What type of op-amp circuitry drives counterparalleled LEDS ?

[Externet]

Saw a few times, counterparalleled LEDs driven by the single output of a single-supply operational amplifier circuit.

                 /-------------------|<|---------------\
Opamp out >------                                       -------- gnd
                 \-------------------|>|---------------/

Second way, the counterparalleled LEDS driven by two opamps :

                  /-------------------|<|---------------\
Opamp1 out >------                                       -----< Opamp2 out
                  \-------------------|>|---------------/

What is the application, logic, use or name of such circuits ? Are the leds supposed to lit only at a specific audio frequency in that configuration ? (The frequency tailored by other components at the op-amp) Or nothing to do with it ? No series resistors were used with the LEDs.

 

[FvM]

Hopefully the used OP can handle continuous short circuit and the rated LED current is below maximum OP short circuit current. Short circuit current undergoes type variations, I would consider it bad design practice and suggest a series resistor.

The circuit has no particular name or application, everything depends on what the input to the OP is and which information you want to visualize with the LEDs. Similarly, I don't see a specific reason for using two OPs, except for having single supply without ground or the single ended supply voltage is too low.

Are the leds supposed to lit only at a specific audio frequency in that configuration ?

Possible with separate active filter stages (additional OPs), using a passive filter in front of the OP won't give much selectivity.

 

[d123]

Hi,

I've seen that done with two 555s, on endless websites, which alternately sink and source for each LED, you can achieve the same with half an L293. I think it's called a half-bridge, not sure if with op amps it's the same name.

Does #1 actually work? Not too able to understand that style schematic... It looks to me like one LED will, the other is like a blocking diode, never lighting up at least - what's the point of that, what haven't I understood?

Maybe bi-directional, or bidirectional - for the more modern hyphen-less person - is how it's often described.

"What is the application, logic, use..." - Looking at #2, maybe to know which direction communication is taking place in, perhaps through a transmission gate? Or just because people seem to like connecting LEDs this way.

 

[KlausST]

Hi,

I ask myself: why OPAMP?
You need to switch between both LEDs....but want to control it linearely?

Klaus

 

[Externet]

Thanks, gentlemen.
From what I vaguely remember, the opamp output being audio AC, turning one led at the positive cycle, the other on negative cycle, showing as both on for valid audio. The purpose was only a visual tuning indicator of some kind, perhaps RTTY.

The lower schematic could be also the case of the opamp being a complementary output type.

 

[KlausST]

Hi,

In my eyes it makes no sense.
Dividing negative and positive signals to two LEDs will have no effect. You will always see both ON or both OFF.

A LED needs about 1.8V voltage. But an audio signal has a huge dynamic.
Lets say 60dB .... (I see you don't like the dB) it is a factor of 1000.

With very quiet signals (but still audible) maybe you have only 10mV
With normal loud signals you have 100mV
With loud signals you have 1V
And with extremely loud signals you have a couple of Volts.

Either you see only the very loud signals or you need huge amplification.

Klaus