I am having trouble with relay board and wonder if anyone could assist
I have wired the relay supplying 5VDC from the Arduino Uno and switching from digital output. I am seeing a volt drop on the switch measuing 4VDC. When disconnected I have 4.8 VDC. The arduino is being feed from 9VC connector supply from a car battery through a 1Amp voltage regulator. If i connect the switch wire to Common 5VDC the switch will operate.
Always show us a good schematic of your proposed circuit.
Show us a good image of your ‘actual’ wiring.
Give links to components.
Yo did not show how the arduino powered?
In the picture you have both USB and adapter connected at the same time. What is the meaning of this? So you can burn the board
The 5V pin on the Arduino is going to the terminal strip.
What all are you powering from his 5V pin ?
Those Arduino jumper wires are poor quality.
At a minimum use proper Dupont jumper wires and use a header housing for more than one wire.
You would of course use male pins in these housings.
Add male pins to these:
Since you are switching 12V, it would be better to use 12V relays. You don't need to use 5V relays simply because the Uno is 5V.
The cause of the voltage drop is probably the Uno's on-board voltage regulator. You are forcing it to supply enough current for 2 relay coils in addition to the Uno itself.
If you used 12V relays, their coils would be powered directly from the 12V PSU, and not the Uno's regulator.
Better still would be to use logic-level, n-channel MOSFETs. These require almost no current to switch.
With either MOSFETs or 12V relays, you will no longer need the DC-DC converter. It's ok to power the Uno with 12V so long as it needs to supply only itself with 5V from it's on-board regulator.
Another solution would be to adjust the DC-DC converter to output 5V and feed the Uno via it's 5V pin and the relays directly from the converter. This will bypass the Uno's on-board regulator.
Your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advice on) your project 
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Nice install but soldering signal pins on the relays - yecch!
Use a terminal shield instead of poking wires into the Uno.
Shrouds are another way to tame flying leads if you have access to a 3D printer.
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