Hello,

This will probably be one more of my newbie questions, but I'm stuck and 
don't know where to go. Here's my problem:

What I'm trying to do:
Control the flow of an R/C servo signal. There is one connector in my board 
(CN1) where the PWM signal and GND comes from the R/C RF receiver and goes 
straight to CN2, which is connected to the actual R/C servo motor. Sometimes 
I want the signal to go straight from CN1 to CN2, and some other times I 
want to cut that path using a transistor and feed CN2 with a PWM signal 
generated by the PIC.

What I'm using:
PIC 18F452 and MMBT123S NPN transistor (and other components not relevant to 
this problem). Port D4 is connected to the base pin of the NPN (link below). 
CN1 PWM signal is connected to the collector pin of the NPN transistor, and 
the emiter pin is connected to the CN2 output connector. In an earlier test 
I had a 2.2K in series between PORTD.4 and the transistor base, but results 
were the same.

What happens:
When I set D4 low, I see the PWM signal on NPN collector (0V to 5V), 0V on 
NPN base and 0V on NPN emiter (as expected). When I set D4 high, things get 
weird. The PWM signal from the R/C receiver turns into a 1.2V to 1.3V signal 
(I can still see some PWM in there), the NPN base (PORTD.4) follows the same 
1.2/1.3V signal that is in the collector, and the emiter has the same signal 
shape, but 0.8V to 0.9V instead.

Assume:
That I'm setting TRISC to 1 and PIC is working fine (at least for other 
things).

http://www.diodes.com/datasheets/ds30292.pdf
I read the transistor datasheet 10 times already, but there are some terms 
in there that I cannot comprehend.

Am I experiencing another impedance mismatching problem? I thought that 
transistors were exactly to solve that. Any alternative solution for what 
I'm trying to do?

Cheers and TIA


Padu 


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