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I've done this with real components, but it won't sim: Often I have needed a 555 output with inverted logic and have achieved that by using the second timer in a 556 as an inverter. But the Circuit Lab sim isn't oscillating. Can any one tell me why? Also, by feeding back the output through a resistor to the trigger and Threshold inputs (and a capacitor) a 50% duty cycle square wave can be achieved -- but the sim won't oscillate. |
April 23, 2013 |
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You cannot run the CL 555 timer at any voltage other than 5V without it giving very confusing results. This should explain at least part of your problem. About the CL 555 timer. The high level voltage at the OUTPUT pin of the CL 555 timer model does not track the vcc supply. It is fixed at 5V. The CL model uses a fixed internal supply of 5V and ignores the voltage on the vcc pin. This means that the internal switching thresholds are fixed at 5/3V and 10/3V. This is incorrect and can be misleading. The circuit below correctly models the output swing of the real 555 timer although it does not accurately model the output stage behaviour vs. load currents. Simulate > Time Domain > Run Time-Domain Simulation https://www.circuitlab.com/circuit/9fu6wh/555-timer-as-astable-multivibrator-oscillator-model-error/ There’s a bug report about this here: and a lengthy discussion about some of the other “faults” on the CL 555 timer model here: :) |
April 23, 2013 |
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