# Unusual Circuit Board Identification Help

#### Zack Morris

Joined Aug 10, 2017
3
Hello everyone, I have just taken apart one of my dads old modems as part of a school project and I need help identifying some of the parts, I have pics attached, I've identified the electrolytic capacitors and a couple of switches but now I,m stuck. Im especially interested in the tiny chips in IMG_0709[1] - can anyone identify the chips in that photo "R620" "D603" "R623" ?

Thank you so much for your help.

Zack

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#### bertus

Joined Apr 5, 2008
21,543
Hello,

D600,D601,D603,D604 and D609 are SMD leds.
They need current limiting resistors.
These are R621,R620,R623,R619 and R725 for the respectif leds.

Bertus

#### Tonyr1084

Joined Sep 24, 2015
6,267
Given that the modem was probably powered by a 5 volt source I'd estimate the resistors (indicated by an R before their number) to be around 500 ohms. Why? Because at 5 volts, and assuming (I said "ASSUMING") they're running about 10 mA (0.01 amps); 5 volts divided by 0.01 = 500 (ohms). Usually there's some sort of numerical value listed on the resistor. But when they're that small it can be hard to read them - impossible to read - or just plain useless to try and label them. However, if they were standard resistors their value would be indicated by three numbers. The first two are direct numbers and the third is a 10X multiplier. For instance, a 501 resistor is 500 Ω. The 5 and the 0 are just as they appear to be. The 1 is how many zeros follow. If it were a 503 resistor then it would be 50,000 (or 50KΩ) resistor. 500 would be 50Ω. 5R1 would be a 5.1 Ω resistor (five point one ohms).

The diodes on your board are likely LED's. Whether they're single color or dual, we can't tell. The KEY factor here is that they are SMD's (Surface Mounted Devices). It's very impractical to attempt to remove and reuse them. For starters, you know nothing about them as scrap materials. The resistors I took a guess at are somewhere around 500 ohms, but that's only a guess. Aside from understanding what you're looking at there's not a whole lot of valuable items on the board.

C209 & C284 are capacitors, but you'll never see any sort of marking on those. You have no idea what their value is, unless you have a capacitor testor. C100 is the electrolytic capacitor you already found. From the picture we can't tell if it's SMD or THT (Through Hole Technology). If it IS THT then the leads are practically too short to be used other than maybe wiring up some leads to the stubby leads coming out of the capacitor and using it as a test piece. ME ? ? ? I'd keep those around. Never know when I'm going to need a toroid (coil) (marked L805). The markings on the board are for the sake of identifying the locations of parts. L805 means nothing as what value it might be. It just means that when you look on the BOM (Bill Of Materials) you can identify which coil it is and according to the BOM what value it is. It IS of some value if you want to (and know how to) wire your own transformer or filter. Be forewarned, they're not like transformers that you might commonly think of. Their purpose is more for filtering than anything else. But there are many other circuits that require a coil. So there ARE some parts of SOME value. The IC's ? ? ? Good luck getting them off. It's not easy for an expert. Let alone a beginner.

So that's what you have. Anything marked starting with an R is a resistor. C is capacitor; L is coil or an inductor; Q is typically (but not always) a transistor of some sort. D - diodes, whether standard diode or an LED (Light Emitting Diode).

Enjoy your new hobby. Learn. Read. Study the math until it's so familiar you can fart it in your sleep. The math is the biggest part. Next is understanding what you have and how those things work. You'll discover what you can do with things and a whole new world will open up to you. Electronics has been evolving and I haven't kept up with it. Wish I did. Tough catching up, but slowly I'm getting there.

#### ebeowulf17

Joined Aug 12, 2014
3,282
Study the math until it's so familiar you can fart it in your sleep. The math is the biggest part.
Speaking of which, you neglected the forward voltage of the LED in your guess-timated calculations for the resistor values. Probably a harmless difference when we're talking about whether or not to salvage smd parts, but it's still better not to give misleading examples to beginners, right?

