Fading LEDs

[littletransistor]

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:

I see. So can I replace the 555 oscillator with the basic and classic 2-transistor, 2-capacitor circuit astable system? These also create square waves too right?

 

[Nigel Goodwin]

I see. So can I replace the 555 oscillator with the basic and classic 2-transistor, 2-capacitor circuit astable system? These also create square waves too right?

No, because you're not using the squarewave output, you're buffering the charge/discharge ramp on the timing capacitor.

 

[audioguru]

I see. So can I replace the 555 oscillator with the basic and classic 2-transistor, 2-capacitor circuit astable system? These also create square waves too right?

An astable multivibrator using silicon transistors harms the transistors if the supply voltage is more than about 6V which is the absolute max allowed voltage for the reverse emitter-base voltage for a silicon transistor. The emitter-base junctions have avalanche breakdown.

The astable multivibrator circuit charges each capacitor to the supply voltage then when the transistor on the other side conducts, the capacitor forces the base of this transistor to a negative voltage.
Here is a circuit with an explanation:
https://hobby_elec.piclist.com/e_ckt7.htm

Instead of using an opamp that doesn't work at a low supply voltage I have one or two transistors:

 

[Nigel Goodwin]

Instead of using an opamp that doesn't work at a low supply voltage I have one or two transistors:

Hey! - isn't that Krumlink's capacitor?

 

[audioguru]

Hey! - isn't that Krumlink's capacitor?

No.
This is a brand new filter capacitor because the one that Krumlink used blew up because it shorted the output of a 555 oscillator.

Krumlink likes shorting things. Krumlink likes blowing up things.

 

[littletransistor]

An astable multivibrator using silicon transistors harms the transistors if the supply voltage is more than about 6V which is the absolute max allowed voltage for the reverse emitter-base voltage for a silicon transistor. The emitter-base junctions have avalanche breakdown.

The astable multivibrator circuit charges each capacitor to the supply voltage then when the transistor on the other side conducts, the capacitor forces the base of this transistor to a negative voltage.
Here is a circuit with an explanation:
https://hobby_elec.piclist.com/e_ckt7.htm

Instead of using an opamp that doesn't work at a low supply voltage I have one or two transistors:

Cool man, that 1st one is pretty nice and simple and I have these components.

I think I can guess I couldn't directly feed the 741 because it ended up with triangular waves below 0V... :shock:

 

[audioguru]

Instead of the output transistors that I used, you could use a 741 opamp if it has a higher supply voltage.
The output voltage of the two-transistors multivibrator doesn't go negative, the base voltages of the transistors go negative.

 

[Hero999]

An astable multivibrator using silicon transistors harms the transistors if the supply voltage is more than about 6V which is the absolute max allowed voltage for the reverse emitter-base voltage for a silicon transistor. The emitter-base junctions have avalanche breakdown.

The astable multivibrator circuit charges each capacitor to the supply voltage then when the transistor on the other side conducts, the capacitor forces the base of this transistor to a negative voltage.
Here is a circuit with an explanation:
https://hobby_elec.piclist.com/e_ckt7.htm

Instead of using an opamp that doesn't work at a low supply voltage I have one or two transistors:

That's nonsense, I've built and run an astable multivibrator for hours from 9V when I was about 12 and it didn't damage the transistors.

 

[Nigel Goodwin]

That's nonsense, I've built and run an astable multivibrator for hours from 9V when I was about 12 and it didn't damage the transistors.

If you're lucky, and you happen to use transistors suitable, you 'may' survive at 9V - increasing to 12V makes it pretty certain to die.

If you were flashing LED's in the collectors bear in mind this also drops the supply by their forward voltage drop.

I first came across this at college, where we were split in to two teams to build a simple 'one armed bandit' from a block diagram. The project had been knocking around for years, but they had never had a group capable of it.

In the end the two teams were amalgamated, with me and another guy doing most of the work - one of his tasks was to build three astable multivibrators. And despite the fact I repeatedly told him, even showing him the reason in a book from the library, he kept blowing the transistors in them. In the end he had a pile of 50-60 dead transistors, but managed to find six that survived in his astables! - this same guy actually became a teacher at college later on!.

For anyone interested, the project was three 7 segment displays (incandescent - long before LED's), using just the horizontal segments. You had to stop the changing segments in a straight line, using a stop button on each.

