 2N3906) transistor per display, it will perform the needed digit select function. Presumably you'll time-multiplex the displays so one 7-segment decoder/driver is all you'll need, together with a series resistor pack and a single digit selector. Four lines from the MCU will drive the digit selector, and four more will be needed to drive the 7-segment decoder/driver. That's only eight lines from the MCU. There are many ways in which this can be done, but these provide a reasonable example.

RE