detecting darkness

Detecting DarknessWe have written recently about how to make a useful dark-detecting LED driver circuit. That circuit used an infrared phototransistor. To add a darkness detecting capability to our solar circuit is even easier, actually, because our solar panel can directly serve as a sensor to tell when it's dark outside.

To perform the switching, we use a PNP transistor that is controlled by the voltage output from the solar panel. When it's sunny, the output of the panel is high, which turns off the transistor, but when it gets dark, the transistor lets current flow to our yellow LED. This circuit works very well and is a joy to use-- it would make a good upgrade to the dark detecting pumpkin to make it go solar with this circuit.

A solar garden light circuitWhile the last circuit works well for driving a yellow or red LED, it runs at 2.4 V (the output of the NiMH battery), it does not have sufficient voltage to drive a blue or white output LED. So, we can add to that circuit the simple Joule Thief voltage booster to get a good design for a solar garden light: A solar-charged battery with a dark detector that drives a Joule Thief to run a white output LED.

Naturally, you'd want to give this a tough, weatherproof enclosure if it were going to be run outside. (A mason jar comes to mind!) This circuit is actually very close to how many solar garden lights work, although there are many different circuits that they use.

Adding a microcontrollerOur last circuit examples extend the previous designs by adding a small AVR microcontroller. We use the voltage output from the solar panel again to perform darkness detection, but instead take it to an analog input of the microcontroller. The microcontroller is potentially a very low current, efficient device that lets you save power by not running the LED all the time, but (for example) waiting until an hour or two after darkness and/or fading the LEDs on or off, or even intermittently blinking for very low average power consumption.

In this example we have the PWM (pulse-width modulation) output of the microcontroller driving a Joule Thief style voltage booster to run the white LED. (This is one of many, many different working designs for this sort of boosting circuits.)

We also made a second version of this circuit, with two red LED outputs to make a spooky Jack-o'-lantern:

To finish it up, we carved a beautiful white pumpkin and added this circuit to make our microcontroller-driven, dark-detecting, solar-powered programmable pumpkin, which faded its eyes in and out one at a time. Note the long leads on the solar panel and wires to the LEDs to reach.

We hope that you might find this introduction to simple solar circuits helpful; let's see those solar jack-o-lanterns!

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A Simple and Cheap Dark-Detecting LED Circuit

Here's a simple problem: "How do you make an LED turn on when it gets dark?" You might call it the "nightlight problem," but the same sort of question comes up in a lot of familiar situations-- emergency lights, street lights, silly computer keyboard backlights, and the list goes on.

Solutions? Lots. The time-honored tradition is to use a circuit with a CdS photoresistor, sometimes called a photocell or LDR, for "light-dependent resistor." (Circuit Example 1, Example 2.) Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated. There are many other solutions as well. Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor. (Our own solar circuit collection is here.)

In this article we show how to build a very simple-- perhaps even the simplest-- darkness-activated LED circuit. To our LED and battery we add just three components, which cost less than thirty cents altogether (and much less if you buy in bulk). You can build it in less than five minutes or less (much less with practice).

What can you do with such an inexpensive light-controlled LED circuit? Almost anything really. But, one fun application is to make LED throwies that turn themselves off in the daytime to save power. Throwies normally can last up to two weeks. Adding a light-level switch like this can significantly extend their lifetime.

Here are our components: On top: a CR2032 lithium coin cell (3 V). On the bottom (L-R): the LED, an LTR-4206E phototransistor, a 2N3904 transistor, and a 1 k resistor. This LED is red, blindingly bright at 60 candela, in a 10 mm package. It casts a visible beam, visible for about twenty feet in a well-lit room. We got the LEDs and batteries on eBay, and the other parts are from Digi-Key, but Mouser has them as well. As we mentioned, the last three cost about $0.30 all together, and much less in bulk.

The LTR-4206E is a phototransistor in a 3mm black package. The black package blocks visible light, so it is only sensitive to infrared light-- it sees sunlight and incandescent lights, but not fluorescent or (most) discharge lamps-- it really will come on at night.

Our starting point is the simplest LED circuit: that of the LED throwie, which has an LED driven directly from a 3V lithium coin cell. (Funny looking example here.) From this, we add on the phototransistor, which senses the presence of light, and we use its output to control the transistor, which turns the LED on.

