How to change politary of a circuit

[1makermold]

How could I change the polarity of this circuit, to use PNP transistors

[Pooldawg]

How could I change the polarity of this circuit, to use PNP transistors

Without seeing what the two rail inputs are doing, it is a little tough.

You can simply swap in the transistor, however your "enable" for the transistor will also be swapped.  When it is a low, the transister will turn on, and when it is a high, it will turn off. Vs now it is off when low and on when hi. 

This simple video may help you:

[MarkF]

Is this for Lionel Model Railroad?  Is it DCC?
This takes some thought.  I re-arranged the circuit to make it clearer to me.

   

[mikerj]

What is the problem with using NPN transistors?

[1makermold]

Yes this is a circuit for Lionel trains that run on 0-18 vac. It is not DCC
 What it does is when it gets a signal of positive dc voltage to the outside rail it turns on the optocoupler and it turns on the whistle, I would like to reverse the polarity so when I send a negative dc voltage to the outside rail it would activate a bell.

[MarkF]

A little research here.  How are you going to generate the signals?

http://www.trainfacts.com/trainfacts/?p=330

https://ogrforum.ogaugerr.com/topic/lionel-sound-activation-button

http://doerry.org/norbert/train/SpeedandDirectionalControlofAmericanFlyerTrains.pdf   Pages 39 and 40.

   

[MarkF]

I simplified version of your circuit

   

[1makermold]

I tried that one i couldn't get it to work, I am sending a signal kind of like the one that you have posted, but I use a 10 vac transformer and a bridge recitfer and a make shift polarized capacitor, when I get some time I will make a drawing of it

[MarkF]

I tried that one i couldn't get it to work, I am sending a signal kind of like the one that you have posted, but I use a 10 vac transformer and a bridge recitfer and a make shift polarized capacitor, when I get some time I will make a drawing of it

There's not a lot of difference between the two.  The problem might have been that the 2N3904 doesn't have enough gain.

Does this circuit have to fit inside the locomotive (or tender)?

One quick thought without any investigating, you might try a second copy of the circuit with the rail wires reversed.
Since it's a DC shift of the AC, just reversing the inputs would see the opposite DC shift.
Really need to see how you are generating the rail power or at least your waveforms.

[1makermold]

 This circuit fits inside the tender, if you switch the wires around the circuit doesn't work