# Need help with a photocell.

#### Mikiver

Joined Nov 27, 2010
7
I'm a noob here and just starting to experiment with simple DC circuits and leds.
I would like to construct a night light that runs a single white led using a photocell. I know that the voltage will need to be more than 3V so I will need to use 3 X 1.5V cells. If the light is not sufficient, I may add more leds and of course adjust the voltage and resistance as required.
I have a couple of photocells with resistances of about 800 ohms in full light to 2K in full dark.
There's many designs on the net and none of them was really what I was looking for. I am also looking for a long battery life. (duh!)
Would someone be so kind as to direct me to a schematic that would suit my needs? Or draw one up for me? Perhaps with a pot so I can adjust the sensitivity?

Mike

#### debe

Joined Sep 21, 2010
1,154
A good source & cheep Solar Garden light dismantled worth about $5 for parts to experiment with. #### Attachments • 295.4 KB Views: 41 • 295.9 KB Views: 34 • 298.3 KB Views: 30 #### Audioguru Joined Dec 20, 2007 11,251 A photocell is 270 ohms in sunlight and 5Meg ohms in darkness. Thread Starter #### Mikiver Joined Nov 27, 2010 7 Thanks Guys. I guess I better check my photocell measurements. It was definately about 800 ohms in my workshop however it was measured under flourescent lighting. For dark, I just covered it with my finger. Mike #### windoze killa Joined Feb 23, 2006 605 Was there anything else in circuit when you did your measurments? What type of meter are you using? Both of these may cause false readings in some situations. The other thing is were you holding the probes on the photocell with your fingers? If so you have just measured your bodies resistance. Thread Starter #### Mikiver Joined Nov 27, 2010 7 Photocell was mounted on the breadboard and alligator clipped to the multimeter (Beckman). Re-checked more carefully and came up with better numbers. I tried this one, but used a 2N3906 transistor and for some reason, it works in reverse. Dark=light is off and vice-versa. It's not my design. Another I tried did the same thing. Is it the transisitor? It's something simple I'm sure but I don't know enough to figure it out. Thread Starter #### Mikiver Joined Nov 27, 2010 7 I think I have a PNP transistor here. That would be the problem correct? #### windoze killa Joined Feb 23, 2006 605 I think I have a PNP transistor here. That would be the problem correct? Yep. 2N3906 is a PNP. Won't work in that config. BUT If you flip it over and swap emitter and collector you might get it to work. OR get a NPN. #### wayneh Joined Sep 9, 2010 16,154 I know that the voltage will need to be more than 3V so I will need to use 3 X 1.5V cells. Not necessarily! The suggestion to get a cheap solar garden light is a good one. I saw them for just over$1 apiece recently since it's end-of-season here.

Anyway, they've almost all gone to using a pulsing DC-DC converter to boost voltage, so that they can operate off a single AA or AAA battery. The LED sees low duty-cycle pulses at 3-5v. This is more than enough to light it up and appear continuously lit to the human eye. They'd be completely dark when connected directly to the battery.

In addition to reducing the battery count, the DC-DC converter is fairly efficient and avoids power lost to current-limiting resistors or other control electronics. So the LED stays lit longer as the battery drains.

The better solar lights include a CdS cell and circuitry for turning the light on and off. Early designs just relied on the main panel.

#### Audioguru

Joined Dec 20, 2007
11,251
Cheap ******* solar garden lights rust away within one year. The screws and switch are iron and even the leads on the LED are iron. The cheap Ni-Cad cell is also housed in iron. Maybe gardens in China are indoors where it doesn't rain. But the cheap solar panel gets badly sunburned after a couple of years. I polish mine.

I replace the cheap Ni-Cad with a modern American Energizer Ni-MH cell (actually made in Japan), replace the screws with stainless steel ones and remove and bypass the switch. Then I seal the whole thing with silicone caulking.

A solar garden light is so dim that it doesn't light anything. It is simply an indicator maybe for a walkway.

