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.

  • 3
    \$\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\$ – Bimpelrekkie Apr 2 '19 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 Apr 2 '19 at 11:50
  • 2
    \$\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 Apr 2 '19 at 11:52
  • \$\begingroup\$ Link to datasheets for the parts you reference as well. \$\endgroup\$ – Spehro Pefhany Apr 2 '19 at 12:34

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.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.