Light sensitive switch

The voltage is NOT the same thing as POWER. How much current or power is your supply capable of giving? How much current or power does your circuit need? An LCD (liquid crystal display) is a component and not a circuit. It would help if you can post the circuit schematics you are working with.

My site

has a system for learning basic analogue electronics and includes stuff like light sensitive circuits and relay outputs which would isolate your light circuit from whatever you want to switch. It would also enable you to double pole switch ie isolate both + and - side of supply to your controlled circuit.

Regards sirkituk

Looking at the rest of your post, I'm guessing that you want to switch off an LCD display module at night?

It's connecting to ground via the ESD protection diodes on the data/control lines. You should disconnect its +5V supply instead, & run all the control lines you're using on it via open-collector buffers, or diodes, depending on what kind of circuits you're connecting to it, that will still be powered u;p when the LCD isn't.

If you're doing it to save power, maybe use a photo-transistor or an LDR to to drive a FET, & use that to drive an opto-relay. Use the relay to switch the 5V to everything else. Shouldn't take more than half a dozen discrete parts.

If you just want to blank the display at night (or in a cupboard, etc), I'd wire a photo-transistor & a resistor into the contrast-control pin on the LCD panel to set the contrast to zero.

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   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      

sirkituk skrev:

The comparator circuit is here

(I used the A part)

I have 5VDC supply from usb (100-500mA), I dont know how much the LCD is drawing.

/Jan

Lionel skrev:

Correct.

I would prefer not to have to modify the LCD module, I brought it in a finished state, and all I really have access to it the +5V, lpt port, and perhaps the wires from the lp port to the LCD circuit.

I tried to run the + from the comparator instead, but that didnt work, so I need some way of disconnecting the + supply when the - supply from the comparator is missing.

What about a transistor, with - to base and + to collector and emitter, would that work?

I want it to turn off at night, since I sleep in the same room and the computer is often on.

So a relay is the best solution?

/Jan

Ah. Now it all becomes clear. You're talking about one of those specially made modules that connect to your PC, & have a built-in backlight, yes?

Well, your first problem is that an LM339 is an open-collector output, & a wimpy one at that, so you can only sink current with it, & even then, it's only rated for 16mA max. So I doubt that it'd be able to reliably drive your LCD+backlight anyway.

It seems to me that what might be simplest would be to disconnect both the backlight (usually one or more LEDs) connections from the rest of the LCD module, & drive that separately with your comparator (16mA /might/ be enough to give you a useful amount of light out of your backlight), or use the comparator to drive a small transistor, & use that to power the backlight.

On most LCD modules that have a backlight, it's generally wired straight to two pins on the connector, for the exact purpose of making it easier to drive it separately from the rest of the module. You'd need to trace it throught to wherever it's getting its power now. (Most likely straight to the +5V connection via a small resistor on one wire, & direct to ground on the other.) Alternatively, you could phone or email the makers, & ask them for the info. If you tell them that you need to isolate the backlight so you can drive it separately, they should know what exactly what data you need. Having rewired the backlight, wire it in place of the LED in "Circuit A" on your diagram, replace the 1K resistor with a 120R resistor, & give it a test. If it works, but is too dim, use smaller resistors until you're happy. If shorting out the resistor isn't enough, you'll need to add a PNP transistor to boost the currnt through the light.

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   W  "Some people are alive only because it is illegal to kill them."
 . | ,. w ,      

Lionel skrev:

yes

Im using LM331, but guess its the same for that one. Right now the comparator works, but by working on the ground level, so it cant turn off the lcd, which gets ground from the data cable?

I had no idea switching a simple 100mA 5V LCD would be this difficult. I will look into changeing the backlight instead.

/Jan

Look through an encyclopedia of electronic circuits in your local library.

R