Design Ideas: February 15, 1996

The simple circuit in Figure 1 monitors multiple (15) pairs of relay contacts and provides visual and electronic indications of contact closures. A series-connected LED, current-limited by a 2.2-kOhm resistor, provides a visual indication of each contact closure and remains lit while the contact is closed. The circuit comprising a current mirror (Q₁ and Q₂) and comparator IC₁ provides an electronic indication (a pulse at IC₁'s output) of each contact closure.

When a set of contacts closes, the current mirror reflects a fraction of the corresponding LED path down the D₁-R₁ branch of the circuit. The voltage at the noninverting input of IC₁ rises immediately, while the voltage rise at the inverting input incurs a delay because of the RC network connected to that input. Because the noninverting-input voltage exceeds the inverting-input level, the output of IC₁ switches to a high state. When the inverting-input voltage eventually exceeds the noninverting-input voltage, the output returns to a low state. The width of the output pulse is approximately

where [delta]V is the change in voltage across resistor R₁ for each contact closure, and V_D is the voltage across diode D₁. A voltage pulse is thus available at the output of IC₁ for each contact closure. To ensure the detection of each additional contact closure when more than one set of contacts closes, the following is a necessary condition: At closure N, the current reflected down the D₁-R₁ branch must produce a voltage across R₁ (VR1_N) that's greater than the voltage stored on capacitor C (VR1_N-1+V_D). When all relay contacts are open, resistor R₂, via the Q₁-Q₂ current mirror, maintains current through the D₁-R₁ branch and subsequently sets IC₁'s input voltages to maintain a logic-low output. (DI #1830).