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Basic Transistor implementation help.

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New Member
I'm making the following circuit to handle the output of my RC transmitter unit (which ouputs a pulse signal from the rear) and convert it to a suitable signal for serial port input. The circuit is described at http://n.ethz.ch/student/mmoeller/fms/index_e.html

So far I've layed out the circuit on a bread board and substituted the one BC547 with a 2N3904 and the BC557 with a 2N3906, which I hope works out.

However, looking at the diagram, only inputs listed are the signal and ground. What drives the transistors? They require voltage if I recall (WAY back to my basic electronics course) for them to actually perform any function. Does the circuit just assume that the designer should know where the power goes? I've been re-learning alot over the last 48hrs, but some pointers in the right direction would be appreciated!

I've really got to dig out my old notes from class on transistors and calculating gain... etc. Theory and implementation do not go hand in hand however. :wink:



New Member
It looks as if it is powered from your RS232 socket, pins 1, 6&7.
Use your search engine to get the full pinout of a 9 pin RS232 connection to find out what's on pin 1, 6&7.
I've only ever used pins 2, 3&5 with 9 pin RS232's, no idea what 's on the other pins.



Active Member
Hi NightFlight,

Also you will have to make sure that your replacement transistors
are the correct gender.
The two shown here are opposites.
Q1 is an NPN type, and
Q2 is a PNP type.

Cheers, John :)


New Member
Klaus, thanks I should have known it was powered from the port. That's extremely handy actually. Perfect for TTL logic, imagine that! *grin*

John, the gender is right, that's the first thing I checked. I went over all four spec sheets I found on the web very closely. They are similar, but not the same.

In the end I didn't end up trying this circuit, I went with a simpler parallel port design, that only makes use of two pins and that works. The code that depends on the input from the port is very chunky because it waits on a signal. I might try out the serial version and see if I can get it to work more smoothly, but unlikely I think.
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