Dusk - Dawn Switch (LDR)

[ThomsCircuit]

I know there is a great deal of poor schematics available but I have been doing a good deal of research before i settled on this one to build from.

So I found a 12 year old article from EEWeb showcasing a D2D switch using a popular (They say) ORP12 LDR. I do not have that LDR. what I have is a KE10720 If you could help me determine if my LDR is a suitable replacement for the ORP12
The SR of the ORP is 515 to 730ns with a peak of 515. The KE10720 SR is 550 - 650ns with an unknown peak. >KE107 Datasheet<

I also do not have the UA741 or the BC109.
I do have the LM311 & the BC547. I'm pretty sure the NPN is a good substitution but unsure of the OpAmp

I will be using this to drive about 24 white LED's (FV 2.5 current 20ma) in series of 3 each from a 12 volt 1AMP transformer. The same transformer will be used to power the circuit. Could you please help me modify this circuit to work with my LDR and OpAmp?

Dusk to Dawn Switch

 

[ThomsCircuit]

I came into this thinking that all the components have pins and those pins connect to each other to make something work. When it comes to the transistor i did not understand why it was needed i just accepted that it was. What I know about the transistor is very little. I do know that in this circuit it will keep the pot from burning up but i dont know how it does that. If i were to guess id say its not the voltage that destroys components but the current.
From what I have read the base voltage is 0.6v current begins to flow. This is referred to as gain.
I suppose if i did not need a pot i would not need a transistor.

If you could help me select the correct resistor for R1 and any other modifications to make this work id appreciate it. These leds are going into a double faced clock. there will be one led placed at each hour segment. I will use the pot to adjust the light so its just bright enough to illuminate the clocks face.

 

[Nigel Goodwin]

I came into this thinking that all the components have pins and those pins connect to each other to make something work. When it comes to the transistor i did not understand why it was needed i just accepted that it was.

It provides MANY advantages, not the least of which is avoiding a far more expensive pot.

What I know about the transistor is very little. I do know that in this circuit it will keep the pot from burning up but i dont know how it does that. If i were to guess id say its not the voltage that destroys components but the current.
From what I have read the base voltage is 0.6v current begins to flow. This is referred to as gain.

No, as it's a darlington it requires double the normal voltage.

I suppose if i did not need a pot i would not need a transistor.

If you could help me select the correct resistor for R1 and any other modifications to make this work id appreciate it. These leds are going into a double faced clock. there will be one led placed at each hour segment. I will use the pot to adjust the light so its just bright enough to illuminate the clocks face.

R1 isn't at all critical, or even strictly required, but 1K should be perfectly fine.

 

[ThomsCircuit]

No, as it's a darlington it requires double the normal voltage.

I chose the darlington because of the number of leds. On a previous project there were just 8 leds. Signal from a counter IC. And a TIP was suggested.

I'm glad I got this right. Even by accident. I'm pleased with my progress. I can understand basic schematics and adapt them to my own.