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Wiring Fundamentals: Digital Signal

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Digital Signal


00:00 - Digital signal sensors have an output value that is only ever one of two values.
00:05 These values are typically known as high and low, off an on, or zero and one.
00:11 A simple example of a digital signal sensor is a switch the driver uses to select between two different engine mapping options.
00:18 Desired boost being a common choice.
00:20 While it's a bit odd to think of a switch the driver toggles as being a sensor, it is reading a parameter and sending that information to the ECU.
00:28 In this example the ECU is reading the state of the switch, either on or off, and adjusting its boost targets appropriately.
00:36 Vehicle speed sensing is another common use of digital signal sensors.
00:40 A hall effect sensor can be positioned close to a toothed ferris wheel that rotates with the vehicle's drive line.
00:47 The sensor then sends an alternating on - off signal back to the ECU as each tooth of the wheel passes a sensor.
00:54 In contrast to the previous example, in this case the ECU doesn't measure the state of the digital input, but the frequency at which it's changing from on to off and back again.
01:05 From this frequency measurement, with some internal math, the ECU can calculate the vehicle speed.
01:11 The most common way for a digital sensor to physically implement its on and off output states is to switch its output pin from being connected to ground when it is on and disconnected from everything when it is off.
01:25 This disconnected state is known as floating and when the sensor is in this state, its output wire is not connected to anything and its voltage level is actually undefined.
01:34 We never want to have a case where a sensor signal going to an ECU is undefined and we eliminate this possibility by using a pull up resistor.
01:43 Pull up resistor work by providing a default state for the ECU input in the case the input pin is left floating.
01:49 Such as when it is connected to a digital sensor in its off state.
01:53 The pull up resistor pulls the voltage signal up to a default value.
01:58 Almost all aftermarket ECUs have the option within their software to enable pull up resistors internally on their digital inputs.
02:06 In the rare situation when this isn't possible, one can be added manually, by connecting a quarter watt, 1000 ohm resistor between the digital input signal wire and the sensor supply.
02:17 Important to note is that this pull up resistor ensures a high voltage level is seen by the ECU when the digital sensor is in its off state, and a ground voltage signal is seen by the ECU when it is in its on state.
02:30 Digital sensors often have either two or three pins and you will need to read the documentation that came with the sensor to confirm their pin out.
02:38 It's common for a two wore sensor to be wired to sensor ground and signal input.
02:43 A three wire sensor is commonly wired to sensor supply, sensor ground and signal input.

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