Newbie...basic circuit question...have mercy! - Page 1
Forum section: electronics / beginners

author: mike519
author_rank: Contributor
timestamp: February 20, 2025, 09:06:52 pm
content: Hi all,Once upon a time, 40 years ago in my teens, I was into electronics as a hobby and knew a lot more than I can recall today. I am just getting back into it with some arduino and basic circuits, the first of which is me putting together a very basic small bluetooth reciever and amplifier to run some small speakers in the bedroom. I am well versed in 3D printing so have a nice case designed but need some help in understand what I need to put into series and what into parallel. Let me explain...1) I have a 24V 2 amp power brick supplying power2) mini 2 pole switch for on/off3) I want to run a blue led to indicate power is on. I have already testing it having a 2.548V voltage drop. Haven't figured out the resistor yet but I know how to do it4) I have an A232 Amplifier Board to power the speakers (30+30 at 4 ohms)...can take 12-24V5) Have a bluetooth 5.0 receiver board that can take 3.7-24V that will, of course, be connected to the input of the amp boardSo, where I am uncertain is how to wire the power to each of the LED, amp, and bluetooth. Instinct tells me I should run the LED right off of the power in (after the resistor of course) which will drop the voltage down to approx 21.5V after the LED which is a little more in range for the amp/bluetooth. Again, my thoughts tell me I should then run the power to the amp and bluetooth in parallel so the voltage is the same for each. Total current drawn should be well under 2amp I would think, even at a healthy volume.But what has me confused and not remembering what I used to do is when thinking about the LED and whether or not it also needs to be in parallel with the other two so that it gets connect directly to ground. In either case, of course, I would have the switch on the negative line.Please tell me I have some memory of this but if not, please put me on the right path.Thank you very much.

author: djsb
author_rank: Super Contributor
timestamp: February 20, 2025, 09:25:23 pm
content: Maybe start off by drawing a basic block diagram that you can share with us here. This will help to organize your thinking. You can just use a pencil and paper or use something like Freeplanehttps://sourceforge.net/projects/freeplane/

author: ledtester
author_rank: Super Contributor
timestamp: February 20, 2025, 09:39:26 pm
content: Quote from: mike519 on February 20, 2025, 09:06:52 pmIt seems you want to put all of your loads in parallel like shown below. "A" and "B" are your amp and bluetooth module and "+" and "-" are their power supply inputs. "24V" and "0V" are your power supply outputs.I've shown it with the switch on the positive side.

author: mike519
author_rank: Contributor
timestamp: February 20, 2025, 10:16:16 pm
content: Starting thinking more that this was maybe what I needed to do so I understand what you are saying, thank you. Just one question though...what is the difference in switching the positive side vs negative. I had thought negative was preferable.Thanks again

author: Zero999
author_rank: Super Contributor
timestamp: February 20, 2025, 10:29:18 pm
content: Quote from: mike519 on February 20, 2025, 10:16:16 pmWhy do you think switching the negative is preferable?Normally the positive is switched, because in many cases, the negative side is earthed. For example in your car, the negative side of the battery is connected to the vehicle chassis, which is considered to be earth or ground in this case. In that case, if you switch the negative to a device, then the positive can still potentially cause a short circuit, if it comes into contact with the chassis.If neither the positive, nor negative sides are connected to earth, then it makes no difference whether or not the positive, or negative side is switched.

author: PGPG
author_rank: Frequent Contributor
timestamp: February 21, 2025, 11:38:46 am
content: Quote from: mike519 on February 20, 2025, 10:16:16 pmIt... depends.Switching negative is easier so in any case when it don't ceres you switch negative.It is easier because:- if you have several voltages than in most cases they have negative in common,- npn, NMOS are simpler to drive as you drive them with signal related to your (common) negative.Example:You have +12V supply and make from it +5V for digital logic, but from that logic you want to switch 12V relay. If you switch negative you just use one transistor (npn or NMOS). If you want to switch positive you have to shift your output signal from 0..5V level to 12V and this costs you extra one transistor.Also if you don't know from what voltage the external load will be supplied a standard solution is to use OC output (so switching negative). In this case your OC output can be used to drive externally powered 12V or 24V door-lock.When the switched load have to have common GND with your device than you have to switch positive side. When? For example when you need to send signals (not using galvanic isolation) between them.

author: mike519
author_rank: Contributor
timestamp: February 21, 2025, 08:34:20 pm
content: Thanks to both replies. I don't know if this is the reason why I always assumed switching negative, but I remember learning it from somewhere (though it sounds like it is not always preferable so I will consider the options more intentionally in the future). Using positive in the case where a common ground is necessary makes sense.I also forgot to add above that I also have a log pot for volume control but the obvious answer there is having it in series with the amp.Thanks all.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 21, 2025, 10:24:46 pm
content: 1. Just switch the positive side; that's the way it's done in such similar circuits probably 95% of the time. There'd have to be a good reason to put the switch on the negative side, and there's no such reason here.2. That pot for volume control: I assume that goes on the input side of the amp? Yes, you'd want to use what's called an "audio taper" pot rather than a linear taper (yes, it's a log taper, but it's not usually known by that term).Be sure to connect the input across the outer terminals and the input to the wiper, like so:That way the impedance seen by whatever's driving the input of the amp (I assume it's something with a line-level output) will be more or less constant regardless of the volume setting.

author: mike519
author_rank: Contributor
timestamp: February 22, 2025, 04:15:54 am
content: So a logarithmic pot isn't always an audio taper? I thought they were names for the same thing. Its not a linear pot for sure...I have a few of them as well. This one is an A10.Will be doing the switching on the positive side now that I have read all the replies. I appreciate you confirming as well, however.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 22, 2025, 05:11:36 am
content: Logarithmic <=> audio taperThe more common term is audio taper.Oh, and for most input sources a 20KΩ or 50KΩ pot would be good.