A simple circuit with LM358

[giorgik]

Hello everyone, I have a LM358N and would like to make a very simple circuit that uses power as a 9V battery. You can help out by showing a simple circuit easy to try on breadboard ?
For example, an elementary non-inverting amplifier that uses as input signal a square wave generated by the oscilloscope (the one from 2V or 0.2V used to calibrate its probes) ?

[LA7SJA]

Since you have one you can experiment with wy not start by reading the datasheet? http://www.ti.com/lit/gpn/lm358-n You should find som schematics to try out.

Johan-Fredrik

[giorgik]

I did this kind of circuit, see Annex, but the oscilloscope displays no square wave on pin 1 of the LM358

[Kalvin]

I did this kind of circuit, see Annex, but the oscilloscope displays no square wave on pin 1 of the LM358

If you compare your input signal amplitude to the voltage divider amplitude, you may find the reason for the anomaly

[giorgik]

because it works as a comparator?
Vref = 4.5V and Vin = 1Vp -> Vout = 0V

[Kalvin]

because it works as a comparator?
Vref = 4.5V and Vin = 1Vp -> Vout = 0V

And when the op amp output will be in the "high" state?
Ps. The component values in your schematics would indicate that the Vref should be 2.5V.

[giorgik]

Vref 2.5V and why ? Vref = 9V / 2 = 4.5 V

[Kalvin]

Vref 2.5V and why ? Vref = 9V / 2 = 4.5 V

Sorry, the font used made the 9 look like 5 in my phone screen.

Anyway, the question remains when the op amp should change its state from 0V to 9V.

[giorgik]

Yes you are right, in this configuration, you always Vout = 0V.
However, the LED should not be lit?

[Kalvin]

Yes you are right, in this configuration, you always Vout = 0V.
However, the LED should not be lit?

Well, you can determine the voltage over the resistor and LED when the op amp output is 0V and when the output is 9V. Then you can calculate the approximate current flowing through the LED in both cases. Then, you will find the answer

[Zero999]

Yes you are right, in this configuration, you always Vout = 0V.
However, the LED should not be lit?

The LED will light when the output is near 0V because its anode is connected to +V.

[Brutte]

I am sure you could easily google how op-amp works to know that the output won't change if both inputs do not cross each other (+- offset voltage of mV range). Yes, using op-amp as comparator is not a fortunate solution.
Another thing is that the output of this particular op-amp (LM324) is not rail2rail. It does reach close to gnd but has very limited performance there (it is slow and weak below 0.6V region). Even with inverting amplifier your output would have been clipped.

I suggest that you should download LTSpice. It is free, easy and powerful simulator (with GUI) from Linear Technologies.
It includes ideal op-amps that money can't buy.

[Seekonk]

It sure looks like a 5 to me.  Anyway that schematic is very educational......Don't believe everything you see.  Pin #1 will never change till the input goes above the reference of 4.5V and that won't happen with a 2V input.  And what is the point of a square wave input to a comparator.  That should be a sine wave of at least double the voltage to show anything.  Stop trying to make that circuit work.

[giorgik]

Ok thanks for your advice