Electronics help needed, ....

... I want to slow start an LED, ie, I throw a switch, and it comes up to full brightness in a few seconds. I was thinking a simple capacitor/resistor, throttling the speed the capacitor charges with the resistor, and putting the LED across the capacitor.

LEDs being diodes though, I have a problem, in that it will 'switch' on a certain voltage, and I don't know what this is. I'm presuming it will be quite low, and the light output quite minimal, so the 'step' won't be too noticeable?

Has anyone done this? Got a circuit/component spec?

It needs to be small as well, and fit inside a diameter of about 3/4 an inch, and 1 inch long.

I know I could use an incandescent bulb to remove the 'step', but I'll need to vary the colour, and may use an UltraViolet LED yet, plus I want decent battery life, and fairly high brightness, so it has to be an LED.

 

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approximately 0.65 to 0.7 volts for silicon.

actually the LED will come to full brightness once the breakover point is reached.
the characteristic curves for diodes suggest that what you want to do is possible but you would need to control the voltage in very minute steps at the breakover point. the slope of the forward biased diode is very steep.

here is a typical characteristic curve for a diode:
http://www.ese.upenn.edu/rca/software/Labview/diodetest/diodetest.html
as you can see the curve is very steep in the forward biased region.

 

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Is that graph for an LED, or a regular diode? Because personal experience using LED torches does show a degradation of light output over time as batteries fade, and common sense says an LED given 0.7 volts isn't going to be as bright as one driven at it's maximum voltage. That graph implies a limit of 1volt, which I just don't buy.

 

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another, more detailed, source:
http://people.seas.harvard.edu/~jon...de_characteristics/diode_characteristics.html

detailed source for LEDs:
http://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/

another source:
http://www2.whidbey.net/opto/LEDFAQ/The LED FAQ Pages.html

 

As voyager pointed out, using an RC circuit as a voltage source will not give you the linear current increase you need because of the LED's inherent barrier potential. An LED's brightness is a function of current across the junction. When batteries in an LED flashlight torch get low, the light gradually dims because their current capacity is diminished, not their voltage capacity (to be academic about it, the voltage does play a part, but it is small).

Therefore, you will probably have luck with a transistor set up as a current source. Then you can regulate the current through the LED to control its brightness, while at the same time keeping the voltage across its junction above barrier potential.

This page is all I could find at the moment to give you an idea for a circuit. It kind of sucks but you should get the idea. For timing the on/off transition, couple your RC circuit to the BJT's base lead. Since the base current will be in microamps, it won't have a noticeable impact on the charge/discharge curves.

 

A thermistor might be just the thing. It exhibits negative resistance.

 

you could also go the digital route and use a binary counter with a R2R resistor ladder.
http://en.wikipedia.org/wiki/Resistor_Ladder

i believe the current limiting route suggested by echo3romero would be a better option.

 

Some form of pulse width modulation would be a lot simpler if you are going to use a microcontroller. Then it would just take the microcontroller and one resister if the micro has a built in oscillator.

 

(rubs hands together) I love threads like this!!

Get's my electronic juices all flowing!

 

How about a couple of 555 timers, one as a vco triggering another as a one shot? Instant pwm.

 

Might be more bother than an 8 pin AVR microcontroller.

 

Yeah but it might be cheaper. And I like analog for simplicity and reliability. But that's just me.

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Programming is a pain anyway. A solution that uses fewer parts would be to use an op amp whose output can swing all the way to ground. Put a resistor and a capacitor with the RC constant that you want on the non-inverting input. Feed the output through the LED, and from the LED run a current-limiting resistor to ground. Tie the inverting input to the top of that resistor. Then the current through the LED will be exactly proportional to the voltage across the timing capacitor. The LED will immediately light up, dimly, then become brighter as it goes. This will work with a power transistor too.

I have single op amps that are suitable but they're the size of a small ant and making the circuit board takes some doing. I don't know of any eight pin op amps that swing down to the negative rail. The LM324 is available from Radio Shack and it fills the bill very nicely. It is the big package and has 14 pins.

 

don't forget that you'll only have about 0.2 to about 0.3 volts to play with.
a diode starts conducting at about 0.5 volts and can be considered "full on" at about 0.7 volts.
this is assuming silicon.

 

The current-limiting resistor goes from ground to the LED. The top of the resistor is connected back to the inverting input of the op amp. The op amp is set up as a voltage follower. The voltage drop across the LED is compensated for. What you get is a current through the LED that is exactly proportional to the voltage applied to the non-inverting input of the op amp. I'll try to get a schematic up pretty soon.

 

To Metakron:

Your suggestion is a good one. It has been a long time so I forget the details, but you probably know them. Something called an "emitter follower" circuit may be possible and only a single transistor instead of an op amp.

 

I know those circuits. The timing capacitor would have to reach at least 2.2 volts. It would start out dark and take at least a fourth of the time to start to light up. One transistor has a pretty low input impedance so it would have to be like a Darlington setup and then you have even more voltage drop. Sometimes that's just fine.

 

You are no doubt correct here, but there seems to be two "fixes."

(1) First and trivial is to see it an initial delay after closing the switch is of any concern. The OP told only the following requirements:

"... I want to slow start an LED, ie, I throw a switch, and it comes up to full brightness in a few seconds. ..."

I bet if the post switch closure was followed by 0.5 (or even 2) seconds of darkness that would be no problem. Also note that the voltage rise on the capacitor will be initially rapid compared to later - fortunately a non-linear increase in LED brightness does not seem to be a problem so long as it is smooth increase. Your Op Amp circuit would be more complex if a linear increase in brightness were required.

(2) Second I assume that a tiny hearing aid battery is to power the system. (Size does seem to be a concern.) Thus it should be possible to use two of these tiny batteries and "float" the system 2.2V higher (or lower?) as needed. Years ago there were op amps in (To5 ?) cans but I gather from your posts that now they are mainly in much larger DIPs etc.

 

Phlogistician didn't say if the room he had for the circuit had to include the battery. I actually own a few eight pin op amps that would fill the bill, but they are in surface mount packages, and the other style that I have some of is actually, like I said, extremely small. By the time you have to use two transistors and associated components you might as well be using an op amp, even a 14 pin DIP. I suppose a 741 or half of a 1458 would give the same performance as two transistors. I just don't know of any eight pin PDIP packages that include an op amp whose output swings all the way to the supply rail. The tiny ones that I am talking about I have actually used in time delay applications in the real world.

 

This thread seems to have developed into a contest to design the most unnecessarily complicated circuit for a simple task, heh.