LM741 Light Dark Sensor

A light/dark sensing circuit is extremely useful and versatile in a wide range of renewable energy projects from automatic lighting to security systems. In our article Light Dependent Resistor we explained how an LDR can be used in simple circuits to control devices according to the ambient levels of lighting - for example, automatically turning o
LM741 Light Dark Sensor - schematic

n a lamp above a doorway at nighttime. The circuits provided in that article were very basic and therefore not without their problems. The main problem is that the automatic turn-on is not sharp - i. e. rather than switching the lamp on fully as soon as ambient levels of light fall to a fixed point, the lamp turns on slowly getting brighter and brighter as light levels fall. This is not a serious problem when controlling a lamp - in fact it may even be desirable - however when controlling other devices (such as a night vision camera) it is not optimal. One way around this is to use a relay, however a typical 12 Volt relay draws 300-500mW of power - a significant waste of energy when a 1 Watt LED spotlight for example is to be controlled. 1st Nov 2007 Update - We have modified the schematic diagram above with the addition of a 220uF smoothing capacitor between the base of transistor Q1 and ground. Without this capacitor, the relay chatter (relay switching on and off many times per second) was terrible around the switch on/off light level. By adding the capacitor, relay chatter was completely eliminated. According to the designer of this circuit, the relay will be closed only when "NO light falls on LDR1", however, in testing this circuit proved to work very well with the user able to adjust the potentiometer (P1) to automatically close the relay at whatever light level they chose. By swapping the postitions of the 10K...

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