As a general rule of thumb, any device's inputs (logic, opamps, whatever) should never be more than one diode drop (0.6V) above or below the device's power rails. Individual data sheets often have more restrictive limits. A common method is to add two diodes to the input, one to each rail, such that they do not conduct in normal operation (both pointed up). In your case, a single 5.1 V zener to GND is good for this.
Aha! I thought that might be the case. So, adding a 3-10K pull-up will make the circuit more reliable, then.
It sounds like I can rely on at least enough current gain for 200 mA, so that's perfect.
So, if the device has internal over-voltage protection, that means I don't need the zener or any other over-voltage protection, right? And it sounds like you're implying that 50mA is more than enough for this application. If that's the case: A couple days ago, as a test, I hooked my circuit up directly to my amp with no speaker in parallel, and cranked the volume to max. That fried my LM393. Do you think that was because of negative voltage? (As opposed to over voltage or too much current?)
Please see new schematic -- does this look good to everyone? (i.e., Is this circuit is adequately protected from over-voltage, too much current, and negative voltages?)
I suggest you move R8 so that it makes contact directly at the node where the transistor's base and R9 make contact, and not at 393's output.
No it is not.
I thought you said the LM393 had over voltage protection built in -- why would I need to add additional protection?
I thought the impedance through the comparator was already sufficiently high. (It's at least high enough to ensure that connecting this circuit in parallel to my speakers steals such a tiny fraction of the current that it doesn't audibly impact the sound quality). So why would I have to add even more current-limiting resistance?
If two back-to-back diodes clamps the voltage to +/-0.7V, then aren't we now allowing negative voltages? I thought we needed to protect against that...
You do NOT want to put any kind of diode (or any other non-linear device) in series with a speaker...this will create all kinds of distortion.My background: My knowledge of circuits is extremely basic. I aced physics in college several years ago so that I could get into med school – then promptly dropped out of med school and forgot everything I learned.So, you’re gonna have to talk to me like I’m five.
The situation: I have an A/V receiver that supports an ‘A’ set of speakers (which are in my living room) and a ‘B’ set of speakers (which are outside on the patio). My receiver is hidden away in a closet, and it allows audio to output to A only, B only, or A and B simultaneously. So, just because the speakers are on inside doesn’t necessarily mean they’re not on outside, too (which, if I’m cranking the volume, is an unwitting nuisance to my neighbors). I need to be able, at a glance, to determine whether the speakers are on out on the patio without having to get up, open the door, and stick my head outside – or make a trip into my closet where the receiver is located. (I know, I’m lazy.)
What I want to do: I want to install an LED (into a wallplate in the living room) that will illuminate when the outside B speakers are on.
What I’ve tried: I found some LED current indicators that I thought might do the trick. I bought this one and this one (0.75A turn-on), strung my speaker wire through them, and tested them out. Long story short, neither worked very well. The NK Technologies inductor has a lower turn-on point, but it’s aperture is smaller, so I could only get my 12-gauge speaker wire looped through 3 times. The CR Magnetics inductor has a higher turn-on point, but it has a larger aperture; so I was able to loop the speaker wire through 5 times. The results were about the same. At high volumes, the LEDs worked pretty well (but still weren’t terribly bright) – but turn the volume down, and the LEDs stopped working. I need the LED to be solidly illuminated regardless of the volume.
My questions:
1. Am I on the right track using an inductor/LED to accomplish what I want to do, here? Any easier way of solving my problem (that of needing to be aware when my outside speakers are on)? I’m open to suggestions.
2. Is there any way to make the indicators I have already bought work in this application? (e.g. Could I use a higher-gauge speaker wire, wrap it several times around the inductor, and somehow wire it in parallel to the thicker speaker wire feeding my outdoor speakers?) Any other ideas for modifying what I’ve already bought? I’m not opposed to some modification and light soldering if someone would be so kind as to provide instructions.
3. How difficult would it be to build my own inductor/LED current indicator with a much lower turn-on point (perhaps 0.05A)? (Remember, my knowledge is way basic – but I’m handy, I’m a quick learner, and I can follow instructions; so don’t be afraid to suggest a project if you think I can handle it.)
4. General question about inductors: does the inductor attenuate the current running through the wire around which it sits? If so, I can’t imagine that would be good for sound quality -- so is it a bad idea to be using an inductor on speaker wire? Or is the effect so small as to not be noticed?
Some (perhaps) relevant info about my receiver and speakers:
Receiver: Pioneer VSX-1130 (100 W in 2ch; likely much less in all channels driven)
Speakers: Polk RC85i and Polk RC80i (8 ohm)
3K seems unnecessarily small, and diverts over 1 mA away from the base, reducing the saturated collector current. 10k to 100K, whatever you have. Another 47K would be fine.
A zener conducts at its zener voltage when reverse biased. When forward biased it conducts like a regular silicon diode, so it is doing the same thing as a 914 to GND. For a circuit like this with only one power supply rail, 1 zener equal to the rail = done.
Try something like this.
Thanks, AK! Here's the updated schematic:
Unfortunately, it wasn't that close to the midpoint. Measuring resistance from each of the outside pins to the wiper pin, I got about 5.7K vs 4K. I tried the 47K resistors, but even maxing out the pot, couldn't get the bias close to the reference. Any other ideas to increase sensitivity? Or is it going to be a trade-off between sensitivity and protecting the circuit?
Give the last circuit I posted a try. It detects mV level signals and requires no adjustments.
I absolutely will give it a try...but I only have one breadboard and I'm so close to having this circuit dialed in, I want to finish it before pulling this spiderweb of leads out. Rest assured, before I do the final build, I'll give yours a try to compare -- I appreciate it.