The circuit diagram looks like this; please ignore the messy handwriting. ;)

When light falls on the phototransistor, it begins to conduct up to about 1.5 mA, which pulls down the voltage at the lower side of the resistor by 1.5 V, turning off the transistor, which turns off the LED. When it's dark, the transistor is able to conduct about 15 mA through the LED. So, the circuit uses only about 1/10 as much current while the LED is off. One thing to note about this circuit: We're using a red LED. That's because the voltage drop across the transistor allows less than the full 3 V across the LED. The full three volts is really only marginal for driving blue LEDs anyway, so two-point-something really doesn't cut it. (Might be able to work around that with a cheap FET-- haven't tried yet.)

And now, let's build it. You can certainly put this together on a breadboard, but there's something more satisfying about the compact and deployable build that we walk through here.

First get the transistor and the resistor. The pins of the 2N3904 are called (left-to-right) Emitter, Base, Collector, when viewing it from the front such that you can read the writing. We're going to solder the resistor between the leads of the Base and Collector of the transistor. Unusual part: hold the resistor with its leads at 90 degrees to those of the transistor while you solder.

Stay safe when you do this: Use Mr. Hands.

After soldering, clip off the excess resistor lead that is attached to the transistor base (middle pin), as well as the excess length of the collector pin.

Next, we add the phototransistor. Note that it has a flatted side, much like an LED does. This pin on that side is the collector of the phototransistor. Solder the collector (flatted side) to the middle pin (the base) of the transistor, again at 90 degrees. The other pin of the phototransistor, the emitter, is left unconnected for the moment. (Here is an alternate view of what that should look like when you're done.)

Finally, we need to add the LED. To do so, we need to know which side is the "positive," or anode side of the device. Regrettably markings of LEDs are not consistent, so the best way to be sure is to test it with the lithium coin cell-- put the LED across the terminals of the cell and, when it lights up, note which side is touching the (+) terminal. (Usually, it's the one with the longer lead.) Solder the "positive" lead of the LED to the emitter pin of the transistor-- it's the one on the left, which doesn't have anything soldered to it. Trim away the excess lead of the LED that goes past the solder joint. Solder the other pin of the LED (the "negative" pin, or cathode) to the emitter of the phototransistor, the pin on the non-flatted side, which does not have anything connected to it yet.

By this point, there are only two pins sticking down below the components: One that goes to the resistor and collector (rightmost pin) of the transistor, and one that goes to the emitter of the phototransistor and to the cathode of the LED.

To test the circuit, squeeze the coin cell between these two terminals, positive side goes to the lead touching the resistor. You can't see the LED on here because these photos were taken with incandescent lighting-- it wouldn't turn on.

Bending the leads to contact the lithium cell a little more reliably, you can try it out a little more easily. In the photo on the right, I cupped my hand over the circuit-- so the LED turned on.

To make this into an actual "throwie," you still need to add some tape and a magnet, but that's quite easily done. This one makes a pretty good nightlight attached to the top of a doorframe-- when the room lights are off, it shines a bright, bright spot on the ceiling.

Where to go from here? While this little circuit can do something on its own, it would probably also be happy as part of a larger circuit. At a minimum, note that if you work with batteries that have lower internal resistance than the lithium coin cells, you should place an appropriate resistor in series with the battery before trying to operate this circuit-- or else you may put too much current through the LED. Certainly, this is one of the easiest and least expensive ways to control an LED with a photosensor. (Unlike, say, this method?) You could also consider crossing it with some more extreme mods, like the Talkie Throwies that know Morse code, or for more extreme hackers, bagel throwies.

Contributed by: Windell on Wednesday, December 12 2007 @ 09:15 PM PST, in EMSL Projects Permalink E-mail Print

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53 comments

The following comments are owned by whomever posted them. This site is not responsible for what they say.

Authored by: westfw on Thursday, December 13 2007 @ 03:20 AM PST A Simple and Cheap Dark-Detecting LED Circuit

Your NPN transistor is in a somewhat odd place; they're usually "low side" drivers. Is the position it's in required by the characteristics of the photodiode and circuit?Is sensitivity adjustable at all? It has seemed to me that a problem with most dark-activated circuits is that they turn on when the room/outside is far from dark, and I end up wasting batteries trying to light up an area that is already quite adequately lit by 'dusk.' It's almost worth throwing a microcontroller at.