#### wayneh

Joined Sep 9, 2010
16,154
Cheap ******* solar garden lights rust away within one year.
Gosh, mine are years old and I've only lost maybe one in a dozen to decay, generally because ants like the enclosures.
The screws and switch are iron and even the leads on the LED are iron. The cheap Ni-Cad cell is also housed in iron. Maybe gardens in China are indoors where it doesn't rain.
Mine have no switches. My humidity level is maybe not as high as your's? Not exactly arid here in Illinois, though.
But the cheap solar panel gets badly sunburned after a couple of years. I polish mine.
I've done that with some of my very oldest ones. More recent ones don't seem to be showing the frosting. I've also had to do my Chrysler headlights.
I replace the cheap Ni-Cad with a modern American Energizer Ni-MH cell (actually made in Japan), replace the screws with stainless steel ones and remove and bypass the switch. Then I seal the whole thing with silicone caulking.
Battery failure is the number one failure mode in my experience. I replace them with aging cells from the house, sort of a trickle-down strategy.
A solar garden light is so dim that it doesn't light anything. It is simply an indicator maybe for a walkway.
True that. I was able to make one a lot brighter by replacing the LED with another cheapo LED from e-bay. I had to add a foil reflector because that LED was narrow-beam, but overall it appears MUCH brighter than it was and seems to last just as long as any of them. But it's still a "be seen" light, not useful illumination.

#### Audioguru

Joined Dec 20, 2007
11,251
Some solar garden lights (Westinghouse) claim to be 12 times brighter! but they cost 35 times more than a cheapo one and are never on sale.

#### retched

Joined Dec 5, 2009
5,208
12 x 0 is still 0.

#### Mikiver

Joined Nov 27, 2010
7
I suppose I should have been a bit clearer with my description. I want to make a light that is fairly intense to light up the key hole in my exterior door. Sure, I could just leave the porch light on but...bear with me, I don't want to do that. A garden light won't cut it. Plus there may not be enough light to recharge the cell(s).
I'll try the circuit in my earlier post with the correct transistor.

Thanks Guys!

Mike

#### blueroomelectronics

Joined Jul 22, 2007
1,757
Why not use a flashlight key fob?

#### thatoneguy

Joined Feb 19, 2009
6,359
My door key has an LED built into it, just squeeze and you can find the keyhole. It's lasted a long time too, you need to squeeze it kind of hard to get the light to come on. It's a single 1220 3V battery directly driving a 3mm LED. Most all key shops in the US stock them.

#### Mikiver

Joined Nov 27, 2010
7
I could do that but I would enjoy making something like I indicated. Cave time in the workshop, learning, plus my wife and late teen kids are pretty careless and it would be more work for me to keep them supplied with special fobs than to create something that was automatic and required no input whatsoever from them. It's hard enough to get them to look after house security. I want to make everything simple and no-brainer. This device is one of many that I have been working on the past while although it's the only electronic one to date.
Turning off the lights???? Are you kidding me??? Is that what the off switch is for?..Yup, we have sensor switches in many rooms. Automatic off for the clothes iron.(it was left on all day and melted a plastic bag but no fire fortunately) Don't get me started on unlocked doors. (Our neighbourhood has seen break and enters in the past and we were hit about 8 years ago.)
Sorry for the rant. Is there a forum for this?

#### retched

Joined Dec 5, 2009
5,208
Sorry for the rant. Is there a forum for this?

#### thatoneguy

Joined Feb 19, 2009
6,359
Include a PIR detector with your light detector so the batteries have a chance of lasting over a week.

The doorknob area would light up when somebody is within a few feet of it, and it is dark out.

#### debe

Joined Sep 21, 2010
1,154
Hi Mike, This is a modified garden solar light mounted on my caravan, the LED is glued where the finger is & shines down the side of the van door. There is plenty of light on dark nights to see the keys & key hole. Been there for a couple of years now. Daryl

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