What is an Active High Device?


7401 AND gate



An active high device is a device that either outputs a HIGH signal when triggered on or that accepts a high signal as input to turn on. It really depends on whether the device is an input or an output device.


Active High Input Device

Let's deal first with an input device.

A classic example is the AND gate logic chip.

An AND gate is an active high device.

This means that it only turns on an output when fed HIGH signals, which are signals above 1/2 of the supply voltage (these are read as logic 1 signals).

Only when both of the inputs fed into the AND gate are at a logic HIGH (1) will it turn on.

If any of the inputs of an AND are a LOW signal (9), then the AND gate will not turn on, so the load connected to its output pin will not turn. Any input where the voltage is less than half of the power supply to the AND gate will be interpreted as a LOW signal.

This is what is meant by an active high device.


Active High Output Device

An example of a device that outputs a voltage instead of reads an input voltage like a logic gate is a PIR motion sensor.

This is a sensor that normally outputs a LOW signal on its signal line when it detects no infrared. When it does detect infrared, it outputs a HIGH signal.

So when not triggered, it normally outputs a LOW signal on its signal line. When triggered on by infrared, it outputs a HIGH signal on its signal line.

So it sends a HIGH signal when triggered on. Therefore, it is an active HIGH device.



Related Resources

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