[ Reply to This | # ]

Authored by: Windell on Thursday, December 13 2007 @ 05:01 AM PST A Simple and Cheap Dark-Detecting LED Circuit

MetaBlog links for this story: [ del.icio.us | technorati ]

Technorati tags: electronics, diy, how to, circuitry, howto, fun, transistor, LED, phototransistor, throwies, make

Hi Bill,We do normally use the NPNs as low-sides, of course. This circuit *is* a little bit of an odd ball, designed for simplicity rather than efficiency. I can't really justify it much better than to say (1) it does work (2) there are a heck of a lot of other ways to make it work as well and (3) compromise is the name of the game here. ;)

The first stage of the circuit is a transistor inverter-- using the phototransistor, which pulls the base of the transistor low when the phototransistor turns on. Exactly how far and how fast the voltage at the transistor base drops is determined by the value of the resistor. The transistor can conduct-- turn on the LED-- only when the base voltage exceeds the sum of the forward voltage of the transistor (Vbe-on) and the LED forward voltage. As long as the phototransistor inverter can pull the transistor base below about 2.5 V, the LED will shut off. So in a sense, we are playing on the characteristics of the components to make this work.

Obviously, you can increase the light sensitivity by increasing the resistor value. The circuit still works okay as you increase the resistance up to, say, 10 k. There is, however, a negative side effect of increasing R. Because the resistance is higher, there is less transistor base current when the LED is on, so the transistor won't turn on as far, and the LED won't be as bright. You could compensate by choosing a transistor with much higher gain than the 2N3904 (not saying much, if you know what I mean). In any case, there is a lot of room for improvement. Our default value of 1k is not all that sensitive-- might leave your LED on at twilight, so it's certainly worth playing with.

---Windell H. Oskaydrwho(at)evilmadscientist.comhttp://www.evilmadscientist.com/


Authored by: Anonymous on Thursday, December 13 2007 @ 05:33 PM PST A Simple and Cheap Dark-Detecting LED Circuit

I am a total newbie to electronics, and barely know how a transistor works, but I ran across something called a darlington pair. Could you just wire two of those cheap transistors together to increase the gain? Then you could use a larger resistor but compensate for the loss in gain for minimal cost. I don't know how that would fit into your compact construction, but it's a thought.


Authored by: Windell on Thursday, December 13 2007 @ 05:59 PM PST A Simple and Cheap Dark-Detecting LED Circuit

A darlington will not work here-- at least if you're using the 3 V battery like we are. It's essentially two transistors in series, trading off a higher voltage requirement for higher gain: it takes twice as much voltage to turn on.

A single transistor with higher gain is a better solution if you want better sensitivity. I have not tried it yet, but the 2N5962 looks like an excellent choice-- it even comes in the same package ("TO-92") with the same pinout. They are in stock at Mouser now for $0.08 each. =)



Authored by: Anonymous on Friday, December 14 2007 @ 03:07 AM PST A Simple and Cheap Dark-Detecting LED Circuit

The transistor is in what is called an emitter follower or "common collector" configuration. The voltage on the emitter will "follow" the voltage on the base, but at roughly 0.7V lower voltage. The downside of this is that you are throwing away a lot of voltage - the 1.5V drop across the 1k resistor plus the 0.7V drop of the vbe of the transistor. The circuit might work with other color LEDs if you increase the resistor slightly and use the 3904 to pull down the ground side of the LED, with the anode connected to vcc, like westfw was referring to. In that case you should be able to get up to 2.8V across the diode if the transistor has enough current gain.


Authored by: Anonymous on Friday, December 14 2007 @ 03:09 AM PST A Simple and Cheap Dark-Detecting LED Circuit

I take it back. The follower only loses 0.5V over the common emitter, since the 1k resistor has no current through it when the phototransistor is dark. So maybe the emitter follower is not so bad after all!


