Variable-reluctance sensors are preferred for industrial and automotive environments, because they sustain mechanical vibration and operation to 300°C. In most applications, they sense a steel target that is part of a rotating assembly. Because the unprocessed signal amplitude is proportional to target speed, a sensor whose signal-processing circuitry is designed for high speed ceases to function at some lower rate of rotation. Hall-effect sensors are preferable for speeds of several pulses per second, but they require the attachment of a magnet to the rotating assembly. They're thus prone to failure when the magnet is broken or damaged. Neither variable-reluctance nor Hall-effect sensors offers fail-safe detection of the processed signal in the event of failure in the cable or sensor. The circuit in Figure 1 is a fail-safe variable-reluctance sensor for low- to medium-speed operation.

The circuit comprises L₁; R₁; and a quad RS-422/RS-485 receiver, IC₁. It provides the complementary, independent output signals V_OUT and . Table 1 lists the resulting fail-safe modes. The supply voltage can be 10V, 12V, or the control system's 24V-dc source. Coil L₁ consists of 2600 turns of #32 magnet wire wound on a 0.8-in. steel bar of 0.2-in. diameter, with 0.125 in. protruding from the sensor face. A magnet attached to the back of the steel bar supplies the necessary magnetic flux. The rotating target causes a change in reluctance and, hence, a change in the amount of magnetic flux conducted. This change produces a corresponding change in the current induced in L₁. R₁ converts the L₁ current to a time-varying voltage. This voltage goes to the inputs of IC₁, whose input-voltage range of ±25V, input threshold of ±0.2V, and typical input hysteresis of 45 mV enable the VR sensor to operate at low speeds.

The separate, complementary outputs come from separate, ESD-protected inputs. IC₁'s outputs Y₁ and Y₂ can source as much as 10 mA. They alternately switch the logic-level, n-channel MOSFETs Q₁ and Q₂, which in turn provide V_OUT and . A low-dropout regulator, IC₂, provides the 5V power source for IC₁. Figure 2 illustrates low- (Figure 2a) and medium-speed (Figure 2b) operation for the sensor. For 5V-supply applications in which you can locate a microcontroller close to the sensor, you need only L₁, R₁, and IC₁ for a direct interface. For 3V applications, replace IC₁ with a MAX3096 IC.

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