Fading LEDs

[littletransistor]

Hey there,

So I have already constructed an astable multivibrator circuit (i ordered a cargo of new components), and I'm going to experiment with some opamps as well.

Could I hook up my u741 opamp to the simple astable multivibrator circuit (2 transistors, 2 caps ones) and from the output, I can see the LED fading on and off?

I checked the Google online for the Fading LED, but all they use is 2x opamps, so will try something different. Is it feasible?

 

[Hero999]

Why not use an MC1458 dual 741?

 

[littletransistor]

Why not use an MC1458 dual 741?

Man, i wish I had these dual-741s.

Any ideas of using a 555 and connected to the 741?

 

[Hero999]

You could always buy some.

Where do you live?

You haven't filled your location int.

If you live in the UK I could post some to you, I've got loads of them I'll probably never use.

 

[audioguru]

You can use an old 741 opamp to buffer the voltage across the timing capacitor of the 555 oscillator to drive fading LEDs like this:

 

[Krumlink]

You could create a Sine wave by using a 555 and a capacitor parallel across the output, it wont look very neat, but it should work. Just use a low enough freq.

 

[audioguru]

A capacitor on the output of a 555 won't change its square-wave into a sine-wave. The capacitor will just short the output and make the 555 smoke.

 

[Krumlink]

Really? I thought a Capacitor on the output (one lead one output, and the other on ground) or any square wave would smooth it out, making the rise and fall more rounded. It should work, that makes no sense. Then just have a LED on the output after the capacitor:

---||-GND
|
LED
GND

 

[rmn_tech]

'Then just have a LED on the output after the LED:'

To quote your previous statement.

'that makes no sense'

 

[Krumlink]

Spelling mistake jeez.

 

[Krumlink]

This is what I mean

 

[audioguru]

The max allowed output current of a 555 is 200mA.

The output of a 555 switches. If a capacitor is connected from its output to ground and its output goes high then nothing limits the capacitor's charging current so the output transistor in the 555 will try to pass a few Amps and will blow up.

I think you also mentioned having an LED from the output of the 555 to ground. It will blow up or the output of the 555 will blow up or both will blow up.

 

[Hero999]

The LED needs a series resistor.

Try:
An AC LED (two LEDs connected back-to-back in the same package) with a series resistor and capacitor in series with it.

A bridge rectifier with a capacitor in series with it and and LED and series resistor connected to the output.

 

[Krumlink]

I Know A Led Needs A Resistor Im Not Retarded! Jeez!

 

[Hero999]

Well you could've fooled me.

Even if you added a series resistor, how you've drawn it the capcitor will just charge up. Remember capacitors block DC current and the diode only allows current one way, therefore creating DC.

 

[audioguru]

A simple lowpass filter is a series resistor then a capacitor to ground. It doesn't filter a square-wave much so the output is a curved triangle wave.
The output of the filter needs a high impedance load so a transistor or opamp must be fed from it to drive an LED.

 

[Krumlink]

Well, I have seen people turn a square wave into a very rounded square wave with a paralleled capacitor. I wish I could find the site

 

[audioguru]

An ordinary opamp limits its output current to about 20mA so you can connect a capacitor to ground on its output and a square-wave might be rounded a little.
But we were talking about a 555 that will try to blow it self up if you short its output with a capacitor.

A Cmos 555 has much less output current than an ordinary one so a capacitor to ground at its output might produce a slightly rounded square-wave.

A common emitter transistor has a fairly high output resistance (its collector resistor) so its output can be filtered with a capacitor to ground.

 

[Overclocked]

**broken link removed**

Scroll Down a bit, you'll see it. You can use 2 741 op-amps in place of one 1458.

 

[Nigel Goodwin]

Well, I have seen people turn a square wave into a very rounded square wave with a paralleled capacitor. I wish I could find the site

Just a crude low-pass filter, it doesn't produce a sinewave, just smooths the corners a bit - and not suitable for feeding an LED.