Apparatus for automatically indicating whether or not a test joint in a circuit is above or below a predetermined reference potential

Claim 1

In a logic circuit testing apparatus, the combination comprising: means for impressing a rectangular wave on the input of the circuit under test; a checking circuit having an input; movable conductor means for connecting said checking circuit input to any one of a plurality of different conductors in the circuit under test, said checking circuit including a first comparator having first and second inputs, said first input being connected from said conductor means; a reference voltage source connected to said second input; means to indicate the output of said comparator; said rectangular wave being a voltage wave having equal periods of high and low amplitudes, said conductor means including a movable conductive probe having a bare conductive end adapted to be placed in contact with various different conductors in the circuit under test, said checking circuit including a resistor connected from said probe to one end of a series circuit including two zener diodes poled in opposite directions, the other end of said series circuit being grounded, said reference voltage source including a source of potential having a grounded negative terminal, a resistor connected in series with a potentiometer from the positive terminal to ground, a zener diode connected in parallel with said potentiometer and having its anode grounded, said potentiometer having first and second wipers, said first wiper being located at a higher potential than said second wiper, a double-pole, double-throw switch, said switch being adapted to connect said first wiper to said second comparator input and to connect said one end of said series circuit to said first comparator input substantially simultaneously while disconnecting said second wiper from said first comparator input and disconnecting said one end of said series circuit from said second comparator input, said switch, conversely, being adapted to connect said second wiper to said first comparator input and to connect said one end of said series circuit to said second comparator input substantially simultaneously while disconnecting said first wiper from said second comparator input and disconnecting said one end of said series circuit from said first comparator input, an amplifier having an input and an output, a dioDe connected from the output of said comparator to said amplifier input, a capacitor connected from said amplifier input to ground, and a lamp connected from the output of said amplifier, said diode being poled in a direction away from said amplifier.