HOBBY CIRCUITS

Resistor key

This circuit turns on and off a relay when two resistors of a certain value are detected. The EXOR gate number 9 takes the input signals from a resistors network. By using a resistor of about 470 Ohm, an high signal is detected at the node A, while the signal at the node B is still low. If the key resistance is less than 470 Ohm both input signals are high. The EXOR gate number 10 works in the same manner. The logic gates network is supposed to latch the relay. When the gate number 11 gets high the output signal switches on and keeps in that condition due to the OR gate number 1. When the input signal backs low the gates number 5 and 6 turn on. The next input will trigger the reset signal through the gate number 7.
Resistor key (pdf)