Hi all,

    I hope you found some time to build the circuit and look for the answers to the questions.

    First of all, did you spot the error on purpose in the text of above circuit ?

    Careful readers should have noticed that the capacitor charges instead of discharges through the two 100K resistors. When you look at the diagram, you see that the switch is connected across the capacitor via a resistor of 5,6K. By pressing the switch, you’ll discharge the capacitor. After releasing it, the capacitor starts to charge. The negative pole of the capacitor will slowly build up a negative voltage through the two 100K resistors. This voltage is amplified in the first transistor and fed to the second transistor.

    The secret of the high impedance (seen at the negative pole of the capacitor) is the use of 2 emitter followers in series.

    The emitter of the first transistor is coupled to the base of the second transistor. The lamp is connected from ground to the emitter of the second transistor. This is the reason why the capacitor mainly charges through the two resistors of 100K.

    So, now for the questions :

    The 10K resistor adds to the high impedance of the circuit. Try to lower it’s value and see the results in the dimming speed !

    So, the lamp is connected between the emitter and the ground to make the transistor act as an emitter follower. Change it’s place to the collector side and watch the difference by observing the lamp. It will stay lit longer and suddenly get dark, due to the amplification difference between the two circuits.

    Aha, the 47 Ohm resistor ! Well, in fact it makes no difference for the lamp and it’s behavior. When we would put the resistor at the emitter side, the voltage will increase also at this side. The current flowing through the lamp remains the same. What will change is the base current, it will be shifted higher.

    Hope you enjoy this little circuit !

    • This reply was modified 4 years, 1 month ago by Michael.
    • This reply was modified 4 years, 1 month ago by Michael.
    • This reply was modified 4 years, 1 month ago by Michael. Reason: Changed font and size styling for easier reading