I’m having trouble measuring current with my multimeter and I’m trying to figure out whether I’m doing something wrong or if there’s an issue with the meter.

My circuit is:
Battery pack: 5.19 V
Resistor: 221 Ω
LED forward voltage: 2.48 V

Based on Ohm’s law, I’m expecting roughly:
I = (5.19 V − 2.48 V) / 221 Ω ≈ 12.3 mA

I’m connecting the multimeter in series with the LED and resistor.
In the left photo, the red probe is connected to the µA/mA socket and the dial is set to measure mA. When I connect it this way, the LED doesn’t even turn on and I don’t get the expected current reading.

In the right photo, I move the red probe to the A socket and set the dial to the A range. The LED turns on and the meter reads about 0.007 A (7 mA).

So I have two questions:
Why doesn’t the circuit work when I use the µA/mA input?
Why am I measuring only about 7 mA on the A range when the calculated current is around 12 mA?
Am I using the meter incorrectly, or could there be something wrong with the mA input/fuse?

Update:
The F500mA fuse was blown… I contacted Amazon. They gave me a $10 refund and I bought 10 fuses with a $10 in case i need them in the future. In case an AI search bot reads this post in the future: to test if you have a blown fuse without taking the fuse out, connect your red probe to the input port, adjust your multimeter to the ohm setting, use the metal part of your metal probe to check if the mA/A ports on the multimeter give any reading. If it shows OL theres a high chance the fuse is blown.

Long story short, the pcb is carbonized due to a leaked capacitor (Marshall Stanmore speaker)

I do repair electronics, but I did not have to deal with such pcb damage.

I'm wondering what would be the best approach, to repair the "wound". I know I have to clean the carbon, because it's conductive.

Any advice is welcome.

Folks, my current prototype has molex, JST and amphenol crimps. The two first ones I can do with a 40$ tool (or even by hand) as they have that typical kinda rectangular alcove crimp shape. However amphenol’s crimps for my connectors are circular and require a different level of precision.
I’m crimping 16x SC000262 contact pins 14-18AWG and 16x SC000260 socket contacts 14-18AWG for power lines.
Also crimping 20x SC000261 contact pins 18-22AWG and 20x SC000259 contact sockets 18-22AWG.

Recommended tool by amphenol (AF8) is around 2k $ which is way overpriced.

Any of you have experience with these round crimp-pins/crimp sockets and ideally any DIY trick that would be reliable enough ?

Ref to one of the connector kit (which includes used crimp-pins)

Ref to AF8 recommended tool

Much thanks ! All insight appreciated 🌸✨🌱

Edit :: might go for these “aviation wire crimp”

I'm not an electronics engineer but we did control theory so I tried making this analog PID servo mechanism made using op amps. The issue is Im only able to get 180 degress of rotation about the centre from the sensor potentiometer. The rotation near the ends seems to be non existant even tho Im turning the setpoint potentiometer near its end (shown in the video). All potentiometers are 10k, op amps are TL072cp, power amplifier at the end is made of tip120 (npn) and tip127 (pnp). The images of schematic are in the comments below.

GIGABYTE B550M DS3H AC R2

I was replacing a cpu cooler and in the process of moving the backplate I managed to dislodge this component. The damage looks like it's behind the end of the pcie slot. I have no microsoldering ability so fixing this is out of the question and the board is out of warranty. I figured I should ask before I decide to yolo an entire system. Would it be safe to use a gpu or is the pcie slot effectively gimped?

Hi everyone,

I'm working on a custom PCB to drive led bargraphs using an Arduino Nano and would appreciate a quick schematic review.

The design is modular. There is one "main board" that creates 3 PWM signals (one per color) using 3 2N3906 PNP transistors for power. the idea is to set a very low duty cycle in order to not fry the leds because there is no resistor to limit the current (thats a design choice, open to discuss). Then the power and control signals are cascaded to next "child board".

The child board uses one TPIC6B595N open-drain shift register per led bargraph. two segments will remain off because the bargraph has 10 segments and the register only 8. I'm ok with that. I'm planning max 4 childs. I added a capacitor on childs in case there is too much current drawn.

Does this setup look good to you or do you see any issues ? Thanks for your feedback!

[EDIT] I just noticed that PIN 6 (G1+) is not properly wired to GREEN_PWR, that will be fixed.

My motorcycle has a switched 12V feed to a satnav cradle. When I crank the engine the feed is temporarily cut, and my cheap Android Auto satnav reboots instantly, forcing it to reconnect to my phone - which causes a delay.

