Simplistic Transistor Usage

[genetica]

Good day

I have a question regarding the uploaded circuit, it is for converting a PPM signal to a USB interface found at http://www.rcdesign.ru/articles/electronics/rcusb (I know its Russian, good for google translator)

The transistor VT1 according to the author:
"PPM signal from the transmitter to the input of ICP (input capture pin) microcontroller through a matching stage transistor VT1. If the amplitude of the signal from the transmitter PPM exceeds 1.5V, you can not establish the elements of VT1 and R3, and to signal to the input of the microcontroller DD1 ICP via a resistor R2"

I do not understand the function of the resistor/transistor configuration or the reference to the "matching stage transistor". Can someone please explain to me?

 

[users]

I do not understand the function of the resistor/transistor configuration or the reference to the "matching stage transistor". Can someone please explain to me?

This cascade is matching a level of a signal - if the signal has low level, then you need this transistor. If level is high enough (1.5 V and more), then you don't need the transistor - use only resistor R2 between TX PPM and ATMEGA.

 

[andrewblog]

I don't know how the circuit works, but I find strange that configuration...

 

[RCinFLA]

The VT1 transistor is doing a level shifting/amplification of the input clock. The collector is using the ATMEG8 internal pull-up resistor which must be enabled for given port, PB0 in this case.

The internal pull-up resistor in the ATMEG8 ranges from 20k to 50K so it is a relative high value resistance. If there is too much stray capacitance on this line or the frequency of the clock is too high it can impact the waveform of the clock. I would probably opt for an external pull-up of a lower value to ensure waveform integrety.