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Fixed: The capacitor was defective. It works well with a different one.

I’m new to electronics and need your help.

I built a circuit similar to this one (see comment for the exact components):

Enter image description here

I expect the LED to remain on for a short time when I release the button, my capacitor discharging and providing current to the base, keeping the transistor open.

It does not. What am I doing wrong?

Adding some pictures of my actual circuit:

Enter image description here

Enter image description here

Enter image description here

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    \$\begingroup\$ I am expecting it to be on for a long time too. With the information you give, something is wrong, but without further info, impossible to say where the problem is. Maybe you built the circuit wrong? \$\endgroup\$
    – Justme
    Commented Jan 7, 2021 at 16:17
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    \$\begingroup\$ Consider uploading a photo of your circuit letting us clearly see the connections and components - we might see something obvious:) \$\endgroup\$
    – Jakob Halskov
    Commented Jan 7, 2021 at 16:32
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    \$\begingroup\$ yep. Nr. 1 reason for beginner transistor circuits not working: mixing up the leads on the transistor, or thinking that all transistors have the same pinout and using a different one without adjusting the connections. \$\endgroup\$
    – Marcus Müller
    Commented Jan 7, 2021 at 16:34
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    \$\begingroup\$ Also note that it's not a good idea to short a capacitor directly to a voltage source. The resulting current could for example brown out your supply or damage your switch contacts. Use a small resistor in series to limit the current to something reasonable. Remember I=C*dv/dt. \$\endgroup\$
    – John D
    Commented Jan 7, 2021 at 16:45
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    \$\begingroup\$ Your expectation seems to be correct. 100uF * 100k = 10,000 ms or 10 seconds time constant. That should be long enough to see something. So there must be something wrong with the circuit. Double-check the wiring and the component values. \$\endgroup\$
    – user57037
    Commented Jan 7, 2021 at 16:54

4 Answers 4

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My guess is that your capacitor has the wrong (too small) value, or you connected a polarized capacitor with reversed polarity.

It is also possible that the base resistor (33k) actually has a much smaller value. You need to verify that both of those components have the correct value and are connected properly.

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I replaced the capacitor with another one (smaller cause that’s all I had) and it’s working so I guess my previous cap was defect..

Thanks a lot to all of you who took some time to help me

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    \$\begingroup\$ I would suspect there wasn't a good connection with the original capacitor on the breadboard. \$\endgroup\$
    – CurtisHx
    Commented Jan 8, 2021 at 17:16
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There are many troubleshooting techniques. Here I would recommend simplifying the circuit until the LED illuminates.

Try the left circuit, if it doesn't work, short the E-C of the transistor (right circuit).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you Mattman, the LED was turning on but as soon as the button was released, it was turning off while I was expecting it to remain on for a short time \$\endgroup\$
    – Dominique
    Commented Jan 7, 2021 at 17:09
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This image shows two NPN transistors. Which one do you have connected correctly?

transistor pinouts

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  • \$\begingroup\$ It’s working now (it was the capacitor) \$\endgroup\$
    – Dominique
    Commented Jan 7, 2021 at 19:53

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