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Can someone identify a problem with circuit?

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Myrmidon

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Greetings all.

I made this simple circuit to turn on 3 led's at night using a comparator. The idea is the solar cell charges the batteries during the day then at night they run off the batteries. An photo resistor does the detecting.

The circuit works ok, however i'v noticed that when daylight comes back in the morning, the circuit isn't snapping off like it should. It works fine when i put some heat shrink over the sensor, then remove it.

Anyone have any ideas why it's not turning off with daylight?

p.s. the solar cell voltage should read as 7.8v.
 

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hi,
An obvious problem is, you do not show a resistor in series with the LEDs.

I'll check your circuit.:)
 
hi,
You have not indicated the type of LED you are using, what is the forward voltage drop of the LED's.

Assume they are 2Vdp and ON overnight they will discharge the4.8V.? to about 2.2V.

As the battery voltage drops so low it will change the setpoint [VR1] operating voltage also the -Vinp of ICa.

This means that the light/dark set point is not the same as would when the 4.8Vbty is fully charged,
thats why you have to shield the photodector, which then creates a bigger 'difference' voltage that ICa can detect.

Also the specified min operating supply voltage of the LM358 is +3V.

Did you design the circuit.?

Include some circuitry to detect when the 4.8V battery falls to its minimum recommended voltage, then it switches OFF the LED's.

Hope this helps.:)
 
an LM10 is an accurate voltage reference plus an opamp. It operates with a supply voltage down to 1.1V and has a low supply current that is half the current of the lousy old LM358.
Its voltage reference will stay the same when the battery voltage drops.

A small amount of hysteresis (positive feedback) should be added to the circuit for a "snap-action".
 
hello im new to your site and forty years old , and just started my hnd
in elec and elect can you do superposition of a circuit with j numbers
ive only done them with ohms (sorry if this is a daft question its an assignment) cheers didnt know where to post this sorry
 
Hey there.

Chuckle, yeah i designed the circuit just as an experimentation with comparators as i'm nearly finished my 1st year in electronic engineering. There is no resistor because i needed it for the led's which i should have specified as being 2 green and 1 blue, all superbright's. I left the resistor out because i believed the current would be safe enough to light the leds to a sufficient level. 20ma for the green and around 13ma for the blue and no resistor also because i needed around a 3-4v drop for the blue led.

It does work, just needs tweaking. I did wonder if it was the lm358 current.

I'll look into the positive feedback also.

Cheers.:)
 
Myrmidon said:
Hey there.

Chuckle, yeah i designed the circuit just as an experimentation with comparators as i'm nearly finished my 1st year in electronic engineering. There is no resistor because i needed it for the led's which i should have specified as being 2 green and 1 blue, all superbright's. I left the resistor out because i believed the current would be safe enough to light the leds to a sufficient level.
What controls the current in the LED's

20ma for the green and around 13ma for the blue and no resistor also because i needed around a 3-4v drop for the blue led.
Whats the forward voltage drop for the other LED's

It does work, just needs tweaking.
You can only use the technical word 'tweaking' after you have graduated.!:

I did wonder if it was the lm358 current.
What does this statement mean,?

I'll look into the positive feedback also.

Cheers.:)


Hi,
Rework your circuit and repost, I would like to see how it goes.:)

Add some voltage levels on the circuit, when the battery is fully charged and discharged.
 
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