D1 and D3 are reverse biased, hence the drain voltage and so the output terminal is pulled to ground via R2 and R3. If both inputs are LOW, D2 and D4 are forward biased, pulling the gate of the transistor to ground, hence the device is turned off. Now the drain voltage of the transistor is provided by input B, while the gate of the device is pulled to ground by input A. The transistor is turned off, hence the potential at the output terminal equals (nearly) those of input A which is the supply voltage.Īt the right of the drawing, input A is LOW, while input B is HIGH. Input B is LOW, hence D4 is also forward biased, pulling the base of the transistor to (nearly) zero volts. At the left of the drawing, input A is HIGH, hence D1 is forward biased and the drain voltage of the transistor equals (nearly) the potential at input A, which is the supply voltage. The output is HIGH if only one of the inputs is HIGH. If neither input is HIGH, the resulting output is kept at low level by the pull-down resistor. The output of an or gate is HIGH, whenever at least one of the inputs is HIGH. The output is HIGH if both input terminals are HIGH, because both diodes are reverse biased and the potential at the output clamp is kept at high level by the pull-up resistor. The signal at the output terminal of the circuit is LOW whenever one of the input signals is low, because the potential is kept at ground level by the forward biased diode(s). Logic gates are used to trigger a device into operation by more than one enabling signal.Ī simple AND gate composed of two diodes an one resistor was introduced at the chapter about H bridges: On the other hand, the lamp should be turned off, if nobody is close to it OR if the sun is shining. The lamp should be turned on, if it is dark AND if somebody is close to it. A motion detection can be integrated as a second sensor to the layout of the circuit. To make the lamp control more energy efficient, another condition can be added before the lamp is turned on: There is no need to turn the lamp on if nobody is close to it. On the other hand the lamp should be turned on whenever it is getting too dark to see. There is no need to read the current value of the photo sensor, but the question is if there is already enough ambient light, so that the lamp can be turned off, saving valuable electric energy. A simple kind of analysis is done by a twilight switch consisting of a photo sensor and a Schmitt trigger.
Whenever sensors are acquiring data, the main intention is to get an interpretation of the sensor output.
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