Quick question about Relays AC/DC Wiring

Hey folks, new here and wanted to ask a question.

I'd like to use an inexpensive driveway sensor system (DC) with an output to an alarm. The unit uses a 9VDC and likely supplies mA to the piezo buzzer. Is there a relay available or is there a way to wire the output from the reciever to operate the on/off switch on a 110VAC power strip?

Understand that the sensor likely has a very short time span, is there also a simple way to set the length of time the AC powers trip remains "on" (a potentiomenter option maybe to be able to set the amount of time)?

I realize this is likely a rather sophomore question for many of you (based on what I've read), but I am rather new to trying to build some of these things for what I want. I can cut/splice/solder/do the basic to advanced stuff in the home and understanding diagrams, however the more detailed understanding of resistors, parts, and putting them together aludes me.

Thanks for any help you may be able to offer.

Here's what I think of it off the top of my head. The driveway sensor is a simple make or break to the buzzer, very basic circuit. You could feed the output from that into a 555 timer, where the buzzer circuit would trigger the 555 which would be set up to turn on for X minutes/seconds and then turn off again until it got the next pulse. A potentiometer could easily be used to adjust the delay, there should be multiple examples of such circuits here in the forums and on the Internet in general. The 555 could easily directly drive an AC relay.

Another solution might be a simple off delay timer. Companies like Square D, Potter & Brumfield, Tyco, etc. make modular timers with the control circuitry built in. One with a sufficient contact rating could switch your power strip and delay it however you wish.

I enjoy building and tinkering with circuits, but in this case a typical general purpose timer can be picked up for $50.00-$75.00. The time and materials to build your own circuit might make this a better solution.

A 555 and relay and it's support components are going to cost less than 10 dollars. IF you can put them together right. Discrete units like Ke5 is suggesting are costly, but they require pretty much zero knowledge, you plug em in they work.

Sceadwian said:

A 555 and relay and it's support components are going to cost less than 10 dollars. IF you can put them together right. Discrete units like Ke5 is suggesting are costly, but they require pretty much zero knowledge, you plug em in they work.

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I totally agree. DIY has its pros and cons. The way I go about deciding, I kind of do a mental accounting for my time and effort and the cost. Say, for example, the materials cost of such a project would run about $20 between your screw-ups and things you hadn't thought of, enclosure, board, even a tool you don't have but need. (Honestly, I think 10 is pretty conservative)...and the pre-fab timer runs $70 for an average quality unit (which is about right). So the difference will be $50. Well, what is your time worth? I know in my case I can work an hour or two of overtime at work and cover that expense. So I probably would go that route just to avoid the work (I figure between the research, design, testing, and final product, a DIY board will cost me at least 3,4 hours. A 555 timer relay isn't all that interesting anyway.

Considering that dozens of people post on this board every day looking for quick solutions without having to do any thinking or learning, this is a good example of where not re-inventing the wheel is a good idea.

I too would lean toward the commercial timer system. They really are basic connect and go devices. With about 10 minutes worth of time on line you should be able to find such units as surplus old stock for around $10 -$30 each. How fancy or basic is up to you.

On the other side of that, if you're on the cheap, don't mind learning and are willing to put some time into it, a 555 timer circuit like this is a good way to start. It really is all about time/money consequences, if you have neither...

Hey all, I really want to thank you for the reply. Apparently I need to go back to school to keep up. You guys lost me at 555 timer, but I like the fact that this project can help me learn. I generally do the same analysis for time/money when it comes to my hobby, however in this case, I am willing to put the time in because I believe I can build a base of knowledge outside of the basics of home wiring and soldering.

As an example I am putting together a quick solution (not something I developed) with a 4.7K resisitor and 470uF 35W capacitor to stop the exhaust valve servo motor from throwing a code on my race bike. It is just a nuissance really, however I am the type that likes things like that to be "clean".

In this case if I can use the resources available on this board, it would seem I'd be able to learn quite a bit.

I hate to ask, but can someone suggest a "short list" of parts (interweb or Radio Shack) and a starting point for a wiring diagram or a repository where I may be able to find a few to dissect. I can reason well-enough, but the vocab alludes me.

Thanks again for the replies, I appreciate the COTS solution as well, and in many cases I'd simply go that route, but for this particular thought I'd like to learn from it as well as have the knowledge to further manipulate/refine it in the future.

One other note, the email notification option didn't seem to work that is why it has been some time since I checked the replies, I will check back more often.

Chris

Actually a good place to start with very little practical knowledge is a 101 in 1 electonics kit that you can get from radioshack and otherplaces. They have a plethora of useful circuits and components that are easy to experiment with and comes with 100+ schematics of useful circuits liek radio tranmitters/receivers, alarms, timers, audio amplifiers, sound generators, and plenty more.

A good website to start reading on is the tutorials provided on allaboutcircuits.com they have an excellent ground up explanation of electronics components and basic circuits. Between that and the 101 projects kit you should be busy for the next month or more. If you outgrow the 101 projects kit too fast you'll at least have a better idea of what you want/need so then you can buy yourself a breadboard and components etc.. to populate it.

