Hello,
Noob here 
I have setup as shown in the attachment 1 - an external device which outputs 4,8V and some small current.
I connect a Arduino Pro mini ( as shown in attachment 2) and the voltage drops to 2.2 V on the output of the external device.
Stuoid question - how I can still connect the Arduino Pro Mini without using another power source?
Any ideas what I can use so Arduino Pro Mini runs only from that external device ? Capacitors?
Thanks a lot for any tips
What, exactly, is this "third party controller" and how much is "a small current"?
It is a Logicdata controller but don't know exactly the model....
The current I measured was around 100mA
If that's a 5V "signal" it can't be used to power anything. It has to be 5V power, and of course, it has to be capable of supplying the required current.
For example, you can power a 2nd Arduino from the Arduino's 5V pin, but you can't power the 2nd Arduino from an I/O pin.
In any case, you are apparently pulling too much current from your controller and there is a possibility of damaging it!
It is a Logicdata controller
Of course, how silly of me. 
edgemoron:
Of course, how silly of me.
I am sorry, last night I didn't know the exact model.
Here is the exact one:
with following tech details:
Technical Data
General
Supply voltage EU: 207-253 V / 50-60Hz
US: 90-127V / 50-60Hz
Nominal voltage EU: 230V / 50 Hz
US: 120V / 60Hz
Standby power, primary (typical) ≤0,3 W
Operating voltage for internal and external electronics
and Hall sensors
5VDC ±15% 250mA
Operating voltage for internal and external electronics
and Hall sensors in standby mode (average)
5VDC ±15% 7mA
Precision of Motor current measurement
@ 100% Output Voltage and 4-8A ±15%
Ambient temperature 0-30°C
Relative humidity (for operation) 5-85% (non condensing)
Storage and transport temperature -40-85°C
Relative humidity (for storage) 5-90% (non condensing)
Protection class (with earth terminal) I
IP class ≥IP 20
Dimensions (L x B x H) [mm]
Tolerances according to DIN ISO 2768-1 c
186 x 100 x 30
You're putting the power into RAW - that goes through a regulator with 1.x V of dropout.
Put the power onto Vcc (and do NOT connect vcc of the serial adapter while it's wired up like that!)
