Hi amaele, Please post links to your circuits when asking for help: it save a lot of messing about trying to find the circuit you are asking for help about! To find out how to do this, please see: :) Your original circuit is here: Now, to fix your problem: Your original schematic used a 'passive' LM324 schematic symbol, i.e. one that has no spice model(s) attached to it. The quickest way to fix this is to go to EasyEDA Libs, select the '5pin Operational Amplifier' symbol and place it on the schematic. Then edit the name from UA741 to LM324. This creates an 'active' LM324 symbol with a spice LM324 .subckt model attached to it. Copy that symbol 3 more times so that there are 4 individual active LM324 opamps in the schematic plus the original passive quad LM324 symbol. Connect the 4 individual opamps up to the relevant pins of the original LM324 quad package symbol and then delete the quad package symbol. Correct the Transient settings from: **Maximum Timestep = 20m** Stop Time = 20m Start Time = 0 to: **Maximum Timestep = 20u** Stop Time = 20m Start Time = 0 Do: **Ctrl+R > Run** The schematic now simulates. However, it produces only a DC output of about +40mV but that does not mean that the circuit is not simulating. That may be what the circuit is expected to do or there could be some other error in the circuit. Without further information about what the circuit is supposed to be doing it is not easy to diagnose any further trouble, Please see your fixed circuit at: :)