# NPN- PNP detecting circuit

Discussion in 'The Projects Forum' started by Engin_Nerd, Oct 31, 2010.

1. ### Engin_Nerd Thread Starter New Member

Oct 22, 2010
9
0
I'd like to design a circuit that can detect when the wrong transistor type is placed. Q1 should be a NPN and Q2 should be a PNP. Both have there own LED that will light up when the correct Transisor type is in place.

When either or both transistors are swapped for the wrong type (Q1 swapped for a PNP or Q2 swapped for a NPN) it's LED should be "off".

Also I need to measure a single output that will be at 5V or less if one of the transisor types is wrong, and at around 16v if they are both the correct type.

I've made a circuit using a 2n6488 (NPN) and 2n6491 (PNP) I thought would work buttt.... it doesn't. Am I biasing the transisors wrong? Is there a better way to do this?

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2. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
102
Good question/problem.

You should first work on a single circuit that lights a LED with a NPN and LED "off" with a PNP.

Once that is working properly, changes it to detect PNP transistor. Then combines the two circuits to get your final circuit.

3. ### Engr Member

Mar 17, 2010
114
5
When you tried swapping the transistors, is it only one LED or both LED that didn't turn on? You said above that Q1 should be NPN and Q2 should be PNP, it doesn't match the circuit you attached. In the circuit you attached, Q1 is PNP and Q2 is NPN.

Looking at your schematic, I think this will be the result:
Q1=PNP; Q2=NPN; result=LED1 and LED2 ON
Q1=PNP; Q2=PNP; result=LED1 and LED2 OFF
Q1=NPN; Q2=NPN; result=LED1 ON and LED2 OFF
Q1=NPN; Q2=PNP; result=LED1 and LED2 OFF

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,165
1,121
Would this be a wonderful spot for a couple of NAND gates?
and one inverter?

5. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
720
I made a "Transistor Identifier" from a circuit found on the web that works rather nifty.

A Hex Inverter is set up as 3 oscillators, for a "3 phase square wave" output. Since the output overlaps, a bicolor LED placed in series with each pin on the transistor will light up one color when biased correctly.

So if two lights are green and one is red, it is an NPN (odd color, red, is base)
If two lights are red and one is green, it is a PNP (odd color, green is base)

I got the schematic off the web, it was titled transistor tester or transistor identifier, IIRC.

A bit of logic could be added and NPN and PNP could be determined, no matter what the pinout is.

Feb 19, 2009
6,357
720
7. ### chimera Member

Oct 21, 2010
122
2
well..firstly..im assuming that you'd want this circuit to be portable--u want to be able to use on different work benches-- for that, bias ur transistors to work on like 9 volts..that way u can use it with a 9V battery ..secondly..keep the circuit simple..make sure u check your connections and see if it works..if not..u can try to implement the above mentioned options by other members--they offer some great ideas..