Authored by: Windell on Friday, December 14 2007 @ 05:32 AM PST A Simple and Cheap Dark-Detecting LED Circuit

I read 50 mV across the 1 k resistor, and slightly over 2 V across the LED. (Not quite fully on; time to order some better transistors.)




I'm somewhat new to electronics and interested in lighting and LEDs. Question: is there a simple (read: inexpensive, small) that could be added to this project? In other words, for example I want the light to go on for X hours then turn off until the next transition from light to dark. That way the battery doesn't expire as quickly but it can be used for applications such as Xmas lights, halloween, etc. where it doesn't need to be on all day (assuming an array of LEDs).



I assume that you meant to use the word "timer" in there somewhere.

You could probably do it by adding a capacitor, resistor, and diode in the right way, but it does start to get complicated. I would suggest instead making the transition to using a microcontroller. Start here for inspiration.

My approach would be to use a single tiny AVR, and to use the LED as the photosensor for it. The internal timer on the AVR is sufficient for whatever timing needs you have.




next step: most folks arent aware that basic led's are themselves photo detectors (in addition to being photo emitters)...hook one up to a sensitive DMM meter to prove it for yourself.

The challenge: if we substitute an led as the detector (let's say because our junkbox has leds, resistors, and transistors lying around...but dont have any photo transistors)...can we redisign a simple circuit that operates in a similar manner using two LEDs...one for input, the other for output?



A photodiode on its own doesn't generate much current; LEDs tend to be smaller and generate even less. The beauty of the phototransistor is that it includes the gain that you need to use that tiny current. You can do the same thing by providing an external transistor for the photodiode (or LED) of course... just one more component.



Authored by: Anonymous on Monday, January 07 2008 @ 03:21 PM PST A Simple and Cheap Dark-Detecting LED Circuit

If you happen to have a microchip around, just that and an LED and a battery will do.Similar to those LED throwies that have the microchip sitting on the LED's legs, the same concept could be used.

Have the microchip reverse bias the LED and then swap the anode to be a high impedance input, then loop over checking the value of the LED, it should go high after x loops, depending on how much light lands on the LED.

In the dark, it will take longer (less light -> less photocurrent) so if you count how many loops it took and then only turn the LED on when the loop count is above the threshhold, you could have the same circuit in only 2 components and the battery.

However, if it got very dark the flashing off when the LED is reverse biased would probably take long enough that it would be noticeable - you could try having a maximum the loop will go up to before it assumes it's dark and stops there.


Authored by: Anonymous on Friday, December 14 2007 @ 05:17 AM PST Bagel Throwies FTW!

I wondered why I was getting more traffic on Flickr - thanks, EMSL!

Steve/Irregular Shed


Authored by: joebar32 on Friday, December 14 2007 @ 11:03 AM PST A Simple and Cheap Dark-Detecting LED Circuit

Could you combine this with the joule thief? Would you just insert this circuit into the joule thief circuit as if it was an on/off switch? Or would the way this circuit plays with current not make it a switch per se? I'm a mechanical with a limited electronics knowledge that doesn't extend too far past the following a recipe stage. I could guess that the low current 'off' state of this circuit might get boosted by the joule thief and end up with an always on circuit maybe.

I'm thinking it'd be nice to use up old batteries in night light or xmas decoration duty. I've got a couple outdoor wreaths that would look good with a few led 'berries' on them at night.


Authored by: Windell on Friday, December 14 2007 @ 12:44 PM PST A Simple and Cheap Dark-Detecting LED Circuit

It is possible.

I *think* I know an easy way to make it work, but I don't have the parts with me to check. So... I shouldn't say anything, except that there is more than one way to make it work. =)



Authored by: Anonymous on Friday, December 14 2007 @ 05:54 PM PST A Simple and Cheap Dark-Detecting LED Circuit

ye i guess u could this is wat u would have to do instead of the 3 volt battery u would hook up what ever size battery u got just as long as it is 1.5 volts then you take off the led and make sure the polarity is correcct , have to connect it instead of the battery in the joule theif


Authored by: Anonymous on Tuesday, January 08 2008 @ 11:13 AM PST A Simple and Cheap Dark-Detecting LED Circuit

The problem I see with this circuit is that when the photocell is turned on, the full 3.6V is across the 1k resistor, which draws 3.6/1000 of an amp or 3.6 milliamps. This is not as much as the LED, but it's a substantial part of what the LED draws, so the battery will last somewhat longer, but will still go dead soon in a few days.

