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I recently purchased an NPN transistor (BC108) for the following circuit. While I can identify the emitter, base, and collector on the circuit diagram, I’m unsure how to identify these terminals on the physical transistor.

An image of NPN BC108 Transistor

An image of NPN BC108 Transistor clearly showing the terminals

Schema

You can read more about the project on page 11(project 3) of this pdf. https://ncert.nic.in/pdf/publication/sciencelaboratorymanuals/classXII/physics/lelm314.pdf

Additionally, out of curiosity, can I substitute a BC547 transistor for the BC108 in this circuit? Would it work without modifications, or are there any important differences I should be aware of?

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    \$\begingroup\$ The answer to both of your questions is available in the datasheets for the two parts. The BC108's datasheet will tell you the pins (although they are also in your attached picture), and both datasheets will contain tables with the performance data of the parts. You need compare them and decide if they are close enough for your purpose. I add this as a comment because it doesn't directly answer your question, but is a general recommendation for a good habit to develop as you work with electronics: know where to get data sheets, and how to use them. \$\endgroup\$
    – Jeremy Impson
    Commented Nov 25 at 22:42
  • \$\begingroup\$ If you have more NPNs wouldn’t you want to use them instead of a reed relay? \$\endgroup\$
    – D.A.S.
    Commented Nov 26 at 2:07

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On BC108 the pin close to notch is the emitter. Next to it is the base. You can see it on picture.

You can substitude it with BC547 since no special transistor properties are needed for this circuit.

Btw the circuit is bad designed because it shorts a big capacitor like 1mF with SW2 switch. It is harmful for capacitor and switch also.

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  • \$\begingroup\$ back when BC108 was a common part capacitors had higher internal resistance. theses days adding 10 ohms in series with the capacitor (or in series with the switch) would help protect the switch. \$\endgroup\$
    – Jasen Слава Україні
    Commented Nov 26 at 2:27
  • \$\begingroup\$ In the PDF I shared, the Procedure given on page 12 does not ask us to close the switch S2, so the capacitor won't be short. They just probably added a useless branch by mistake. \$\endgroup\$
    – Achyuta Dixit
    Commented Nov 26 at 3:00
  • \$\begingroup\$ @AchyutaDixit Ya. Anyway, the charge should be removed completely before the test to get precise result. Also, if the test has to be repeated for some reason. \$\endgroup\$
    – Michal Podmanický
    Commented Nov 26 at 3:09
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There is a pin diagram of the BC108 transistor given right below the schematic you have included.

The label "from below" means that you should look at the transistor from the pin side, orient it like in the drawing, with pins facing you, and then the pin labels on the drawing will describe the physical transistor.

These transistors were designed in the early 1960s, and went on the market in 1966 :)

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enter image description here

For the BC108 the sequence is also CBE with E on the flag.

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