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photocell to turn LED on questions

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Ok i'm off on another project. i'm a newbie so take it easy on my. I'm looking for a circuit to take a photocell and have it turn on an led using 2 aaa 1.5volt batteries and trun on the LED around Dark. can anyone point me in the right direction?
 
You know those solar garden lights your neighbor has down the street... :) Just kidding, you are looking for a dark-activated switch. Several circuits in this forum already (posted three different ones myself...). You didn't mention what kind of LED you plan on using. If it's white, 2 rechargables won't work, disposiables won't last very long, you'll need a boost converter. Red or amber will work okay. If you're in a hurry, search the forum or the web, plenty of circuits. I'll search the hard drive, and post the on I've been using a little later (already in the past posts some place...).
 
Give these a try!

My only concern is if the 3V is enough, and if so for how long.
 

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Team_NightStalker wrote: "how do i fine tune it? i don't want the LED to come on until it is almost night fall."

The 50k pot allows for some light sensitivity adjustment, but that really is a problem with many of the store-bought side-walk lights and this circuit as well.

I ended up using a rechargeable batt. with a 741 op-amp and a Photo-transistor instead. I ended up with a very sensitive circuit.

The following circuit is what I had come up with:
Note: The 10k pot is needed for the sensitivity adjust.

Maybe some of the real guru's can simplify this circuit.
 

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That's fine but it won't run from a 3V battery.
 
How about this!
The TLC25L2 can operate on a single supply voltage as low as 1.4v.
 

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i added a resistor on the one side of the photocell to the other ground and it helped some but not alot. the lower the resistor should make it turn on when darker correct? i used a 470. what else could be used to turn the LED on when darker out?
 
TEAM_NightStalker,

The first circuit I posted had R2 as a 50k pot, this should be the main control for sensitivity. Changing R3 to a 470 ohm resistor may help, but minimally. I've played with resistor values and did not get the desired results with this circuit.

The 2nd circuit post, maybe the the 3rd circuit post are far superior to the first in controlling when the LED turns on.
 
iONic said:
How about this!
The TLC25L2 can operate on a single supply voltage as low as 1.4v.
But the circuit still won't work!

R4 and R5 form a potential divider so when U1 goes high the base voltage will only be 0.5V so the transistor will never turn on!
 
RadioCrap war-surplus junk never has any spec's.
Operating voltage?
Operating current?
They don't know so you also don't know until tou smoke it.
 
It has worked for me... Maybe between a new 9V being close to 9.6V and a crappy transistor, it works.

So, why don't you go ahead and give the guy a circuit, Hero999, go ahead...
be a hero!
 
iONic said:
TEAM_NightStalker,

The first circuit I posted had R2 as a 50k pot, this should be the main control for sensitivity. Changing R3 to a 470 ohm resistor may help, but minimally. I've played with resistor values and did not get the desired results with this circuit.

The 2nd circuit post, maybe the the 3rd circuit post are far superior to the first in controlling when the LED turns on.


ahh see i dont' have a 50K pot, just 2 100K resistor paralleled. maybe i can find something in my old radio. hehe
 
iONic said:
It has worked for me... Maybe between a new 9V being close to 9.6V and a crappy transistor, it works.

So, why don't you go ahead and give the guy a circuit, Hero999, go ahead...
be a hero!
The origional circuit with the 9V will work.

Your modified version with the 3V battery won't work.

You removed R7 so the LED will overheat, you should have reduced its value to between 47:eek:hm: and 68:eek:hm: depending on the forward voltage.

With the 9V version R4 and R5 form a potential divider to stop Q1 turning on when the 741 is low as it's minimum output voltage is 2V which is above 0.6V, the turn on voltage of Q1.

Unfortuantely with the 3V version the potential divider formed by R4 and R5 will mean the voltage at Q1's base will only reach 0.5V when the TLC25L2's output goes high.

Remove R5 and make R4 2k and it should work.

Another thing you could do is connect a 100k resistor between pin 6 and pin 3, this will introduce so hysteresis and prevent the LED from flickering slightly when it turns on or off.
 
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