# Home school circuit analysis class suggestions needed

#### dl324

Joined Mar 30, 2015
15,511
How soon do you need to clean it?
I've found that sooner is better. A short bristle acid brush helps with residue removal.
I just think I’ve always had crappy old solder.
I haven't used any solder I didn't like.

#### MrSoftware

Joined Oct 29, 2013
2,073
Flux is your friend, never too much flux when it's time to solder. To clean the flux, get some of these brushes and use scissors to cut the bristles shorter to make it stiffer. Get one of these for the alcohol. Soak the brush, brush the board, then lay a clean dry paper towel over top of the board, soak the brush again and sweep it over the paper towel. The first part dissolves the flux, the next part actually removes it. Repeat a few times as necessary. Some people use kimwipes, I'm cheap and paper towels work OK. Flux remover spray also works good, but it's a step up on the chemical scale compared to alcohol so I'm not sure if it's the best to use around kids.

Flux in good shape comes right off. Flux that has been on for a while, or that has been burnt will be more difficult to remove.

#### rpschultz

Joined Nov 23, 2022
286
Ok. Last night I started with isopropyl and an old tooth brush, that wasn’t aggressive enough. So I found similarly sized nylon brush that worked better.

#### dl324

Joined Mar 30, 2015
15,511
Ok. Last night I started with isopropyl and an old tooth brush, that wasn’t aggressive enough. So I found similarly sized nylon brush that worked better.
It looks to me that you've soldered the pots, resistors, and caps on the wrong side of the board.

Most of the joints look bad. Most have too much solder and it didn't wet. The solder shouldn't look like a bubble. It should have concave profiles.

I like to lay wires over pads to make them easier to work with:

Didn't realize Harbor Freight had acid brushes - price is decent. I bought 1 gross a couple decades ago from some company that has since gone out of business and won't be needing any time soon.

I have brushes with bristles cut down to about 1/16" and 1/4". I put some rolled Kimwipes in heatshrink for cleaning around leads.

#### rpschultz

Joined Nov 23, 2022
286
Thanks for the soldering tips. I do feel like I’m getting better, learning how to use flux. Yes there were joints it didn’t suck down like I wanted and I probably added too much solder and not any additional flux. Will keep trying…

I had to put the caps on the opposite side as the pots because of height considerations within the 1590a enclosure. The resistors could go on either side, but a previous enclosure design had a jack in that area so I soldered them underneath.

#### dl324

Joined Mar 30, 2015
15,511
Thanks for the soldering tips. I do feel like I’m getting better, learning how to use flux. Yes there were joints it didn’t suck down like I wanted and I probably added too much solder and not any additional flux. Will keep trying…
I very rarely need additional flux. If the board and component leads didn't have oxidation, the flux in the core of the solder should have been sufficient.

If the solder didn't flow through the hole, either you didn't apply enough solder or you didn't get things hot enough.
I had to put the caps on the opposite side as the pots because of height considerations within the 1590a enclosure. The resistors could go on either side, but a previous enclosure design had a jack in that area so I soldered them underneath.
If you didn't design the board to have the pots on the component side, pot rotation will be backwards unless you wired them as rheostats.

#### rpschultz

Joined Nov 23, 2022
286
If you didn't design the board to have the pots on the component side, pot rotation will be backwards unless you wired them as rheostats.
yes. I know the opamp needs to be the the correct side. And I *think* I accounted for the pots when I designed it so they aren’t backwards. I’ll find out soon enough…
They are designed as pots not rheostats.

#### dl324

Joined Mar 30, 2015
15,511
You should try reflowing the bad joints. If you can't turn them into good joints, you might have to add solder, or remove solder and try again.

You have so few components that point-to-point wiring wouldn't have been very difficult. The layout I posted was for 4 discrete flip flops. I didn't want to have to etch a board before I could use the circuit, so I hand wired (and expanded the layout on-the-fly to accommodate 6 flip flops so I wouldn't have wasted space).

#### rpschultz

Joined Nov 23, 2022
286
You have so few components that point-to-point wiring wouldn't have been very difficult.
I actually tried that, see this post for pictures. I soldered all the components on prototype board, but doing the point to point wiring would have been impossible for the size board I had, to keep it in a 1590a. So creating the pcb layout made sense.

#### dl324

Joined Mar 30, 2015
15,511
doing the point to point wiring would have been impossible for the size board I had, to keep it in a 1590a.
With point-to-point wiring, you could make it smaller than the board you made. You were constrained to 2 layers for routing. That constraint wouldn't exist with a hand wired board.

#### MrSoftware

Joined Oct 29, 2013
2,073
The joints look like the parts themselves didn't get hot enough. When soldering, you're not "melting the solder", you're getting the parts (pads on the board and leads on the resistor/cap/...) hot enough to melt the solder. If the surfaces don't get hot enough to melt solder then the solder won't flow onto the parts properly. To fix the joints, put a little bit of flux then apply a hot iron, keep the iron there until it flows nicely. If you have to hold the iron there for a long time, turn up the temperature and try again. With practice you'll learn what temp works best.

You'll also discover that joints surrounded by a lot of copper with take more heat as the copper will wick the heat away.

For your first board ever, nice job, if it functions properly then I would call that a success. Like everything, practice makes perfect.

