Need help with a basic circuit

[Logik]

Hello. I'm new here and I need help.

I'm making a simple non inverting amp using a LM741 amp op.

I'm shure about the theoric circuit and about the physical connections.

I need a gain of 5 and the circuit will work on 12 volts to light up a small led.

The input is 0-2 volts PWM @ 10 kHZ and I want the led to light up. This idea works, but the led lights up really dim. Tha't why I wanted to put an amp op and to rise the voltage to be able to light the led up.

The problem, however, is that as soon as I put the power on, with no signal, I get 10V at the output (pin 6).

I connected the +Vcc to +12V and the -vcc to ground.

I wonder if that has to do anything?

When I connect the signal, I see the PWM signal on top of the 10V and it tops out at 10V.

It's as if I had a 10V offset in my signal.

I looked up the input of the circuit on the scope and I have avour 200mv of noise.

I looked up the output and the 10V is a straight line with a bit of noise over it.

Any ideas?

 

[Logik]

This is a scan of my circuit

**broken link removed**

I wanted a gain of 5 so I took RF = 4K Ohms and R1 = 1K Ohms

 

[Optikon]

A bright LED will need about 10-20mA of current.. Can you 741 provide that? Check the datasheet. So You will have a 10V PWM signal.

Dont put 10V directly across the LED. You need to have a current limiting resistor in series with the LED. Your diagram did not show the hookup of your LED which, is important. Show us the WHOLE schematic please.

Your basic circuit looks ok. If it is not working, you have a wiring error.
Double check your wiring. And when you are convinced its connected properly, check it again.

Your opamp output might be railed (feeback pin not hooked up??) -check it.

Is your 0-2V input PWM smoothed? Whats the PWM frequency? If it is too high, the 741 at a gain of 5 might be having problems. - Check it.

Keep in mind the LED could be dim because it is on the threshold of destruction. Make sure you are current limiting 10-20mA. (I doubt the 741 can put out that much at 10V) - check it.

 

[Logik]

Okay, I may have mised a few details.

The circuit in error is actually the one shown here. The led is not connected yet to the circuit.

When I put the power on Pin 7 and 4 and no signal AT ALL on Pin 3, I get a 10V constant at the pin 6.

I've checked the wireing 3 times and it all looks okay.

My pwm is at 10 KHz.

what do you mean about my op amp being railed?

Thanks!

 

[Logik]

I'm thinking that maybe my 4K and 1K resistors are too low and this could cause something like that?

 

[Nigel Goodwin]

I'm thinking that maybe my 4K and 1K resistors are too low and this could cause something like that?

No, it doesn't really matter - it's more likely that you have left pin 3 disconnected - connect it to 0V, and your output should become closer to zero.

However, an opamp is rather overkill for driving an LED? - a single transistor and two resistors would be enough (and one of the resistors is the current limiting resistor for the LED).

 

[Logik]

I'm thinking that maybe my 4K and 1K resistors are too low and this could cause something like that?

No, it doesn't really matter - it's more likely that you have left pin 3 disconnected - connect it to 0V, and your output should become closer to zero.

However, an opamp is rather overkill for driving an LED? - a single transistor and two resistors would be enough (and one of the resistors is the current limiting resistor for the LED).

Would you have a circuit for that?

I'm not used to working with transistors... :?

 

[Nigel Goodwin]

Check from my PIC tutorials, in particular the third graphic' "'Sinking' current from a lamp." - you just need to replace the lamp with your LED and series resistor.

 

[Logik]

Thank you!

I tryed it and it works fine!

I had tryed something similar but I was putting a resistor instead of the lamp and the LED was in parallel to the ground.

So basically I would get the opposit results :roll:


But thank you very much!

 

[Logik]

Now I'm pushing my luck, but I'm pretty shure it can easily be done.

Would it be possible to put another led for when it's not working?

The way I was putting it worked, but I still see a bit of the light, except it's very dim.

I would like it to be completely off or completely on.

This way I would put a red led and a green led.

Thanks again I really apreciate your help!

 

[Nigel Goodwin]

Now I'm pushing my luck, but I'm pretty shure it can easily be done.

Would it be possible to put another led for when it's not working?

The way I was putting it worked, but I still see a bit of the light, except it's very dim.

I would like it to be completely off or completely on.

What is your PWM signal doing? - if your PWM isn't turning OFF, then the LED won't turn OFF either - it's only doing what it's been told to!.

 

[Logik]

Now I'm pushing my luck, but I'm pretty shure it can easily be done.

Would it be possible to put another led for when it's not working?

The way I was putting it worked, but I still see a bit of the light, except it's very dim.

I would like it to be completely off or completely on.

What is your PWM signal doing? - if your PWM isn't turning OFF, then the LED won't turn OFF either - it's only doing what it's been told to!.

Actually, my module is to test if the PWM is really being broadcast or if the connection isn't working. It's a test module...

I would like that when there's no signal, a red led lights up and when the signal works, a green led lights up.

The green led works fine, now if it was possible to work the red led out it would be great. Other wise it's still okay.

But looking at the schematics, I don't think it's possible for the red light to completely shut off then there's no signal.

 

[audioguru]

In my Ultra-Bright Chaser projects, the LEDs were still very dim when the PWM brightness control was turned all the way down. The pulse was so narrow that I couldn't see it on my 'scope. Adding C7 at the base of the driver transistor fixed it.
Sorry for the JPG fuzzyness. I was still a pc noobie.