A simple circuit with LM358 - Page 1
Forum section: electronics / beginners

author: giorgik
author_rank: Contributor
timestamp: November 21, 2015, 03:51:16 pm
content: 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) ?

author: LA7SJA
author_rank: Regular Contributor
timestamp: November 21, 2015, 04:07:47 pm
content: Since you have one you can experiment with wy not start by reading the datasheet?http://www.ti.com/lit/gpn/lm358-nYou should find som schematics to try out.Johan-Fredrik

author: giorgik
author_rank: Contributor
timestamp: November 21, 2015, 05:17:41 pm
content: I did this kind of circuit, see Annex, but the oscilloscope displays no square wave on pin 1 of the LM358

author: Kalvin
author_rank: Super Contributor
timestamp: November 21, 2015, 05:27:07 pm
content: Quote from: giorgik on November 21, 2015, 05:17:41 pmIf you compare your input signal amplitude to the voltage divider amplitude, you may find the reason for the anomaly

author: giorgik
author_rank: Contributor
timestamp: November 21, 2015, 05:47:35 pm
content: because it works as a comparator?Vref = 4.5V and Vin = 1Vp -> Vout = 0V

author: Kalvin
author_rank: Super Contributor
timestamp: November 21, 2015, 05:54:58 pm
content: Quote from: giorgik on November 21, 2015, 05:47:35 pmAnd 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.

author: giorgik
author_rank: Contributor
timestamp: November 21, 2015, 06:04:31 pm
content: Vref 2.5V and why ? Vref = 9V / 2 = 4.5 V

author: Kalvin
author_rank: Super Contributor
timestamp: November 21, 2015, 06:07:03 pm
content: Quote from: giorgik on November 21, 2015, 06:04:31 pmSorry, 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.

author: giorgik
author_rank: Contributor
timestamp: November 21, 2015, 06:11:20 pm
content: Yes you are right, in this configuration, you always Vout = 0V.However, the LED should not be lit?

author: Kalvin
author_rank: Super Contributor
timestamp: November 21, 2015, 06:14:48 pm
content: Quote from: giorgik on November 21, 2015, 06:11:20 pmWell, 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

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2015, 06:15:32 pm
content: Quote from: giorgik on November 21, 2015, 06:11:20 pmThe LED will light when the output is near 0V because its anode is connected to +V.

author: Brutte
author_rank: Frequent Contributor
timestamp: November 21, 2015, 06:17:21 pm
content: 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.

author: Seekonk
author_rank: Super Contributor
timestamp: November 21, 2015, 06:27:05 pm
content: 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.

author: giorgik
author_rank: Contributor
timestamp: November 22, 2015, 06:53:31 pm
content: Okthanks for your advice