# Odd pin voltage on Arduino Single-Sided

I recently build 2 Arduino Single-Sided boards and am experiencing a very odd behaviour on both boards.
Without the ATmega chip installed and power applied to the board i have measured the voltage between pin 7 (vcc) of the ic socket and pin 19 of the ic socket and am reading approx. 3.6 V.
It must have something to do with the led because between pin 8 (GND at ic socket) and in between R3 and the led(LED 13) are 0V but between pin 8 and in between pin 19 (ic socket) and the led are approx 1.1V

I already checked like 5 times for unwanted solder bridges across the board and so on but couldn’t find any.
I’m completely at a loss here. Could anyone plz explain this to me?

edit:

``````just to be clear:

led
gnd o---------------------o---[1kOhm]---o---|<|---o nothing connected

|    <-> 0V           |
|    <-> 0V                         |
``````

CKoch: Without the ATmega chip installed and power applied to the board i have measured the voltage between pin 7 (vcc) of the ic socket and pin 19 of the ic socket and am reading approx. 3.6 V.

Pin 7 is connected to +5.0 Volts (Vcc). Pin 19 (Arduino D13) is connected through a resistor and LED to ground. It makes sense that the voltage between the two will be 5.0 Volts minus the voltage drop across the LED. I guess the voltage drop across the LED is 1.4 Volta which leaves 3.6 Volts.

It's completely normal if you measure it that way.

With the chip in place the voltage on Pin 19 will be close to 0 if the output is LOW and close to 5V when the output is HIGH. The resistor is there so the current through the LED does not exceed the limits of the LED or the output pin.

ok, but what about the case without the chip installed and measuring from pin 8 how come there is a voltage with nothing connected to the other side (see diagram above)

Are you powering the board through the ICSP header? Could there be something connected to the SCK pin of the ICSP header which goes to Pin 19?

nope i'm powering the board with a 12v wall plug only

by the way: i checked the complete trace of pin 19 (without power) against all neighbouring traces ... no connection where there schouldn't be any

Does the 1.1v across the LED go to 0 when you press the Reset button? If so, there is probably a short at/near the ICSP header between pin 3 (SCK/P19) and pin 5 (Reset/P1) which has a 10K pull-up resistor.

nope still there, regardless of whether resetbutton is pressed or not

(deleted)

top side with atmega inserted

bottom side

btw: yeah i’m aware that it isn’t pretty

i modified the second board to have a jumper between the resistor and the led
modified like that voltage is normal as expected (so i rule out a short of some kind)