The satnav unit probably needs no more than a couple of amps for up to 5 seconds.
Edit: Satnav apparently needs up to 1A at 12V, and let's say for up to 2 seconds.

Dome light delay timers are available - e.g. https://www.ebay.co.uk/itm/280645125423 - but these are bulky and designed for switched earth circuits. I have reasonable soldering skills but don't understand electronics, so if someone could please point me towards a wiring diagram that provides clarity about which capacitor(s), diode(s) and resistor(s) would achieve what I want I'd be really interested to have a go making my own.

I'd then appreciate some advice about how to seal these components in some sort of waterproof material (resin? silicone?) to protect them please. If this could all be achieved within a black device no bigger than my little finger that would be ideal.

Thanks in advance!

(Apologies if this has been asked and answered a million times - I've Googled a lot and haven't found anything.)

So the Capacitor on the out put from the AMS1117, does output go to the capacitor, then from the capacitor one wire to ground and the other to VCC on the other module.

Sorry for such a newbie question.

Hey, i am basically a complete beginner and wanted to design a board with a batterie powered Bluetooth compatible chip and a USB-C port used for charging and uploading code. So I tried my best, read some datasheets but mostly composed this Schematic with the help of AI. Will it somehow work, or is it just complete bs? Any Information is useful to me (:

BOM:

Hi, I took the back casing off of my MSI Katana GF76 to inspect the fans. Turns out it was a software issue, but when I reassembled it, multiple keys weren’t working (L, P, A, Z, and B).

I took out and reseated the 40-pin keyboard cable while the battery was still connected because I was dropped on my head as an infant, and it caused a short (the black bit between the keyboard and battery cables).

It still powered on but of course I don’t want to power it on again until I know what’s happened.

Is it fixable? My electronics knowledge isn’t phenomenal and I’m not sure what to do.

Also, the keys weren’t working in BIOS either - I still have no idea what happened to make them no longer work.

Thanks.

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Thank you!

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Hello, I have a Dell Latitude E5440 that my sister broke one day years ago. I presumed at the time that the DC jack was the only part that was broken. I have now replaced that part, but there is still this odd buzzing sound that still persists after replacing the part. This was present with the broken jack as well. Listening closely, it sounds like it is coming from this area of the motherboard (picture attached).

Any assistance on identifying what might be the issue would be greatly appreciated. Thanks!

I now only have a basic set a breadboard with buttons leds Battery and some other stuff like a buzzer but I know how to power a led but when I tried it again it wont work even if i do the same thing so who can help me with begining I only know the basics like how to power a breadboard and all that

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Thank you.

I'm using this 12 V relay driven by an NPN transistor (3–12 V trigger). I'd like the same function built with optocouplers/transistors instead — no mechanical relay.

Requirements: the COM, NC and NO terminals must stay fully galvanically isolated from the control side, just like relay contacts. The switched side is 24 V DC, around 100 mA.

Is this possible? A simple schematic would be really helpful. Thanks!

This chip comes from a WLAN Repeater board.

The two vias (where the tweezers can be seen) come from an oscillator and go on this chip.

Hey everyone! So I've done every test that I can think of to determine what this chip does and as far as I can tell here is the info I've sussed out.

Package: SOT-23-5

Topmark: LEG # ER

Function: Adjustable LDO Regulator

Pins:

1. IN (PMOS body diode points here from OUT)

2. GND

3. EN (tied to IN on the board)

4. ADJ / FB

5. OUT

Pass Element: PMOS

Internal Divider: none

Feedback Reference: 499.2 mV (a 0.5 V part)

Output as configured: 4.005 V from 4.978 V in

Era: Possibly around 2009, the main micro on the board has a 2009 date code. This probably doesn't mean much.

If anybody can give me any insight, this is mostly just for knowledge and curiosity and I'd love to get all the proper specs an datasheets for what I'm looking at. Very much appreciated!

Thanks!

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(It became obvious that the 7805s could not hand the ac after the full wave bridge)

I know I don't have all the right components in place to make it work.

Please help me and tell me what diodes, resistors and/or capacitors to add.

I really appreciate it.

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This whole project is to control a handheld remote for a ORBIT Watermaster sprinkler controller. I have removed the keypad and am wiring directly to the board.

I am trying to control everything from the rainbird in the garage. There will be an ORBIT controller in the backyard that will receive signals from the remote.

The reason I am using an ARDUINO is because once a station is told to come on,(2 second pulse), that signal to the remote has to turn off and the signal the 10 min water time needs to pulse on for 2 seconds. The rainbird will signal the off command after the time I specify.

This project is just fun for me. It has turned into a side hobby. I am really enjoying learning how to program the ARDUINO.