#### Tonyr1084

Joined Sep 24, 2015
6,267
better not to give misleading examples to beginners, right?
Yeah, I tend to do that. Forget to account for the forward voltage. Sorry.

#### Zack Morris

Joined Aug 10, 2017
3
Given that the modem was probably powered by a 5 volt source I'd estimate the resistors (indicated by an R before their number) to be around 500 ohms. Why? Because at 5 volts, and assuming (I said "ASSUMING") they're running about 10 mA (0.01 amps); 5 volts divided by 0.01 = 500 (ohms). Usually there's some sort of numerical value listed on the resistor. But when they're that small it can be hard to read them - impossible to read - or just plain useless to try and label them. However, if they were standard resistors their value would be indicated by three numbers. The first two are direct numbers and the third is a 10X multiplier. For instance, a 501 resistor is 500 Ω. The 5 and the 0 are just as they appear to be. The 1 is how many zeros follow. If it were a 503 resistor then it would be 50,000 (or 50KΩ) resistor. 500 would be 50Ω. 5R1 would be a 5.1 Ω resistor (five point one ohms).

The diodes on your board are likely LED's. Whether they're single color or dual, we can't tell. The KEY factor here is that they are SMD's (Surface Mounted Devices). It's very impractical to attempt to remove and reuse them. For starters, you know nothing about them as scrap materials. The resistors I took a guess at are somewhere around 500 ohms, but that's only a guess. Aside from understanding what you're looking at there's not a whole lot of valuable items on the board.

C209 & C284 are capacitors, but you'll never see any sort of marking on those. You have no idea what their value is, unless you have a capacitor testor. C100 is the electrolytic capacitor you already found. From the picture we can't tell if it's SMD or THT (Through Hole Technology). If it IS THT then the leads are practically too short to be used other than maybe wiring up some leads to the stubby leads coming out of the capacitor and using it as a test piece. ME ? ? ? I'd keep those around. Never know when I'm going to need a toroid (coil) (marked L805). The markings on the board are for the sake of identifying the locations of parts. L805 means nothing as what value it might be. It just means that when you look on the BOM (Bill Of Materials) you can identify which coil it is and according to the BOM what value it is. It IS of some value if you want to (and know how to) wire your own transformer or filter. Be forewarned, they're not like transformers that you might commonly think of. Their purpose is more for filtering than anything else. But there are many other circuits that require a coil. So there ARE some parts of SOME value. The IC's ? ? ? Good luck getting them off. It's not easy for an expert. Let alone a beginner.

So that's what you have. Anything marked starting with an R is a resistor. C is capacitor; L is coil or an inductor; Q is typically (but not always) a transistor of some sort. D - diodes, whether standard diode or an LED (Light Emitting Diode).

Enjoy your new hobby. Learn. Read. Study the math until it's so familiar you can fart it in your sleep. The math is the biggest part. Next is understanding what you have and how those things work. You'll discover what you can do with things and a whole new world will open up to you. Electronics has been evolving and I haven't kept up with it. Wish I did. Tough catching up, but slowly I'm getting there.

Thank You very much for you in depth reply, I'm just taking it one day at a time and hopefully someday I can accomplish my dream of becoming a professional circuit designer.

Zack

#### Zack Morris

Joined Aug 10, 2017
3
Hello,

D600,D601,D603,D604 and D609 are SMD leds.
They need current limiting resistors.
These are R621,R620,R623,R619 and R725 for the respectif leds.

Bertus
Thanks a lot Bertus; I have access to a lot of boards from old household appliances, right now I,m just studying a lot of them but I do wonder about the feasibility of taking some of the components off these boards and re-purposing them, I know that most of these discrete components can be purchased for almost nothing, but still it gets the imagination going!

Zack

#### bertus

Joined Apr 5, 2008
21,543

#### Tonyr1084

Joined Sep 24, 2015
6,267
According to Bertus; to name a few.