I think you have it backwards. When the comparator output is low, the transistor should be off.i'm not sure about rf but i think r4 is to pull the output of the chip low so when the chip output is on (too bright) it stops the transistor base from being connected to ground thefor not activating the ralay. however when the output is low(off) the base is coonected to ground turning the relay on
You don't need the Rf if you want to light up the LED in a dark room/off in lighted ambience.
However, if you want to adjust the brightness of your LED according to brightness of the room, this circuit is for that purpose, hence the RF is to attenuate your voltage from the divider between the LDR and VR1. This is quite difficult to achieve since the transistor is current controlled, unless you're using MOSFETs.
R4 is to provide virtual grounding for the signal coming out from the op-amp, I suppose. I think it can be omitted.
Are you locked in to a particular type of amplifier in the circuit? If so, what is the part number? If not, a comparator will work better.So R4 should be a very high value resistor?
So how does RF work exactly? Does it just add some extra current to the base or how exactly does it help? I"m not saying I don't want to use, I would just like to know how it works.
Not to nitpick, but "as stated" this is incorrect. The brightness of his LEDs will be unaffected by the circuit. The "responsiveness" of the comparitor to light input, however, can be adjusted.
The brightness will be unaffected because the output is a relay switch closure. His LEDs will not be analog, they will have two states ON/OFF.
Are you locked in to a particular type of amplifier in the circuit? If so, what is the part number? If not, a comparator will work better.
Post a complete schematic, with component values, if you have one. Otherwise, we can create one for you.
No, offset voltage is a measure of the imperfection of the op amp. Ideally, it is zero. Input offset voltage is the differential input voltage required to make the output voltage equal zero.Could anyone explain the concept of Input Offset Voltage. I was looking at the TL081 Datasheet, and I've read a little about it online, but I don't quite grasp it.
Is the offset voltage part of what makes negative feedback work on an opamp?
Yes. Keep in mind that the datasheet spec is for the maximum you might expect from any part. If the spec is ±3mV, it could be anywhere in between. For your circuit, input offset voltage is irrelevant.So if the differential is 3mV to get the output to a true 0V you would need one input to be say 5V and the other to be 5.003V?
The (+) and (-) inputs of the opamp are drawn backwards. Rf is supposed to provide positive feedback, not negative feedback. When the LDR is dark and a high resistance then its voltage is low and you want you want the (+) input to be low to turn off the transistor and relay.
Rf provides a "snap action" when the circuit is at its threshold voltage.
R3 and R4 form a voltage divider to reduce the output-low voltage of +1.2V of the opamp to below +0.4V so the transistor can turn off.
The minimum output voltage of most opamps is 1.2V above the negative supply voltage that is 0V in this circuit. A "rail-to-rail" opamp has outputs that go to 0V and to the positive supply voltage when the load current is very low.