Help with a circuit again

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1moldmaker
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Help with a circuit again

Post by 1moldmaker » Tue May 28, 2019 5:20 am

How can I change the polarity of this circuit by using PNP transistors
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1moldmaker
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Re: Help with a circuit again

Post by 1moldmaker » Wed May 29, 2019 5:27 am

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.

dyarker
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Re: Help with a circuit again

Post by dyarker » Wed May 29, 2019 1:15 pm

bell.png
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NPN to PNP, reverse diodes, reverse polarized capacitors, pin 1 of optocoupler stays positive.

Cheers,
Dale Y

1moldmaker
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Re: Help with a circuit again

Post by 1moldmaker » Wed May 29, 2019 5:07 pm

thanks, never even thought of switching the leads going into the optocoupler

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haklesup
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Re: Help with a circuit again

Post by haklesup » Thu May 30, 2019 12:33 pm

I guess that changes the trigger point from the falling edge to the rising edge of the outside rail. you may need to change the values of R3 and R4 if the bias current to turn off the Darlington PNP is different.

1moldmaker
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Re: Help with a circuit again

Post by 1moldmaker » Fri May 31, 2019 5:14 pm

Im using the pnp equivalent of a 2n2222a, its a 2n2907a, I am going to bread board this circuit tonight,
thanks again

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