I have a circuit that uses a PNP transistor to enable or not the current to flow and turn on a LED. The point is, is it a good engineering practice to conect the base of the transistor to the Vcc of my NE555P, so that it will be grounded through the ~15kOhm resistance of the IC and saturated whenever there is no voltage in it, and when there is 12V in Vcc it will open the transistor as I desire? Another solution would be using 2 NPN transistors in a logic that would do the same job.

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    \$\begingroup\$ I have a circuit... To us you don't until you include it here. Please include the circuit, I'm too lazy to convert your text into a schematic in my head. \$\endgroup\$ Commented Apr 2, 2019 at 11:45
  • \$\begingroup\$ I agree with @Bimpelrekkie. Schematic would be really helpful. What I understood is that you want to use PNP transistor as a "key", that is, just turn it on and off. This is normal situation for the transistor. \$\endgroup\$
    – Tako
    Commented Apr 2, 2019 at 11:50
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    \$\begingroup\$ There are a number of ways to use a bjt for similar purposes. Your use of English to describe the situation is ambiguous and difficult to fully parse. It would help a great deal if you would supplement it with a proposed schematic. Use the schematic editor available to you to add a detailed example. We'll be better able to understand you, then. \$\endgroup\$
    – jonk
    Commented Apr 2, 2019 at 11:52
  • \$\begingroup\$ Link to datasheets for the parts you reference as well. \$\endgroup\$ Commented Apr 2, 2019 at 12:34

1 Answer 1


No, it's a terrible idea. There's more than just the three 5k resistors connected between Vcc and Gnd inside the 555. In fact, the PNP B-E junction will pass enough current to operate the 555, so it will never actually turn off.

You could do it with a P-channel MOSFET, however.


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