Roff

I know there have been discussions on these forums of LEDs as light sensors, but I don't remember seeing one used like this.
Fig. 1 here is a night light that uses the same LED for sensing and for illumination. I breadboarded it, and it worked. I'm not sure how useful it would be, but it demonstrates the principle nicely.
Fig. 2 supposedly works in the opposite fashion, but I didn't test it.

EDIT: Here is a more complicated, earlier version of the same idea.

__________________
Ron

Hero999

I've had this kind of idea before but I've never implemented it as I haven't needed to.

The 7555 timer idea looks good but it does have a high standby current at 7.5mA. There must be a way to build it so it uses next to no current when in standby mode.

I haven't analysed the more complicated idea but it looks more efficient.

__________________
What's so bad about Microsoft?

Get Opera it's simply a superb browser.

RODALCO

Cool circuits, I see if i can make time to build one.

__________________
There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

ronsimpson

Roff,

When that second circuit came out in EDN I built one using a very small PIC. The OP-90 can be replaced by the comparator or the ADC in the PIC. I think the LED produced 20 to 30mV if you keep the load very low. That is only 1 to 2 DAC steps.

audioguru

No.
The 7555 is Cmos. It, the LMC555 and the TLC555 have a typical operating current of 100uA with a 5V supply. The max current is 250uA.

An ordinary 555 has a typical operating current of 3mA with a 5V supply, 6mA max. The current is a little less when the output is high.

__________________
Uncle $crooge

blueroomelectronics

Do you know if the 7555's power consumption goes down if you hold it in reset?

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com

audioguru

The Cmos 7555 IC has a current consumption spec that has no note. So it could be with the output high, low or with it actually timing.

Cmos logic ICs draw almost no current when their outputs are high or low.
The ones with a Schmitt-trigger input draw some current in their input Mosfets when they are active which might also happen in a Cmos 7555.

__________________
Uncle $crooge

Hero999

I know that, what I was talking about has nothing to do with the power consumption of the IC but the circuit as a whole. You need to look at the bigger picture rather than just what the 7555 is doing.

Look at the schematic, R1 and R2 dissipate power even when pin 3 is low. This means the circuit will consume 9/1210 = 7.438mA which is far too high for standby operation.

Attached Images

Night light.gif (8.9 KB, 53 views)

__________________
What's so bad about Microsoft?

Get Opera it's simply a superb browser.

Hero999

I've modefied it to use less power.

Now it should only use 257.44A.

I haven't tested it but it should work.

Attached Images

Low power night light.gif (6.6 KB, 42 views)

__________________
What's so bad about Microsoft?

Get Opera it's simply a superb browser.

audioguru

Good, you reduces the wasteful current in the resistors.
But the battery will still need to be replaced every week and there isn't enough wide-angle light for a night light.

It will just be a guiding light like airports use to mark the runways at night for landing aircraft.

__________________
Uncle $crooge

Rolf

I have several < $ 2.00 LED night lights, they light is greenish in color. And they give off enough light to allow you to move around without bumping into things. Because the light is greenish you can look straight at them at close range and not loose you night vision. Like over the counter in a small bathroom.
I have never understood why car headlights are white and not blue! :-)

audioguru

Your greenish light panels are not LEDs. They are luminescent panels.
LEDs are glaring and don't light up a wide area well.

__________________
Uncle $crooge

Rolf

Don't you think I know the difference? Maybe the light is more blue.
I know the ones you are talking about but that is NOT what I am talking about.
I forgot to mention the obvious that they have diffusers on them.
I have lived with them for several years now and they are great.

Hero999

So stick a diffuser on the end or the LED and use a nice and chunky 6V SLA battery to power it.

__________________
What's so bad about Microsoft?

Get Opera it's simply a superb browser.

Rolf

Here is a picture of what I am using. There are several different diffuser designs.

http://www.pbase.com/sinoline/image/94748057

I have blown up part of the text on the packaging for the doubters.