question about LED in circuit

[mcnscott01]

I currently have a circuit that has a photoresistor in it, so as you know when it's bright the LED is bright, when its dim the LED is very dim. Is there a way to make it opposite?

I could make it work with a comparator, but that just turns it off and on, what I need is the LEd always on, just dim in the the light, bright in the darkness, if there's a part or schematic I'm overlooking, I would appreciate the tip, thanks!

[MrAl]

Hi,

Depending on what LED you are using and what photoresistor
you are using sometimes you can get away with putting
the photoresistor in parallel with the LED and running another
fixed resistor to the positive voltage supply.
It also depends on what power supply you are using (batteries or
a wall wart or something).

Usually you need some gain however, like a transistor.
You would run the PR (photoresistor) from V+ to the base
of an NPN transistor, perhaps through another resistor, then
connect the LED to the collector through another resistor.
You have to adjust the resistors a bit to get the results you want.

For better control over the LED brightness you might use a voltage
to current converter, which uses one op amp.

It would be good to know what your photoresistor does...what
resistance it has during bright and dark conditions, as well as
how much current your LED requires and it's nominal operating
voltage, as well as what kind of power supply you are using.
This would help to come up with a circuit that would
work well with your LED and PR.

[Chris Smith]

A simple transistor and voltage divider net work.

The Led [bright] goes through a conventional transistor and on to resistor and then ground.

When the Photo transistor /Photo resistor has no light going through it there is no interfering power going to the transistors base and resistor net work on the LED. [Led is on]

When light strikes the photo receptor it produces power through it to short out the transistors base which grounds the LED. [Led is OFF now]


Led...Transistor...xxxx...photo trans tie in point......Resistor .....ground.

= Bright led to ground when the tie point xxxxx has no pos + at its lead.

When the photo trans is struck by light, it allows a positive signal to go to the base of the standard transistor [-] making it positive instead of negative canceling out its ground, and thus the LED is turned off.

Two positives at this point = LED is off.
OFF and Ground = Led is on

Add in appropriate resistors for the correct bias.

[GoingFastTurningLeft]

Make sure your LED doesn't light up the photoresistor in the dark

[mcnscott01]

thank you for the replies I'll put them into use.

[Bob Scott]

Make sure your LED doesn't light up the photoresistor in the dark

Yes, that would be funny to me but:

I have a Vivitar Japanese SLR 35mm camera with multicolor LED indicators in the viewfinder sight. It tells me not to go below F8 even in complete darkness, because the light from the indicator LEDs interferes with the incoming room light/darkness.

How stupid could they be to actually put this camera into production? I thought Vivitar was a half-decent photographic equipment based company. Not top of the line, naturally, but half decent.

Bob

[Chris Smith]

Black electrical tape, a buck a roll!

[mcnscott01]

I've been told I mislabeled my led, I should have called it the proper term "7 segment display". The next part of the project is multiplexing 3 "7 segment displays", My partner and I decided to use an MC14489 so that we don't use up so many pins on our PIC. I problem again is the lighting and dimming issue. My question is can I apply the transistor trick again?

According to the data sheet for the chip, pin 8 is called Rx which is tied in with Vss with a potentiometer, we used the pot and it works great, however, I think it would nifty if I can still get it to dim in bright light, and brighten in the dark. Will I damage the IC though if I do the transistor trick? Thanks!

MC14489 Data sheet
http://www.cs.net/lucid/mc14489rev4.pdf

[MrAl]

Hi,

You are working with a seven segement LED display?
Wouldnt you want that to brighten in the light and dim in the dark
to help visibility?

[mcnscott01]

I think that's how 7 segments are designed to work, however, I don't want it too dim in the dark, I guess I just like it more if it's brighter in the dark, (bad eyes you know )

[Chris Smith]

If all of the led segments use one common ground, then Yes, the photo resistor/ transistor idea will still work.

It can even work if they all use a common positive feed together and modulate their negative returns on the PIC. [opposite]

[Engineer1138]

If you're concerned about pin usage to mux three 7-segment displays, you may look into a concept called "Charlieplexing" You can run 3 of those displays with just 7 output pins.

[haklesup]

If you just want it to be bright all the time, just cut the photoresistor off and replace it with a fixed resistor which gives the brightness you want.

If the photoresistor also controls another function, leave it intact and rewire the LED to a seperate fixed resistor.

If you really want variable brightness, you need to use something active like a transistor or op amp. I would use an op amp in an inverting amp configuration. I would substitute the photoresistor for R1 or R2 (need to figure that out yourself). Varying the resistance of the photoresistor would change the gain of the amp. If the signal you are amplifying is just a DC reference, you will get a variable voltage at the output.