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Op-Amp as Comparator to switch a Relay which will drive an Indicator. 30K resistor found at Op-Amp Inverting input would be replaced with the Photoresistor. As light increases on the photocell it's resistance falls. Simulate this by selecting the 30K, and using the outer dual swing the resistor's value down toward 20K ohm. Comparator sees that the voltage from the sensor has crossed the reference point, which is set by the two resistors acting as a Voltage Divider, found at Op-Amp Non-Inverting Input. This causes the Comparator to swing to an Output High state. The +V Output Signal is sensed by the Base of the BJT Relay Control. That additional voltage on the Base causes the Transistor to change states as well, allowing enough voltage to be seen by the Relay so that it is activated, thereby powering the Indicator. It's a good, solid circuit in my opinion. There is nothing super complicated or mysterious about it, which is helpful to the hobbiest in the beginner to intermediate range, as each function of it's operation is understandable and measurable. This is a fairly basic circuit but really a great one to experiment and practice with on a breadboard, as it needs to be tuned according to environmental conditions and actual components to be used. These conditions can vary greatly, so it would be unusual to see this circuit built by two different guys having the same value set. So grab some components, pull out your breadboard and make sure you've got a decent battery in your multimeter- and let the fun begin! =^)
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