Arduino voltage drop

I am new to arduino community and a new arduino user as well!
I am trying to connect a proximity sensor (6-36V)with the board as the attached image shows.
I constantly see voltage with a poly meter by connecting + to5v and - to GND of the BREADBOARD
•when I connect the arduino via usb:
I connect the sensor to 5v, gnd and analogpin
I see voltage drop to about 4.4Vand the small led on the sensor is dimming. I read from the analog pin 24% ( proximity percentage when I’m literaly touching the sensor)
• when I’m using 9v battery for the sensor and I don’t connect the battery’s gnd to the arduinos gnd, the led lights up but I get no info. That’s because the circuit isn’t close. Well if I connect the arduinos ground to the battery’s it dims and I get 0-5%( proximity percentage
•if i connect external power on arduino 12v:
• I use 5v and the same connection as the picture, then the voltage drop and the info remains the same
• if I use the VIN on arduino I get 11.1 V, the sensor works and I get 99% proximity when I touch the sensor with metal.

I know that the sensor need more voltage but I have voltage small drops even when I use just a led and a resistor (connected to 5v,GNd) and when I’m trying to drive dual relay rated at 5V. At the same time I switch 2 dual relays(4 switches) and they turn on their small onboard leds(4) so I go down to about 4 voltages. They do switch but while and after the switching the voltage drops.
My questions are:
• are the voltage drops normal?
• can I use vin for the sensor and the relays? why shou I use 5V and not VIn which outputs 11 v(again voltage drop0.1V)
Thanks in advance and sorry for the long post

Most common "commercial" proximity sensors work best around the 24V range.
Some "hobby" sensors work around the 5 volt range.

You need to supply that directly to the sensor but before you connect the sensor to the Arduino you may need to us a voltage divider on the OUTPUT of the sensor to limit that range to a max of 5 Volts.

The output is often a proportional voltage hence the need to restrict the feedback to the Arduino before you damage it.

Then a common ground between the Arduino and the sensor or you may also still get unpredictable results.

Do you think that by using these relays (Arduino voltage drop - General Electronics - Arduino Forum ) a drop of voltage is normal? I’m connecting to the 5v pin and Gnd pin of arduino the vcc and gnd of the relay.
While they are off I see almost 5.1V on arduino. If I switch them on the relays’ onboard leds turn on and the voltage drops to 4.3.

Relays generally draw more voltage as they use a coil
Exceptions as SSR which tend to use opto coupling or similar approach.

You linked back to this post so not sure what "relay" you are trying to use.
Almost all sensors have a voltage drop of some kind and only the specifications for an individual item can determine what that is.

Hello and thanks for your fast replys :smiley:
Im sorry for the link. That's the correct one, From the datasheet i have found that they draw about 74mA .

What do you mean by saying "Almost all sensors have a voltage drop", when i connect the sensors parallel with the voltage source shouldn't they have the same voltage(5V)? Or is this just theoritical, and due to wire resistance i have voltage drop. And should i care about these drops? Or should i use external power for these relays?

Moreover, if i use the Vin to take 11V out of arduino, can i use this voltage, with probably a voltage divider for my relays? I want to avoid this divider,and any external power as i want to keep the project as simple and small as possible .

Do you have a link to the sensor specs?
Your picture doesn't show a relay. We have no idea what you're doing with the relay.

That relay board should work if you wire it correctly and it shouldn't cause a voltage drop unless you have a "weak" power supply.

I connect the sensor to 5v, gnd and analogpin
I see voltage drop to about 4.4V...

If the Arduino's 5V power is dropping, that's a problem and "something's wrong". (A lower voltage on an input pin is OK.)

• when I'm using 9v battery for the sensor …

The Arduino can be damaged by more than 5V (more than Vcc) into any input pin. Unless the sensor's output is somehow limited to 5V, don't give it more than 5V!

You can probably add a voltage divider (2 resistors) to knock-down the voltage, but let's see the sensor's datasheet first.

I suggest you work on one thing at a time! Work on the relays and sensor separately until you
get them both debugged.

Those cheap relays can draw a lot more than you think especially at the start up.
I do recommend a suitable supply for them more in terms of current than voltage.

You can feed both the Arduino and the relays with seperate feeds from the same supply.
I also recommend keeping Arduino board supplies below a 10 volt threshold for a long and happy life.
Clones tend to be more tolerant but true boards can often spit out the voltage regulator at above 10 volts leaving you with a dead board.

Also the 74 mA is above the Arduino pins 40/45 mA thresholds so that is just looking for a problem.
OK for a quick test but I would not run them directly for a long period.

That is almost half a watt at peak. Having used these types myself in the past I had hit an miss issues until I gave them thier own supply with enough current plus a margin of extra for reliability.

This is the datasheet

In this site :
it states that they use just 4 ma from the pin.

Connecting these relay modules to the Arduino’s 5V pin means both the trigger current (IN) and the current to drive the relay coil are being supplied by the Arduino board. From the datasheet for the SRD-05VDC-SL-C you can read that the relay coil takes ~72mA to operate. Added to this is the current required by the IN input to trigger the coil. For the optically isolated relay that is about 4mA and perhaps a bit more for the non-isolated one. (Measure it for your module).

In any case each of these relay modules will take <100mA from the Arduino’s 5V supply. So if a USB supply is used to power the Arduino,you can attach up to 4 of these modules. Indeed that is just what the 4 channel relay shield from Seeed Studio does

DVDdoug, thank you for your reply. Im measuring with a polymeter between 5V, GND, and just with the relay i get drop 0,6.
i also get small drops on 5v+GND with a red led and a resistor connected to GND and Digital PIN. is this normal?

PS. im trying to ask more general questions and not just on my project,because i do have general questions. Like why Connecting a relay rated 5V on 5V arduino pin powered with 12 V ,1.5A DC adapter (or USB ) have voltage drops? Relays turn on and off. They draw Currect when they switch . However I have drops after the switch situation. After the switch the relayremains switched and have a led turned on.


Not the best of pictures.

I think that “instructables” article has confused you somewhat. It does contain some blunders but is modestly useful. You can connect a relay module which contains a control transistor and possibly an opto-coupler, to an Arduino pin; you cannot connect just the relay itself.

If you are going to attach modules - such as relays - to the Arduino, you need a 5 V power supply. Powering from the USB port will work up to a point, but you simply cannot use the on-board regulator to power any extra devices such as relay modules, it is unsuitable.

A common way to power something is to use a 5 V USB “phone charger” connected to the USB port. This can supply up to 500 mA for the system. This is limited not by the phone charger, but by the polyswitch protection device on the Arduino UNO. If you need more current, you find a 5 V fully regulated power supply and connect it to all the devices requiring 5 V - including the Arduino itself via its 5 V pin. Note, the Arduino is not supplying power, it is using it.

Thanks a lot, you made a lot of stuff clear to me.

you simply cannot use the on-board regulator to power any extra devices such as relay modules, it is unsuitable.

Why cant you power using power jack back you can using usb?

So do you recommend Powering arduino via usb port and a charger or via a 5v dc adapter through +5vpin rather than the input jack.
Just for 2 dual relays modules, a rf receiver and a couple of switches.
receiver: 5V ,4mA


Why cant you power using power jack back you can using USB?

Because it has a piddling little regulator with virtually no heatsink. A USB port can at least supply 500 mA.

So do you recommend Powering Arduino via USB port and a charger or via a 5v DC adapter through +5vpin rather than the input jack.


Just for 2 dual relays modules, a RF receiver and a couple of switches.
receiver: 5V ,4mA

Especially for 2 dual relay modules! :astonished: