View attachment 307447

 

 

It looks correct, are the transistors the correct type?

 

It looks correct, are the transistors the correct type?

One to left is 2N3906. To the right 2N3904. On power up LED turns on. That’s it, LDR does nothing and POT does nothing. What you’re looking at is the second completely different board/components and jumper wires. I did this to eliminate odd chance a part or wire might be the problem. The first try acted the identical way?

 

Might be too sensitive on power up.
Try connecting a 100nf or higher value cap across the base- emitter of Q1.

 

One to left is 2N3906. To the right 2N3904. On power up LED turns on. That’s it, LDR does nothing and POT does nothing. What you’re looking at is the second completely different board/components and jumper wires. I did this to eliminate odd chance a part or wire might be the problem. The first try acted the identical way?

What is the min-max resistance of the LDR? Q4 turns on when there is ~0.7V at the base node. Assuming your circuit is wired correctly, this means that if the circuit is always on even after power reset, that node is getting at least 0.7V all the time! Bad news for a latch circuit. In my circuit, I had the same problem from a power spike on power up that I solved with C1.

I haven't tried this circuit but my LDR measures 0ohm is full light and ~200kohm in full dark. This is a massive range for my room facing south which is why I used a red LED in my circuit to prevent false triggering. In your case though I think you have the Q4 base biased incorrectly.

I also read that matching transistors is a problem for these circuits.

 

Might be too sensitive on power up.
Try connecting a 100nf or higher value cap across the base- emitter of Q1.

I did as you suggested. It now works. I am using a 330 ohm resistor in place of the 2.2K. I seems to not be so sensitive ambient light turns on the LED but the pointer laser pen does turn it on. I think I need the LED to be brighter. I need to use a 2032 button cell because of space constraints. I have a dual coin cell holder to get 6v. Would much of a change be needed to the circuit?

 

I wouldn't think so but one could reduce R1 to 47 ohm if you need the LED brighter.