What is causing the unexpected behavior in my Darlington pair circuit?

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The unexpected behavior in the Darlington pair circuit using 2N2222 NPN transistors is likely caused by the influence of static charge and electromagnetic noise from the environment. Touching the base lead introduces a small voltage, which, combined with the high current gain of the Darlington configuration, can turn on the transistors and light the LED. The presence of a capacitor connected to the base can also sustain the LED's illumination after the initial voltage is removed. Additionally, interference from 60 Hz mains noise can exacerbate this effect, especially in urban settings. Isolating the circuit from external noise sources can prevent the LED from lighting up solely by touching the base.
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I've built a Darlington pair from 2N2222 NPN transistors on my breadboard, and I find that just my touching of the base lead in the circuit cascades both transistors and let's current flow in the circuit. What's causing this? See the video below for details.

 
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jono20 said:
I've built a Darlington pair from 2N2222 NPN transistors on my breadboard, and I find that just my touching of the base lead in the circuit cascades both transistors and let's current flow in the circuit. What's causing this? See the video below for details.



Your video don't work. But my guess is since you put them in darlington, β of 2n2222 is about 100, two of them darlington give 100000. So if your body is charged up to so voltage and touch the base, you provide enough current to make the pair conduct enough to light up the LED. Just a guess.

Think of if you put a cap from the base to ground, just a small 0.1uF. If you apply a small voltage enough to light up the LED, then you remove the voltage. The voltage from the cap will hold the LED on for a while also. Just static charge.
 
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Your body picks up noise which is mainly 60 Hz voltage and your hand applies this voltage to the base of the transistor.

This turns the LED on and off rapidly and you see light from when it is on.

It is similar to the effect you get if you touch the microphone input of an amplifier. In that case, you get a loud burst of a buzzing noise.

This comes from living in a city where mains noise is everywhere.
 
Is it a result of the DC transformer being plugged into the wall then?
 
It could be. Any alternating magnetic field, and the nearer it is, the worse, usually.
 
That seems to be the case. If I completely isolate the circuit and use charged capacitors as the power source, I can no longer light the bulb by touching only the base, I can by touching the base and positive capacitor lead, though.
 
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