newb transistor question


a few questions for you gurus:

To clarify, A PNP transistor is "triggered" when its base is pulled low correct?

Is there another way to get this sort of circuit behavior with a NPN type? Basically I want it so that when part of the circuit is open it turns on a light (closes another part). I drew out a schematic with a PNP transistor that should work. But is there some logic trick to get a NPN to work this way? Am I over thinking this?

If I do need a PNP type transistor, does anyone have any recommendations? I'm wondering a common equivalent to the TIP 120, but of PNP type. is it the TIP 125?

Cheers and thanks in advance!

Transistor biasing. Haven't done that in a decade or two. Gotta go pull that out of the backup tape in the basement archives. NPN transistor will conduct when the base/emitter junction is forward biased, meaning the base is at a higher voltage than the emitter. You want it on when the connection is open? Put your load between Vcc and the collector. Then maybe you tie the base to Vcc through a couple 10K ohms. That will keep it on until you pull the base low to shut off. Would that work for you?

Ahhh, backup tapes. Perhaps they'll never grow obsolete.

Seems to make sense. Thanks, I'll give it a shot!

Maybe that could help:

hmm I was thinking a not gate of sorts, but figured I'd need some type of integrated circuit (looks like the 4049 has 6 independent NOT gates). however the wiki shows a way to create one with two transistors > however this does take two transistors. im looking for the lowest cost solution, so using transistor biasing with a single transistor might be the ticket. or even a PNP type for that matter. alternatively, I could rethink my problem and probably do it with more wire and no transistor at all.
The NOT gate does have an element of elegance however.

Thanks for pointing me in the right direction guys.


quick question kBit,

so the NPN type transistor current direction will be inverted, since the load will be between the collector and the ground (Vcc), and the emitter will be connected to Power V+?


since the load will be between the collector and the ground (Vcc), and the emitter will be connected to Power V+?

No Vcc is +V

Emitter is tied to ground.
Load between +V and collector.

When base is higher than emitter by ~0.7V that "diode" goes into conduction, current can flow thru transistor, collector goes to ground (OK, 0.7V?) Anywho, you've got the load connected between Vcc and "near ground" = "ON"

When base goes to ground, base-emitter "diode" no longer in conduction, transistor = "OFF", collector voltage rises up to Vcc.

For future reference, if you put the load between the emitter and ground, the output will follow the input, not the reverse of it. (And if I understand, you currently want the reverse.) But, assuming you are using a bigger power supply on the load than the Arduino, be careful that your transistor base doesn't rise to a voltage above the Arduino's Vcc (5V). Powering ICs via their output pins is not a situation known to increase silicon life span. :stuck_out_tongue:

Here's some pictures...