I can't see how this will work

[KTW]

I found this schematic somewhere and in looking at LED#2 on the right with the photoresistor wired in parallel how could it possibly control the LED when the LED is wired in circuit with the power source?
I would think the light would stay on day and night.

 

[duke37]

The current is limited by the resistor.
In the first case, the LDR goes conductive when lit and allows current to pass to the LED.

In the second case, the LDR goes conductive when it is lit and passes the current around the LED so it does not light.

 

[KTW]

This one doesn't work like that Duke, stays on regardless of brightness, do you see a mistake?

 

[duke37]

The LDR is not perfect, it needs to go to a low enough resistance when lit.
Measure the voltage across it and see if it comes down to less than the voltage which is needed to light the LED.

Edit: A transistor could be employed to make the effective resistance much lower.

 

[KTW]

I'm seeing 2.88 volts with the LDR uncovered and a slight increase to 2.9 volts when covered.

 

[KTW]

A transistor could be employed to make the effective resistance much lower.

I have a feeling it's missing something.

 

[duke37]

Get a small npn silicon transistor.
Connect the LDR between base and collector.
Connect the assembly where the LDR was with the emitter towards the negative supply.

 

[KTW]

That's not working either, I have another schematic with a variable resistor controlling the LDR so I'll pick one up and try that .I'll keep you posted, thanks for the suggestions.

 

[KTW]

I have it working with .9 ohm resistance before the LDR and it's close to what I want but it's still operating in too much light.
Is there anything I can use to drop the resistance a little more?

 

[duke37]

I do not see why the amplified LDR should not work. We need information to see what is happening.

Measure the resistance of the LDR at zero light, at the level where you want the LED to change and the resistance at maximum light.

 

[KTW]

In bright light the LDR reads 1.9K
No light 160K
Where I would like it to operate is between 125K and 160K.

I replaced the 330R with a pot down to 1 ohm and it's better but not what I want.

 

[Colin Mitchell]

You really need a schmitt trigger

 

[KTW]

Thanks Colin, I'll look for a schmitt trigger schematic where I can incorporate a 555 timer.

Kevin.

 

[Colin Mitchell]

Try this transistor Schmitt Trigger

 

[KTW]

I can try that, what do you think of this one?

 

[duke37]

If P3 out is high there will be no current limit for the lower LED - POP. Add another 470Ω resistor.

 

[KTW]

Ok, will do.
Thank you both, very helpful.

 

[BobK]

Or move the one resistor to go between pin 3 and the junction of the two LEDs.

Bob

 

[duke37]

Or move the one resistor to go between pin 3 and the junction of the two LEDs.

I think not, if there is no resistor in the LED chain, then the current from the supply will be unlimited.

 

[BobK]

I think not, if there is no resistor in the LED chain, then the current from the supply will be unlimited.

You are right, what was I thinking?

Bob