| In the AND circuit, the
cathodes are held low by the "pull down" resistors.
The diodes are forward biased and the output is held low. If two of the diodes are taken high to reverse bias them, the remaining diode still holds the output low. A and B and C must all be taken high to make the output go high. |
| In the OR circuit, both
anodes and cathodes of the diodes are held low, and the output is low.
If A or B or C is taken high, then the associated diode is forward biased and the output goes high. |
Copyright Graham Knott 1999