A sensor for producing an electrical signal parameter responsive to the mean square of an applied alternating voltage has a rectifier element, and has a zener diode element electrically in series with a light-emitting diode element across the direct voltage which the rectifier element produces. The polarity of the direct voltage applies reverse voltage to the zener diode element and applies forward voltage to the light-emitting diode element. A photoconductor is optically coupled to receive electroluminescense which the light-emitting diode element emits.

A common mode rejection coupler having a pair of optical isolator circuits for receiving input signals which are the complement of one another. The optical isolator circuits generate output signals which are the complement of one another and which are normally isolated from a common mode voltage existing between the input and output of the isolators. The complementary outputs of the optical isolator circuits are shaped and one output is inverted so that both signals are in-phase with one another. The signals are coupled to a voltage divider circuit the output of which is coupled to a Schmitt Trigger. The output of the Schmitt Trigger is in a first state when both signals are high and in a second state when both signals are low. Should a spurious voltage spike be generated at the output of one of the isolator circuits due to a changing common mode voltage, the resulting input signal to the Schmitt Trigger will not change a sufficient amount to cause a change in state of the output of the Schmitt Trigger.

A method and apparatus for testing a plurality of electrical circuit devices (100a . . . 100n FIG. 4) to determine individual device failures wherein the input, (110-117) output (118-121) and power leads (130) of each device are interconnected with corresponding input, output and power leads of the other devices to form common input, output and power leads. The ground lead (140 . . . 140n) from each device is individually connected with test apparatus (102). Test signals are introduced onto the common input (110-117), output (118-121) and ground leads (130) and signals, responsive to the test signals, are monitored on the individual ground leads (140 . . . 140n) by test apparatus 102 to determine individual device failure.

An input signal discrimination method is provided wherein the on/off state of an AC input signal is discriminated. An AC input signal (1) is input to a processor (21) through a photocoupler (13). The processor (21) counts pulses in a time for which the AC input signal (1) remains above a predetermined threshold level, and a time for which the signal remains below the predetermined threshold level. When the number of pulses counted while the signal remains above the predetermined threshold level is larger than a previously calculated number, it is decided that the AC input signal is on, and when the number of pulses counted while the signal remains below the predetermined threshold level is larger than a previously calculated value, it is decided that the AC input signal is off. Consequently, the on/off state of the AC input signal can be accurately discriminated in a short period of time.

Disclosed is an input circuit using a photo-coupler for circuit isolation which comprises a photo-coupler and an indicator for indicating the presence of an input signal both of which are adapted for an AC input signal as well as a DC input signal of either polarity. Typically, the input end of the photo-coupler comprises a pair of light emitting diodes which are connected in parallel with each other directionally opposite and the indicator comprises a pair of light emitting diode chips which are connected in a similar manner. Thereby, the input circuit can be adapted to input signals of all kinds and the indication of the presence of an input signal is accurate because the indicator is directly driven by the input signal and is not affected by the condition of any external circuit.