How to build a relay control circuit based on variable voltage input? 1

[BFuddled]

I hope that someone would be willing to help an electrical-engineeringly-challenged person with the need to build a seemingly simple (?) circuit, preferably out of discrete components. Or, what the heck, if I need to use an IC, I’ll just have to learn how to solder without melting it down into a blob of melted plastic. ;-)

I need to control a relay that will use an external & isolated power source for the coil. The power supply for the coil can be used to power the circuit that I am seeking to build. (Coil voltage can be either 12VDC, 24VDC, 24VAC, or 120VAC - I’m not picky, just desperate.)

The relay needs to be triggered based upon the output of a sensor unit. The sensor unit output consists of a milliamp signal, varying in voltage between 0-5 volts DC. This milliamp output must see (at most) only a very high resistance across it, with no interfering signals fed back into it.

I need to be able to trigger a relay based upon the sensor output voltage, and I need to make the control circuit adjustable, i.e., I must be able to alter the voltage level at which the relay is triggered (via a variable resistor?)

Even better, but obviously more complicated would be a circuit that also had an adjustable hysterisis built into it, so that the relay could be triggered to operate at, for example, 1.0 volt, wait for the signal to reach 1.5 volts to shut off, and trigger again when voltage drops to 1.0 volts again.

Does anyone out there know where I could find a schematic of such an animal? Is there a commercially available (inexpensive) unit that would work? Or would anyone be helpful enough to actually figure out how to do this?

Much MUCH appreciation in advance for any help with this!

Thank you,

B. Fuddled

 

[cbarn24050]

Hi, a 555 timer chip can be used for this.

 

[BFuddled]

Cbarn24050:

First off, I would like to thank you for your reply. I can assure you that I’m not lazy, just ignorant, so I did a site search on the 555 timer, and came up with some interesting reading material & other web pages, most of which is Greek to me.

I will be the first to admit that I am COMPLETELY out of my element when it comes to designing any electrical circuit. I can rewire my car, or even build something out of a kit – aren’t I an accomplished one, huh? (Self-depreciating humor fully intended.) I have no clue on how to adapt that IC for my application.

I guess what I really need is for someone to actually design a schematic for me to follow, monkey-see, monkey-do style. I just don’t know where to get this sort of help – I was hoping that someone on this site may have already put something like this together & be willing to share. Hey, I’ll even send a bottle or two of wine to anyone that can put this together for me.

Thanks Again!

B. Fuddled

 

[DRWeig]

Hi BFuddled,

Kele has a device (UCS-121 or UCS-221) that does this trick, and if you need to scale the input, there is a UAT-1 that can do that too.

Have a look, the web is

http://www.kele.com,

and it's in the product catalog under output transducers.

Let me know if you need more info.

Old Dave

 

[BrianR]

You have not said what you want to switch, AC or DC and what voltage and current. However here are some ideas. Use a standard industrial optically isolated solid state relay. Typical specs are:

Control input: 3.5VDC min @ 3.5mA,
Optical isolation between input and output: 2.5KVRMS,
Output will switch AC and/or DC at almost any current you want (depending on the relay chosen).

I suggest you visit a local supplier of industrial control equipment as the range of these things is large.

To use a 0-5VDC trigger with hysteresis, a comparator is needed. This is an inexpensive IC with half a dozen simple components. There is a large range of these too, but look at NS Semiconductors web data on an LM311 IC for example. They offer circuits too.

 

[jbartos]

Suggestion: Since there are some application aspects of the original posting withheld, it is recommended to search for application notes. Majority of manufacturers provide excellent documentations of their products, which is highly commendable, however, when it comes to several typical integrated applications of their product, one is literally stuck. I think that the best start is to search for some close application notes pertaining to your sensor applications. Visit

http://www.eetasia.com/ARTICLES/2001SEP/2001SEP12_AMD_AN.PDF

or type "sensor application note" in your browser which will give you good return on temperature sensor applications, etc.

 

[rodar]

Try a panel meter made by Simpson. It is called a
Hawk meter. It can be purchased in a variety of input
configurations and it has relay outputs with
programmable set points. The display value can
be scaled to read in units for whatever process
you are measuring. I think NEWARK sell these.

 

[xray]

BFuddled:
I think I read your messages clearly and you would like the schematic handed to you. I have a little time this week and could email you a completed schematic by Friday 6/7. If you have use of such a proposal let me know. My email address is drollinger@montana.net.
I think your output is mA as you have stated and not voltage as you also have stated. In my limited exposure to output sensors, the outputs range from 5ma to 20ma. The current output is roughly linearized, by a small amount of circuitry, to an appropriate voltage signal.
Again if I can help please email me.

Sincerely;

Cy Drollinger

 

you should think about using an opamp for this problem. it has a very high input resistance, drawing micro-amps from the source. it has a low output resistance, good for energising a relay. it is cheap, typically 50p(UK) from RS for two opamps on one IC. i have designed a circuit for my employers that sounds very simmular to what you require. it monitors a voltage output from a fuel level sensor. when the fuel tank is empty a value of 3 volts is available from this source. the opamp 1 triggers and energises the fuel refill pump relay. when the tank is full a value of 7 volts is available from the source. this value triggers opamp 2 and de-energises the relay, thereby stoping the fuel refill pump. if this sounds interesting or usefull to you then respond to this posting and i will email you the scematic with further details. have fun tinkering!!!!..elecrtonerd.