Immediately its output will go high switching OFF the transistor, the relay and the lights. With an increase in the ambient light the resistance of the LDR will gradually fall and after a certain level the input of N1 will become logic low.This means that at this instant the output of N1 is logic low, the relay is activated through T1 and the lights (load) connected to the relay contacts are switched ON. In the absence of light or when its dark, the LDR offers an infinite resistance and thus the input of N1 is kept at logic high due to the voltage received through VR1.A LDR is in fact a resistor which changes its value with a change in the intensity of light falling on it. A LDR (Light Dependant Resistor) which is also connected at the input of N1 is used to sense a difference in light levels.This adjustment will set the level of darkness at which the system will change state. A reference voltage can be set at its input with the help of VR1.As shown in the diagram a single NAND gate N1 from the IC 4093 is configured as an inverter and a voltage monitor.
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