555 ics hysteresis for dark activated

Posted on Feb 4, 2014

This is a circuit that can be used for dark activated means the relay will be activated when thelight intensity fall below a certain threshold. Without hysteresis, the relay will be activated and deactivated if the sensed brightness fall under or rise above a single point of darkness level. With hysteresis, the darkness level for activating and de

555 ics hysteresis for dark activated
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activating the relay will be different, and this solve the relay oscillation problem when the light intensity is swinging up and down around a single point of no hysteresis activation level. Here`s the figure of the circuit; We can employ the hysteresis of a 555 IC to improve the sensingof a drop in light, since the internal 555 circuit has 1/3 and 2/3supply voltage thresholds. We have to use a LDR or CDS cell with out 2 to 8 k resistance at desired light level. At the dark, the resistance of the LDR will rise and activate the relay if the voltage at pin 2 reach 2/3 of supply voltage (8V). After the relay is activated, more light is needed to make the LDR decrease its resistance until the voltage at pin 2 falls below 1/3supply voltage (4V).

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