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Can someone help me. LED doesn't turn on

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 the led 1 and 2 doesn't turn on 

Message 1 of 3

Did you calculate how much current they you get across two 1KOhm resistors in series? How much current do they need to light up (probably more than 6mA)?

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Message 2 of 3


Hi danizunich and altenbach,



The values of resistors as pointed out by altenbach may have merit but the main problem might be somewhere else.


Switches S1 and S2 are both controlled through the A key. S1, however, is normally open while S2 is normally close. Analyzing the circuit, we can see that the two LEDs cannot be lit simultaneously.


With S1 closed (S2 open), LED1 will glow if the voltage in the capacitor is low enough for adequate current to flow through this first LED. As the capacitor charges up the light emitted by LED1 will become fainter. When the A key is pressed the switches toggle states, S1 opens turning LED1 off, S2 closes providing a current path from the capacitor to R2 and LED2. If the voltage in C1 is sufficient to forward-bias LED2, this LED turns on. As the capacitor discharges the light emitted by LED2 becomes fainter.


To obtain this predicted behavior the following should be done first:


  • Initial voltage (IC) of capacitor C1 should be enabled (checked). The initial voltage, in most cases, is left at the default value of 0 V.
  • Initial conditions in Simulation settings should be set to User defined.


Very likely, the capacitor is not initialized and Initial conditions in Simulation settings is set to Determine automatically (the default). This results to strange behavior of the circuit.



Best regards,

G. Goodwin


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Message 3 of 3