My second (and this time successful) attempt at what I naively assumed would be a straightforward thing to implement: to test when two voltages are 'equal':
The INA128 (which has very low offset voltage) is configured to give a gain of about 500; any significant difference in X and Y input voltages gives rise to a significant voltage (positive or negative) - the TL071 op amp inverts the output of the INA128; the output of this op amp and the direct output are put through the Schottky diodes - hence regardless of the sign of the voltage difference, the base of the first transistor is driven high when X does not equal Y. The base of the second transistor goes high when X = Y, as does its emitter!
The actual circuit has two independent X = Y channels:
In the scope display below, the horizontal axis is 20 microseconds / division; the triangle wave (i.e. X) (5 kHz) is 20 mV / division; Y = 0 volts. The X = Y output (which appears on the computer's front panel) is 0 to 10 V. In practice, the circuit detects X = Y when X is within +/- 10 mV of Y, for X and Y in the range -10 V through +10 V.
The DG403 chip (shared between channels) provides normally open and closed switching - these outputs also appear on the front panel.