#### rpschultz

Joined Nov 23, 2022
286
One of my goals for this class is for students to build a circuit of their own as a Final Project. Something relatively simple, talking-electronics 555 timer stuff or similar. There's also some simple kits on Jameco for cheap (<$20) too. Like to be simple enough to do some basic circuit analysis of it. I've debated the difficulties of teaching soldering to 13-17 year olds. I've also considered wire wrap, that while still possible, is becoming increasingly difficult to source accessories for reasonable prices. A nice soldering kit ($19) is cheaper than any wire wrap tool ($25 min) and the sockets are not cheap. My most recent idea is this: 1. the default construction method for the Final Project will be breadboard. Neat. Organized. 2. But if a student wants to learn how to solder, I'll give them a soldering kit and they can practice at home. I probably would have a special 60-90 minute class ONLY for those who WANT to learn, after class some week in late Feb or early March. The class is 3:30-4:30 on Mondays. I could envision staying after for a handful of students who want to learn, 4:30-5:30 most weeks. Anyway, that's my most recent idea that may or may not have any merit. #### dl324 Joined Mar 30, 2015 15,511 But if a student wants to learn how to solder, I'll give them a soldering kit and they can practice at home. As your students will all be minors, I'd advise you to get parental permission before giving them an iron to take home. Burns from a soldering iron can be very severe. I burned myself once and I don't ever want to do that again. Might even want to CYA and get approval for in class use. #### MrSoftware Joined Oct 29, 2013 2,073 I started soldering in elementary school (it was by all accounts a hot mess at that age), I'm sure teenagers can pick it up right way with instruction. How about design a circuit board that writes their initials in LED's and runs on a 9V, or something similar that they can keep with them. Kids can use your favorite free program, send them all to AllPCB or whatever your favorite prototyping house and then the kids can solder in the parts when the boards come back. Super easy, a bunch of LEDs and a bunch of resistors and a 9V battery end. If you want 9V battery leads, I've got literally thousands, I'll send you however many you want (get them off of my hands lol), just message me the address. Thread Starter #### rpschultz Joined Nov 23, 2022 286 That’s a cool idea. It’d force them to add up diode Vfs, etc. a bunch of parallel circuits probably. I’ll add it to my list. Thread Starter #### rpschultz Joined Nov 23, 2022 286 The semester is over half way over, things are going... well. A couple kids have dropped, but the remaining 11 seem to enjoy it and are learning. We've done a lot of diodes (LEDs), resistors and pots, spent 3 weeks on Arduino and last week we transitioned from series circuits (where everything has the same current) to parallel (same voltage applied) and KCL. The kids are accustomed now to breadboarding and fairly well versed in using Eagle to simulate simple circuits. Typical classroom time looks at a circuit 3 different ways: traditional math, breadboarding and simulation. The kids like this hands on approach and different methods that all give the same or similar results. 6 weeks left. Next week is Voltage Dividers, building on last week where they used Ohm's law to manually figure out the potential between 2 resistors by first calculating the current and then the potential across each resistor. After that, I'd like to do 2 weeks of transistors, very simple NPN stuff. Then maybe use an op amp in the same way a transistor is used to gain up something. I've got tons of 2N3904 NPN transistors and their kits have a ua741 general purpose op amp. Jameco has 4Ω 20W speakers for$1.50 and electret mics for <$1. I could get each kid a speaker and mic for around$2 each. Could we breakboard the mic into a transistor with fixed gain and then a speaker? After that, an opamp and a pot to control volume? Would this work, am I missing something?

Any help on the simplest circuit that would work for this would be appreciated! Thanks!

#### rpschultz

Joined Nov 23, 2022
286
The other thing I'll mention is that their breadboard kit has a 5v supply based on 4 AA's that is very convenient, but we also could do 9v and battery snaps. The ua741 has a minimum supply voltage of 5v.

#### MrChips

Joined Oct 2, 2009
28,153
Sorry. I would have to say no.
You have two problems, impedance matching and gain.

The input of the amplifier has to have high impedance, higher than that of the microphone.

The output of the amplifier has to have low impedance to match that of the loudspeaker.

You need high gain, in excess of 200.

The transistor or 741 opamp will not meet those requirements.

#### AnalogKid

Joined Aug 1, 2013
10,225
The ua741 has a minimum supply voltage of 5v.
Actually, that's +/-5 V, or 10 V overall. A common technique is two 9 V batteries in series, with the centertap being the circuit GND.

ak

#### AnalogKid

Joined Aug 1, 2013
10,225
Could we breadboard the mic into a transistor with fixed gain and then a speaker? After that, an opamp and a pot to control volume? Would this work, am I missing something?
Yes, it will work well enough to demonstrate the concepts. BUT, it will have limitations, which also is valuable learning.

A 1-transistor mic preamp circuit is very common; one transistor and two resistors. If you have a scope and a sine wave source, you can tweak one stage for minimal visual distortion, and then cascade two stages with less gain in series for the same overall gain to show less distortion. You could then compare this to a 741 amplifier stage. There will be a lot of wailing and gnashing of teeth on this forum about all of the 741 issues. Ignore the rabble; this is concepts for children, not Abbey Road Studios.

The 741 can then be a speaker driver. As you increase the signal amplitude, the 741's output current limiting will kick in, more learning. If you spring for a higher impedance speaker, such as 32 ohms, things will improve.

A more complicated approach is to make the 741 the only amplifier stage, and use external transistors as a current-boost stage on the output. Again, lotsa schematics on the innergoogle for this. Plus, the output stage now will not have current limit protection, so they can learn about how much heat it takes to melt your fingerprints.

ak

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