I have the following circuit, and I am required to explain how changing various components will affect the LED flashes. I understand that if the capacitor or resistors are changed it will make the LED flash faster or slower, however I'm unsure as to exactly why or if a relationship exists between these factors.

If the capacitance is increased, it takes longer for the capacitor to charge and discharge --> longer time between the LED being on and off?

Will the change of resistors affect the LED a lot? If so, which one will have the greatest effect?

Does changing the voltage of the power source affect anything?



2 Answers 2


The LED can only be turned on when the BC557 transistor's base is connected to ground(it's a PNP transistor which is being used as a switch).

The base of that transistor itself,is being controlled by BC547(NPN transistor) which connects the base of the BC557 transistor to ground to light up the LED.Base of BC547 is being controlled by charging and discharging of the capacitor.I suggest you read about capacitor charging time which will explain the change in frequency due to change in capacitance or resistance.You can also look up LED flasher circuit explanation with different types of circuit design.

  • \$\begingroup\$ if I increase the resistance of all the resistors would it take longer for the capacitor to charge? \$\endgroup\$
    – Lisa Ban
    Commented Nov 4, 2017 at 0:26
  • \$\begingroup\$ yes,look up charging time of a capacitor in google \$\endgroup\$
    – OM222O
    Commented Nov 4, 2017 at 9:42

Another thing you might try -- in addition to learning about capacitor charging time as suggested above --is to actually test it with different resistances, for example. Replace the resistors with potentiometers and vary their values and note the relationships between the various values and the LED blink rate. Experimental data, backed up by numerical predictions, should give you a good answer to this question. And practical experience backed up by theory and math is often the best way to get a deep understanding of a topic -- or circuit.


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