Circuit multiplexes automotive sensors

Adil Ansari, Delphi-Delco Electronics, Kokomo, IN -- EDN, 1/20/2000

Often, a µC limits the number of input-capture lines to accommodate the various types of automotive sensors with pulsed outputs, such as vehicle- and engine-speed sensors. The circuit in Figure 1 uses discrete components to multiplex two sensors with open-collector outputs into a single output, thereby sharing one input-capture line of the µC. The µC selects the sensor whose output you will measure. You can apply this approach to sensors whose outputs are amenable to time-sharing and do not require continuous monitoring, such as position sensors. In Figure 1, Sensor 1 and Sensor 2 are outputs from two sensors using npn transistors with open-collector outputs. To enable Sensor 1 or Sensor 2, Q_1Aor Q_1B, respectively, must turn on. A logic-low signal from the µC on the Select input turns off Q₂ and Q_1C. When Sensor 1 input goes low, D₁ forward-biases, and Q_1A turns on, providing a high signal on MUXED_OUT. When Sensor 1 input turns off (high-impedance state), Q_1A turns off, providing a low signal on MUXED_OUT. Therefore, when the Select input is low, MUXED_OUT produces pulses that are inverted but synchronized with the Sensor 1 pulses. At the same time, Q₃ and Q_1D are on, turning off Q_1B and disabling the Sensor 2 input.

Similarly, when Select goes high, Q₂ and Q_1C turn on, turning off Q_1A and disabling the Sensor 1 input. At the same time, Q₃ and Q_1D turn off, allowing the Sensor 2 signal to turn Q_1B on and off when Sensor 2 switches on (low) and off (high-impedance state), respectively. Therefore, MUXED_OUT produce pulses synchronized with the Sensor 2 input. You can change the values of R₁, R₄, R₅, and R₆ to meet the sensors' requirements. D₃ clamps MUXED_OUT to CMOS/TTL levels. The use of the MPQ3906, containing four pnp transistors in one package, minimizes the number of components. Similarly, you can obtain arrays of 1-kW resistors in a single package. (DI #2469)