Does anyone know of hiring compnies?

hello everyone I am a new member here. I just graduated last summer with
an Electronics Certificate of Achievement and Technology and am having a hard time finding an entry level job in electronic engineering or internship.

Can anyone please help me?


Thank you
Nestor

Sceadwian, thanks for the suggestion, I had a few of those when I was a small child, I understand the basics behnd the components, I don't know the colors for the bands of resisitors like many folks in the industry do, but I don't feel like I need to buy a kit with spring clips, I am talking about switching 110VAC which is more involved than those kits.

Just hoping someone had a quick lesson in relays and could point me in a specific direction for this, I thought using a very low amp DC trigger to switch 110VAC (about 5amps or 550 watts - I'm not turning on a microwave here, just a few lights, maybe a small fan) might be sothing that was fairly common.

Thanks folks.

Seriously Nester? Really? You know they make this thing, it is called a New Thread button...try it.

I'm sorry kubricky, but you seem to be posting under a major miss assumption, switching 110VAC with a relay is an order of magnitude more simple than even a basic 555 timer circuit or alarm/buzzer etc..

You apply DC or AC to the primary coil, it draws X current (listed on the relay) and the secondary is capable of Xvolts and Xamps of current. That's it nothing else to worry about except for maybe a flyback diode on DC coils. Check out a webiste such as Digikey which lets you search for relays based on their specs.

Just as an example I did a quick search for 5 volt relay that could control a 7amp 110VAC load.

16 bucks.

**broken link removed**

Scaedwian. That is basically what I was looking for. I picked up a few relays at Radio Shack and figured I'd just dive in, I have a breadboard kit and figured I take a look at making something work (sans the pot to control other items) again, it isn't the basics I need but the basics of the components. Getting in to the stuff doesn't scare me (that does not mean I don't respect 110V AND DC), but what I mean to say is that I don't mind experimenting.

I guess my real reason for the question is that I figured you simply buy a relay (for example the one I purchased has a "trigger voltage" of low amp DC and control 8amp 110VAC NO) and hook up the DC and AC properly. However in the past I have found that DC and AC in one device is frowned upon (or so I am told) in the event of shorts, etc.

I really appreciate your input and your guidance, thanks again.

I am going to put together something this weekend and see how it works out.

I guess my real reason for the question is that I figured you simply buy a relay (for example the one I purchased has a "trigger voltage" of low amp DC and control 8amp 110VAC NO) and hook up the DC and AC properly. However in the past I have found that DC and AC in one device is frowned upon (or so I am told) in the event of shorts, etc.

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FYI....

No, generally speaking most relays are designed for that very purpose, switching AC with a DC signal. (In broader terms relays are for switching one circuit with a signal from another circuit and can be AC-AC, DC-DC, AC-DC, or DC-AC)...But DC switching AC is found in the majority of applications.

Relays serve the PURPOSE of isolating AC from DC and therefore help mitigate shorts, not cause them.

Relays without extra protection are generally safe unless there is catastrophic external failure they're electrically isolated already. Opto isolation can be used to further remove the controlling circuit electrically from the mains voltage if need be. Generally speaking not an issue.

Outstanding...thanks for the info and steering me in the right direction. I did tear down the driveway item and metered it this weekend and found the trigger signal to be extremely low so I'll have to find a relay that will actually switch 5A 110VAC with a small DC signal input. I've done some research on the 555 timer circuits and that may be the way to go for this project.

Thanks again.

555 timers are very useful for relay control because most of them have a pretty healty ability to supply current.

Okay, so it would seem that the smaller relay I was going to use isn't needed, however I am having some issues because I am looking at this from a mechanical engineering logic point of view (which I understand for instance I look at a diode as a one way valve) vs the electrical engineering point of view.

1 = ground
2 = trigger
3 = output (I am using a 9V battery - so I can get the full 9v)
4 = reset
5 = Control voltage (???)
6 = Discharge (???)
7 = Threshold (???)
8 = Vcc (or supply voltage to the 555)

I belive 6 and 7 make the RC (yes, I took the time to understand -- not well -- the resistance network) which controls the time the "circuit" will remain on.

The output from the sensor box is clean me a very "dirty" signal, but it is enough to trigger the NE555, however it only works with one side of the output connected to the trigger and one side connected to the Vcc (pin 8)

The output pin then trips a large relay that can control up to 10A 110VAC however the circuit does not reset.

And does the reset pin (#4) act as a reset for the 555? What I mean to say is if I connect this would it allow me to reset the circuit (i.e., wait for another trigger?) and how is the reset connected (I am using a momentary switch).

Thanks.

Based on the info above -- any thoughts from anyone?

kubricky said:

Based on the info above -- any thoughts from anyone?

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You are going to have to provide more information, some drawings of how you have things wired up, and all your voltage measurements