Requesting Feedback/Circuit Review for a Notification Circuit  - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: June 23, 2012, 10:40:01 am
content: Over the past few months I have taken upon a big project for myself as a beginner hobbyist. I was asked to create a circuit that will notify people in two ways - light and sound. After a big amount of research I have finally got my circuit. I started off by finding different building blocks to the circuit and then finally I have put them all together to work as a notification system. The circuit is attached as an image. I would like some review on this circuit since it will be built and used a lot to notify people in different rooms in my house. So first of all lets talk about how it works:1. The first building block at the top of the image is the timed power on switch. It basically relies on a relay to do the switching and a capacitor and resistor combination to stay on in order to keep the other circuits powered. This stays on for about 15 seconds, but I am not sure what formula determines how long it stays onplease advise.2. The second building block is the astable multivibrator using a 555 which is the light notification with two LEDs placed opposite to one another so they flash alternatively.3. The last building block is the sound notification which is optional and turned on by the switch seen. To activate the sound the user simply has to press the push button and the capacitor will discharge through the transistor which will turn off the transistor closer to the buzzer and with therefore turn the buzzeron for a certain time. The time is also selected by the capacitor and resistor combination and once again I am not sure about the formula, please advise.Is there any way to improve my circuit?Did I miss any key points?Any recommendations for when I install this circuit in the house? It will be connected throughout the roof, will the mains wiring for the lights interfere with the circuit wiring?Should I try to make a dedicated PCB for this circuit?-Thanks in advance to everyone!

author: 6502nop
author_rank: Regular Contributor
timestamp: June 23, 2012, 04:10:55 pm
content: Not asking for much, huh? Well, okie dokie!1> That's called the "R/C Time Constant", and it is determined by plotting how fast the cap charges against how fast it discharges through the resistor. Since your circuit has no charge resistance to speak of, then it's simply how fast that cap can get to full voltage. Once the switch is opened, then the charge on the cap bleeds off through that resistor, and how long that takes is a function of the charge on the cap, the value of the resistor, and the cutoff voltage of the transistor (about 0.7V for most).2> Cool, but the top LED is backwards. It'll never light up.3> Errr. You've already got a 555 in there. How 'bout making it work for a living? Grab a 12V buzzer from Radio Shack, and put it behind a switch attached to pin 3 of the 555 (it'll source/sink quite a bit of current!). Now you can switch on/off the sound, and it'll be synced with the LEDs.QuoteSee attached, "555alrtsys.png"!QuoteWell, if you run a low voltage a long distance, you'll get voltage drops (especially DC), so throw some filter/bypass caps on the V+ to compensate. If you use twisted-pair wiring for your project, the mains wiring shouldn't be a problem.QuoteSure. Although if you keep it down to simple things like transistors or 555s, then just a couple of sections of perf board will do.nop(You'll notice I'm getting the hang of gEDA...)EDIT: Just noticed that those LEDs would blow up! Resistor added.

author: Mint.
author_rank: Frequent Contributor
timestamp: June 24, 2012, 03:36:28 am
content: Hey 6502nop!Thanks a lot for your time and effort into drawing and redesigning my circuit. I think it is a good idea to use the 555 timer, the caps will probably start to leak later on and then will not be so good for use.

author: 6502nop
author_rank: Regular Contributor
timestamp: June 24, 2012, 01:02:45 pm
content: Not a prob-... Oh, wait, I had a problem!Attached is version "B" of the schematic, with the pull-up resistor added. Try to forget you ever saw rev. A. Clear your mind. You're getting sleepy... sleepy...Now, about those caps: Use Rubycon, Panasonic, Nichicon, or United Chemi-Con (UCC), and you'll be fine. This isn't a critical circuit for caps, so any GP will do, but if they're getting shoved in your attic, make sure they've got a suitable rating as far as temperature is concerned. The ceramics are never a problem, but the electrolytics should be 105C. If these are staying inside the rooms, then 85Cs will do, but I just play safe and order 105Cs for everything I build.nopWe now return you to your previously constructed breadboard...