But by making a few changes, the battery life can be extended a lot more, to months. One way is to replace the 2N3904 with a 2N7000 enhancement mode MOSFET. This is a few cents more than the 2N3904 if you buy them from FairchildDirect.com or Mouser. Instead of a 1k, replace the resistor with a 1 meg resistor. The current drain will then be a thousand times lower when it's off. The 2N7000 requires more voltage at the gate, so to solve this, the LED must be moved from the emitter and put in the 'collector' which is called the drain in the 2N7000 (the pinout is the same as the 2N3904). Since the gate of the 2N7000 draws almost no current, you can use something other than a phototransistor. A LED might work instead. But since it can generate a voltage when it's illuminated, the LED may have to be put in backwards to _reduce_ the voltage at the gate.

Another way is to add another transistor to reduce the base bias needed. I'll have to write up a blog on this later (it's at watsonseblog.blogspot.com). But if you've experimented with the 2N3904 or other transistors, you will just *LOVE* the 2N7000! The gate is so sensitive that you can turn it on with just a touch. And since it is almost infinite input impedance, the gate can be used to detect all sorts of things like E fields. But for timers it's incredible because you can turn it on for minutes or more with a several megohm resistor and a 100uF capacitor. It's like a super transistor.Later...


Authored by: Windell on Tuesday, January 08 2008 @ 12:14 PM PST A Simple and Cheap Dark-Detecting LED Circuit

Obviously a FET is a better way to go, and I even considered the 2N7000, but I didn't have any at home when I built this. (It really was a five minute project. Even as far as NPNs go, the 3904 is not a particularly good choice, as we discussed earlier-- but it *does* work, and it is very common.) At some point I'll write up a few other variations on this-- including the joule thief version.

In any case, the efficiency of the circuit as drawn is not as bad as you claim, nor is the lifetime of the circuit you describe as good as you claim.

First point: When it's "off" it's drawing 1.5 mA, not 3.6 mA. Because this is *not* a FET, the on-state resistance really is not zero, and the maximum current through the resistor is lower than you claim. Also, it's a 3 V battery -- NOT 3.6 V-- , so the maximum current that you could get, even with an ideal FET, is 3 mA, not 3.6. The phototransistors vary in their maximum on-state collector current, where the poorest bin has a maximum of 1.2 mA. Mine are from Digi-Key and aren't relabeled as to what bin they are from, but under bright light, the highest current that I was able to measure through the resistor was 1.5 mA, as stated in the article.

Second point: Your claim about extending the lifetime to months does not make sense. In this version of the circuit, the "off" current is 1.5 mA, ten percent of the 15 mA that it draws while on. Now matter how you cut it, the power consumption is still dominated by the amount that it uses while the LED is on-- eliminating the small amount used in the daylight will not cause the lifetime to dramatically increase. You know as well as I do that the only way to dramatically increase the lifetime would be to dramatically decrease the average power consumption while the LED is on.



Authored by: Anonymous on Tuesday, January 08 2008 @ 11:18 PM PST A Simple and Cheap Dark-Detecting LED Circuit

I'm sorry, my mistake. I read that the coin cells were from Ebay, so I got on Ebay and looked at some of the offers there. For some reason, I got the impression that they were 3.6V. But I just got through reading the specs (I googled CR2032 datasheet) and it said that the V was 3V. This one, or any other similar: data.energizer.com/PDFs/cr2032.pdf

I'll address the other points in my Jan 8, 2008 blog at watsonseblog.blogspot.com.


Authored by: Anonymous on Friday, January 25 2008 @ 03:08 PM PST A Simple and Cheap Dark-Detecting LED Circuit

Anyone willing to post some specifics about parts to order, say from mouser or fairchild? When I search those part numbers, I get no fewer than 10 + choices each.

Thanks in advance.

Super Noob


Authored by: Anonymous on Monday, February 04 2008 @ 06:49 AM PST A Simple and Cheap Dark-Detecting LED Circuit

Which change will it be in the circuit if we want more leds to light? Because i can't find the phototranzistor can i put a photodiode?and if yes which one?


