Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

NPN transistor to PNP transistor

Status
Not open for further replies.

FireAce

New Member
Hey guys, new here, so greetings to all. Kindoff stumped about a simple circuit I'm working on, and need some help from the pros.

In general, I have a PNP transistor running 2 3MM LEDS. Power off at base, LEDS turn on. Base has a 10K Res to pull down.
An NPN transistor running off a 4017 chip is supposed to power the base of the PNP transistor to turn it off, but therein is my problem. I cant get the PNP to turn off, even though I have 5v running into it. It will turn off If I remove the 10K res, but as you know, the res is needed. Any advice?

Note: I cannot remove the NPN, it is also powering something else.
 
A PNP transistor turns on when it's base is pulled toward ground. So you have to pull it up toward +V to turn it of.

How are you connecting the NPN?

Could you posts a diagram of your circuit?
 
The Emitter of the NPN is connected directly to base of PNP, and I did confirm voltage at base. So thats what I cant understand, I have 5-6v at base of PNP and it still wont turn of. PNP is an A999 I pulled from old stereo system. Could that be the problem? Circuit is running at 6v (4 AA batteries).

I did up the circuit in Yenka (a free software) and everything works fine. I could "Print Screen" and post if necessary.

Thanks
 
I'm a little confused and it's kind of hard to understand what you're trying to achieve with no schematic. When the output from the 4017 is high do you want the LEDs on or off?
 
Project1.jpg

Heres the specs. You will notice Led #2 is on, powered from NPN.
Another NPN from same IC powers another PNP transistor. In the attachment, you will notice Led #1 is off, but that is not true for actual project. I cant get LED #1 to actually turn off.
 
Try increasing the value of the 10 KΩ pull down resistor to something like 50 KΩ and see if that makes a difference. What is the part number of the PNP?
 
I have tried different res, no difference. The PNP is an old A999 I pulled from old stereo. There could be the problem, but I dont know. It will turn off when I remove the resistor, but as you know, the res has to stay in place to pull down
 
If it is an old part, it is possible the transistor could be bad. If you have another NPN you could connect it like this to accomplish the same thing. Logic low = LED on and vice versa.

pnp_npn-jpg.44065
 

Attachments

  • PNP_NPN.jpg
    PNP_NPN.jpg
    9.5 KB · Views: 1,415
Hi VNE, your a genius. I threw it into the Yenka simulator and it seems to work fine as you suggested. The problem I seem to have now is, there is actually 2 NPN's connected to the IC, one to deal with the LEDs turning off on Low output, and one to actually power a small circuit (shown as Red LED in my schematic above)). It seems the circuit you gave me draws the other NPN too low to actually power the circuit.

Thanks for the help, I've been learning a lot, but have a ways to go yet. Have been tinkering since I was a little kid :)
 
Sorry, the diagram I posted was incomplete. When the output from the 4017 goes high it turns on an NPN which I think is drawing the output pin down to Vbe. Using a base resistor should fix the problem.
 

Attachments

  • PNP_PNP.jpg
    PNP_PNP.jpg
    31.1 KB · Views: 475
Awesome! This simulates just fine, and I have had very good success with NPN transistors. i buy em bulk! Gonna have to grab some resistors from Radio Shak and I'll let you know how it went! Thanks so much!
 
Hey VNE, I threw it all together, seems to work great! Thanks so much.

Note: My PNP diagram I had above may have worked, I found a misplaced wire coming from the 2 3MM LEDS that I thought was grounded, but in fact wasnt directly grounded.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top