Eng.Remon

hi i have a problem . when i setb some pins connected to a lcd or a relay nothing happen . ? i guess the problem in the output current so is there any way to get more current when i set the pins !?. i checked it wid a led and it works properly but not with the device it self . !!!!!!!

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Eng.Remon

Oh btw i tried a transistor and pull up resistor . it only works wid a photo coupler ic. or 2 inverters in sequence . which will make the pcb large and not logical solution in fact :S .

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Hero999

Please post some schematics as it's hard to visualise what you're talking about.

You do need a transistor buffer to operate the relay but you also need a diode in reverse parallel with the relay to absorb the back EMF.


http://www.uoguelph.ca/~antoon/gadge...ys/relays.html

There's nothing critical anout this circuit, here's another example.

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Eng.Remon

umm For example i want to connect the lcd with the micro controller. so nothing would happen if i connect it directly to the ports. but if i put the 2 inverters which leads to the same output it works :S so i guess the problem in the current coz the voltage is ok . i tested it with a led and it is right .also it works when i put 2 inverters in sequence .

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Hero999

That's nice but without a schematic, it's hard for us to help you.

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Eng.Remon

schematic for what ?! it is at89c51 and p1 is output for the lcd and there is p2.0 to p2.2 3 control pins for enable and read and write . but when i connect them to the microcontroller pins no output . but when i put 2 inverters to give the same output as if i just get from the uc it gets output .
If u need schematic tell me what schematic u want.

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3v0

Herro999 is not being unreasonable. It was only in your last post that you told us what uC you are using. Port drive current differs so it makes a difference in regards to you problems.

When talking about the relay give the schematic with the uC and relay. Unless you are using a tiny relay with a low current coil you need a transistor.

The LCD does not require much current but can be hooked up several different ways. Again the schematic.

You can draw simple schematics using a paint program. Or if you want to get a leg up on making PCBs use Eagle's schematic editor. It has an export function that allows you to create an image you can post.

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HarveyH42

So far, I've only used ATTiny... but doesn't the LCD need 5v logic levels? Relay needs a driver. What's you power source? Regulated? Decoupling capacitor? Got a diode across the relay? A schematic of your circuit would have answered all these questions and more.

I usually simulate a circuit on the computer before I build, but my program doesn't do MCUs, so I do a quick sloppy sketch to keep my pins straight while wiring it up on the breadboard. I've gotten help here just off a scan of the sketch, and many times from poor digital photos.

The LCD may not be intialized proper, wrong clock or delays. Contrast might not be set, backlight... You most understand that without pictures, and very few details, its tough to visualize what you have done, and to see what went wrong.

donniedj

Eng.Remon:
LCD's have more conditions compared to a Relay to be properly driven and high current on the logic lines is not one of them.

1. Do you mean the LCD backlight leds are working with the addition of 2 inverters?
2. Are you monitoring the voltage out the microcontroller pins with a voltmeter or o-scope and the LCD load unattached and attached?

Eng.Remon

donniedj yea it deos. and i monitory the voltage on the pins of uc using voltmeter and it is around 4 to 5 volt . i ll try to attache the schema which works wid .

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