Op amp selection

[goolash]

I am trying to decide what sort of op amp to use for an LED lighting project. It is a variable 6-12 Vdc to 3.5-0 Vdc very slow changing signal conversion (the 6-12 Vdc is an analog signal created by a temperature sensing device), so I have no bandwidth requirements, but the opamp needs to use +12Vdc/gnd as the supply voltages, and 12Vdc reference and input. They will be driving a 3.5 V forward voltage, 30 mA forward current LED. The response requirement is slow enough that I can use a second or third op amp in parallel to provide extra current capacity to drive the 30 mA LED at full on.

I'm limited at the moment to what I can pick up at radioshack or similar, LM741, LM324 quad, TL082/TL082CP dual, or LM339 quad comparator.

 

[kchriste]

Post a schematic. It will help eliminate misunderstandings. None of those OpAmps are the so called rail-to-rail types, which it appears you want, but may not need.

 

[goolash]

I'll scan them and put them up, once they're in a legible state. I doubt I'll need rail-to-rail. I plan on supplying +12V only, with the V- connected to ground. The output voltage will range from 0 to 3.5V only, to drive a 3.5V LED. If it can't quite output 0V exactly, that's fine, this project does not require any level of high precision, just a ramping voltage.

 

[MrDEB]

maybe a voltage compater circuit??

sensing 6-12 vdc??

 

[goolash]

**broken link removed**

There it is. The purpose of the overall design is to gradually dim a blue LED as Vf rises from 0 to 6Vdc, while gradually brightening a green LED, then gradually brighten a red LED from 6 to 12Vdc, while gradually dimming the green LED. The net effect will be a changing color that runs the color spectrum.

The input labelled 12Vdc is a constant, regulated 12V power supply, which will also supply 12V to the + supply pins on the op amps.

This is still a rough drawing by all means, and amongst other things I'm wondering if the auctioneering diodes are necessary or not, and whether I should go for resistors in the megohm range instead of k-ohm. I should have spent more time studying op amps back in my electronic fundamentals course.

 

[kchriste]

The circuit isn't going to perform as you expect due to a few different things:
The blue green and red LEDs will not output ANY light at 1V, so there will be no gradual shift in brightness below a certain voltage. You'd need to determine the minimum voltage for the minimum brightness first. A better method would be to drive the LEDs with a variable current source of 0-30ma.
Referring to the "blue" circuit at the top, U2 is connected wrong for a buffer. The output of U1 should goto U2s + input only and U2s output should connect to its own - input. The + & - inputs, on the same OpAmp, should not be connected together. U4 and U6 are also wrong and should be fixed the same way. Instead of an OpAmp buffer, I would use a transistor to boost the current for the LED.
You should also drive the LED through a current limiting resistor to convert the voltage output of the OpAmp to a varying current.
The green LED will never dim as the voltage rises past 6V.

 

[goolash]

Yeah, I've been seeing some of these things in a cheap java-based circuit simulation I've been running. How large of a resistance would be good to put in front of the diode? Enough to draw the full forward current at 12Vdc?

 

[kchriste]

Since the OpAmps aren't rail to rail, the max output voltage will less than the supply voltage. The LM324, as an example, typically has a max output 1.5V less than the supply voltage (10.5V for 12V supply) when sourcing apx 5ma. Here is an example of using a transistor to boost the OpAmp's max current and a resistor to limit the max LED current:

 

[goolash]

Ok, so I took your suggestions, and with the help of a java-based circuit simulator, came up with the following:

**broken link removed**
With input @ zero.

**broken link removed**
Input at 6Vdc.

**broken link removed**
Input at 12Vdc.

I notice already that I forgot to tie in one of the 12Vdc references, so I will have to go back and re-calculate the op amp resistors, but this seems to demonstrate that it's possible.

 

[goolash]

Well, the simulations all panned out, but when I actually assembled the circuit, the op amp began operating like a switch. The output voltage is 3.4V until I reach about 4V input, and then it jumps to 10.3V. Is the LM324 the wrong op amp to be using here?

Update: nevermind, after checking and rechecking, I noticed I failed to jumper the feedback resistor to the op amp output. Now it works more like what I intended, but still does not ever turn off completely. I am assuming this is due to a small amount of leakage current across the transistor, despite being un-biased? Also, I've found that the best output obtainable on the LM324 using a 12V Vcc is about 10.3 Vdc, so I will be recalculating my resistors for that value.

 

[MrDEB]

here is maybe a better solution

while not using an opamp this circuit does what your asking.
instead of using one multi color led use a red, blue and green one.
I am designing a circuit where the leds light up a cabinet but fades from all blue to all white and a mix of both.
Electronics-Lab.com Blog 4029

 

[goolash]

Yeah, I was thinking about that, but I don't know how to write assember, nor do I have any means of burning my own proms.

 

[MrDEB]

what prom??

it uses a 555 and a 4026
no programming involved

 

[MrDEB]

oups 4029

about $10 or less in parts
that includes a pc board

 

[MrDEB]

if you are planning on building

I have the expresspcb file as I am using same circuit for my cabinet lights that fade from white to blue and combo.
using 32 leds total (8 per array x 3 arrays

 

[kchriste]

MrDeb,
I don't think that is what goolash is looking for. He wants the LED to change color as the voltage from a temperature sensor varies. ie: the LED changes color with temperature.