Authored by: Windell on Monday, February 04 2008 @ 07:14 AM PST A Simple and Cheap Dark-Detecting LED Circuit

Hmm. Looks like you might be redesigning this circuit from scratch!



Authored by: Tarles on Monday, February 04 2008 @ 04:16 PM PST A Simple and Cheap Dark-Detecting LED Circuit

As I was trying to get the components to build this I wasnt able to find LTR-4206E and instead I bought BSP 103-2 which works but only when it is near a bright light.

Any ideas anyone? I would like to have it really sensitive, so if anyone can help me I woould appriciate it.

Thanks in advance.


Authored by: Windell on Monday, February 04 2008 @ 04:22 PM PST A Simple and Cheap Dark-Detecting LED Circuit

Do you know what kind of device the BSP 103-2 is?

I don't see any datasheet around for anything similar sounding except the BSP102-- which is a mosfet, not a phototransistor.



Authored by: Anonymous on Friday, May 09 2008 @ 12:02 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

I used a 5mm phototrans from radio shack. The only one avaliable... Don't know the part number. It was clear not dark. Also switched to a 100k resistor and bumped up the volts to +5. Make sure you have the polarity right on the phototrans. Mine would work reversed but was significantly less sensitive. Took me days to figure that one out. Configured that way it and correctly biased, it is SUPER sensitive. I used it in conjunction with another NP3904 transistor as sort of a Darlington pair to saturate the base voltage. It works great as a dusk to dawn LED solar charging night light thingie. I can light a candle across the room and point the photo trans at it and it emulates the candle flame... Fascinating effect.


Authored by: Tarles on Wednesday, February 06 2008 @ 02:23 PM PST A Simple and Cheap Dark-Detecting LED Circuit

Nice,

still no reply?!

I wasnt able to find that Phototransistor LTR-4206E here and this one which I have needs to real close to light if I want LED to be off.

Can I make phototransistor to be less/more sensitive?

please help me guys

Thanks


Authored by: Windell on Wednesday, February 06 2008 @ 02:28 PM PST A Simple and Cheap Dark-Detecting LED Circuit

I replied last time, and I'll reply again, perfectly clear: No one will be able to help you out with your particular case if you don't provide sufficient detail.

You might also consider reading the other comments for discussions of ways to change the sensitivity, you know.



Authored by: Anonymous on Saturday, February 16 2008 @ 10:25 AM PST A Simple and Cheap Dark-Detecting LED Circuit

Hi, thanks for the circuit. With a little trouble I was able to get it to work. I spent an embarrassing amount of time trying to make it work in my florescent lit office before realizing the LTR-4206E doesn't turn off with florescent lights.

I have a couple of questions. Sorry if they sound dumb, but I'm a bit new to electronics.

1. Photo active components like photoresistors, photodiodes, phototransistors, etc - they all seem to be dark off and light on. Am I correct? Is there such a thing as a dark on light off device? Would such a device be more efficient in this case? Would the current to turn the transistor on be less than "wasted" current in keeping it off?

2. Is there a good reference for transistors, telling what base current is necessary to achieve what collector/emitter current with efficiency, loss, etc?

3. Transistors switch and amplify current, right? Not voltage? The reason I'm asking is I'm wondering how hard it would be to make this device with an LED series array.

Thanks.


Authored by: Anonymous on Thursday, May 01 2008 @ 01:47 AM PDT A Simple and Cheap Dark-Detecting LED Circuit

Hello all,I realize that this may be a somewhat elementary question for most of you, but Id appreciate any and all answers to it as this circuit could be really useful to me. my question is simply if its possible to use this same circuit when powering 3-6 LEDs from a 3v DC power source. If not, is there a way to modify it so that it can perform under those conditions?

Thanks,michael


Authored by: Anonymous on Saturday, May 31 2008 @ 02:09 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

yes you can. simply wire all of those led's in parallel.


Authored by: Anonymous on Wednesday, July 09 2008 @ 12:07 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

I got all the components that are needed for the project, but at the moment to do that doesn't work because the LED is always on, I would like to know why.I appreciate any help


Authored by: Windell on Wednesday, July 09 2008 @ 12:09 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

Are all the components exactly the same as we're using?



