>> The reason it looks complicated is because you are using a 
>> current-amplifying device (the BJT) to amplify voltage.
>
> Uh... does it matter?

Well yeh, that's why the resistors are needed.  Generally you want to 
amplify voltages, not currents.

> Let's assume that a positive potential represents logic high, and a zero 
> potential represents logic low. Take a pair of relays connected up so that 
> by default the circuit is closed, and energising the magnet opens the 
> circuit. (This is easy to arrange with 3-pin switch type relays.) Connect 
> the relays in parallel between the positive rail and the signal output 
> terminal. Connect each input to one side of one magnet coil, and connect 
> the other sides to the negative rail. Done.

I don't understand what is connected to the negative rail?

Don't forget that the output must be connected to either the negative or 
positive rail (and not both!) under all input conditions.  It sounds to me 
like in your circuit, the output will just be floating (ie not connected to 
anything) when you charge both relays.

Post a reply to this message