It consisted of:

3 x astables (one for each channel)
3 x bistables (for the stop buttons)
3 x ring of three counters (for cycling the displays)
3 x drivers and displays
Logic gating to drive a WIN/LOSE light.

This all had to be built using just discrete components (it predated IC's really anyway, although I think RTL was out?), each module had to be built on a seperate matrix board, with all components and wiring on the top surface. The entire device was built under a perspex cover so visitors on open day could see the internal workings, and play the machine.

If most people haven't heard of a 'ring of three counter', neither had we back then! I went to the library and found out about them in a book!.

 

[audioguru]

That's nonsense, I've built and run an astable multivibrator for hours from 9V when I was about 12 and it didn't damage the transistors.

If the collectors have LEDs then the emitter-base junction gets 6.8V with a brand new 9V battery which might be low enough not to cause damage.

When I worked for Philips the first thing I was told was to never allow an emitter-base junction to experience avalanche breakdown because it forms hot spots on the tiny junction that is not supposed to dissipate heat. The hot spots destroy the current gain of the transistor.

In a two-transistors multivibrator the capacitors discharge with fairly high currents into the avalanching junctions.

I found an article that explains it:
http://adsabs.harvard.edu/abs/1976MiRe...15..619M
"The operation of silicon planar transistors under emitter-base avalanche breakdown conditions can cause drastic reductions in current gain. Having investigated the dependence of the degradation rates on avalanche current for two types of n-p-n transistor, the mechanism, carrier injection into the protective oxide is explained, and experimental data from tests on 300 transistors is compared with published theory."

 

[Hero999]

You're right, they were LED flashers so the LEDs dropped the voltage to a safe level.

 

[Roff]

Here's a simple LED fader you can make with one op amp and one transistor, plus a few resistors and a cap.

 

[Overclocked]

No.
This is a brand new filter capacitor because the one that Krumlink used blew up because it shorted the output of a 555 oscillator.

Krumlink likes shorting things. Krumlink likes blowing up things.

Pyrotechnics Maybe a more suitable hobby for him then

 

[littletransistor]

Okay, I already tried hooking up an astable multivibrator to the opamp using audioguru's suggestion, and it works. but actually, what's that opamp configuration, a negative feedback? The LED fades, but not smoothly.

Also, the one which doesn't involve an opamp (only another 2n3904 + cap) worked too, but the fading is too fast to be seen. (it looks like it's just blinking).

 

[Overclocked]

Okay, I already tried hooking up an astable multivibrator to the opamp using audioguru's suggestion, and it works. but actually, what's that opamp configuration, a negative feedback? The LED fades, but not smoothly.

Also, the one which doesn't involve an opamp (only another 2n3904 + cap) worked too, but the fading is too fast to be seen. (it looks like it's just blinking).

Lets see, I think its called a Integrater. It converts a Square Wave signal to a triangle Wave. You can slow it down by modifying the RC time constant (ie, make either R or C larger). Ive built the one I posted a couple posts back, and its pretty smooth. It basically looks like what the <now> old macs used to do when turned on.

 

[Krumlink]

No.
This is a brand new filter capacitor because the one that Krumlink used blew up because it shorted the output of a 555 oscillator.

Krumlink likes shorting things. Krumlink likes blowing up things.

Yup

I built a simple audio amp using a lm386, and I had a capacitor on it backwards, and with 12 Volts going through it backwards it exploded very loud like and fireball like

 

[Sceadwian]

I'm building an LED fader for an RGB LED that's going into the base of a large crystal my stepson has using a tiny11. 1 tiny11 3 resistors and 1 RGB LED. Total cost of parts was less than 2 dollars. Completly programmable and there are three pins left to use for a second RGB led or a few buttons to add useful features. I'm going to have a single button for input just so the currently cycled colour can be stopped if desired. I bought a 10 pack of tiny11's just for random uses like this, can replace tonnes of analog circuitry.

 

[audioguru]

WalMart had a fading LED globe with 3 colors of LEDs. It used a microcontroller and you could see the fading jerk between the steps.
My analog fading 3 colors of LEDs fade smoothly.

 

[Hero999]

Digital would be fine if you have enough steps but analogue is eaiser to go and gives better results.

 

[HiTech]

Hey! - isn't that Krumlink's capacitor?

LMAO!! "Krumlink's Kapacitor"
Sounds like something right out of an Isaac Asimov sci-fi novel.

**broken link removed** **broken link removed**