Authored by: Anonymous on Thursday, July 10 2008 @ 09:30 AM PDT A Simple and Cheap Dark-Detecting LED Circuit

Yes Even I test all components with a multimeter and all are ok.


Authored by: Windell on Thursday, July 10 2008 @ 10:55 AM PDT A Simple and Cheap Dark-Detecting LED Circuit

If you know how to test a transistor with a multimeter well enough to be sure that it's good, then (1) I'm impressed (2) surely you can find the problem in your circuit without our help!



Authored by: Anonymous on Thursday, July 10 2008 @ 07:22 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

Well thanks, sorry for any inconvenience.

My multimeter is the MUL-040 from STEREN.


Authored by: Anonymous on Tuesday, July 22 2008 @ 07:36 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

What would work for both sun and florcent (CPF) lightsThanks for the idea and it does with with a standard flashlight


Authored by: Windell on Tuesday, July 22 2008 @ 09:44 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

Just use a phototransistor without the dark (visible-blocking) lens.



Authored by: Anonymous on Wednesday, July 23 2008 @ 02:19 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

Thanks. I ordered some and will write back with my findings.


Authored by: Anonymous on Tuesday, September 16 2008 @ 07:09 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

How would you do this wiht a PNP transistor? Like the 2n3906?


Authored by: Anonymous on Wednesday, October 15 2008 @ 04:42 AM PDT A Simple and Cheap Dark-Detecting LED Circuit

who invented the dark detector circuit and when was it invented?


Authored by: Anonymous on Monday, November 24 2008 @ 11:35 AM PST A Simple and Cheap Dark-Detecting LED Circuit

How would the circuit be diffrent if we had a photoresistor, instead of a phototransistor ?

thanks


Authored by: Anonymous on Saturday, January 31 2009 @ 01:55 AM PST A Simple and Cheap Dark-Detecting LED Circuit

how can i make dark/light detector using a general purpose transistor?


Authored by: Anonymous on Saturday, February 21 2009 @ 03:38 PM PST A Simple and Cheap Dark-Detecting LED Circuit

I've been looking for these parts on radioshack, and i wanted to know if i can use radioshack part 276-142 in this project. Is 276-142 the same thing as the phototransistor used in this project?


Authored by: Windell on Saturday, February 21 2009 @ 04:32 PM PST A Simple and Cheap Dark-Detecting LED Circuit

Not exactly the same, but I would guess that it would work.




i have aL14F1 photo transistor can anybody tell me how to connect it and the details of the pins [ i dont know which one is emmiter n which one is collector] ... plz


Authored by: Windell on Saturday, February 28 2009 @ 12:19 AM PST A Simple and Cheap Dark-Detecting LED Circuit

You might try looking at the datasheet for it.




i connected the whole circuit ...... i put a L14F1 photoresistor .. but the led turns on even in day time ... i checked the connections all were fine ... plz help ..


Authored by: Windell on Saturday, February 28 2009 @ 11:09 AM PST A Simple and Cheap Dark-Detecting LED Circuit

That's a very different type of phototransistor that you're using-- a darlington --I'm not suprised that it behaves differently..



Authored by: Anonymous on Sunday, March 01 2009 @ 04:51 AM PST A Simple and Cheap Dark-Detecting LED Circuit

how do i fix a darlington in the circuit ?????? L14F1


Authored by: Anonymous on Friday, March 06 2009 @ 03:25 AM PST A Simple and Cheap Dark-Detecting LED Circuit

What kind of changes might need to be made to this circuit if I want to use white LEDs? I think that they require a bit more voltage. Could I make a simple part substitution and be able to use white?

Thanks,

Scott


Authored by: Windell on Friday, March 06 2009 @ 03:42 AM PST A Simple and Cheap Dark-Detecting LED Circuit

See this post in the forum.



Authored by: Anonymous on Monday, March 23 2009 @ 02:28 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

i have just started electronics and this is the first project i have made. Is there anyway of making it more sensitive to light?


Authored by: Windell on Monday, March 23 2009 @ 04:30 PM PDT A Simple and Cheap Dark-Detecting LED Circuit

Yes; you might want to start with the method that we mentioned above.


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detecting darkness

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