Frequency "Limit" Relay

[Brandnew]

Hi,

I'm trying to find a relay that will open (or close) after a certain frequency is achieved. Ideally the relay would be adjustable to allow for fine tuning but this is not necessary. I'm looking to find a relay like this or I will build my own if need be!!

I have a signal from a VSS that is 0-4.5V (DC). The frequency from this switch comes in a squarewave and varies with speed. When the frequency gets above 30Hz I want the relay to open.

Any ideas where to find a relay like this?? Thanks!!

 

[dbaird]

Try Moore Industries Model FDY (

http://www.mooreindustries.com/products/data_sheets/fdy.pdf?Subject)

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[automatic2]

low pass filter rectified into a comparator?. Easily adjusted

 

[Brandnew]

Thanks for the suggestions.

How much is the FDY?? That looks like that might be a little spendy!!? I want to be as cheap as possible on this

automatic2, could you explain that a little more??


Does anybody think programming a simple microcontroller would work for this?? That would be pretty easy to adjust, easy to build and pretty cheap.

 

[47gandalf]

If cheap (inexpensive) is what you want, I agree that any-old-micro can do this. I'd look at the MSP430 or PIC12 family since they are under a dollar at quantity one.

Assign one pin of the processor to drive the relay, one to input the frequency signal, and write a very short program to measure the frequency and make a comparison with the threshold (30 Hz) to determine when to actuate the relay.

You haven't said whether the relay can be driven directly off the port will need an external means of amplifying the processor port pin's output power to the level required to pull in and hold the relay. If external is needed you might look at ULN2003 or family member. Even though you only need one "channel", the part is so cheap and so perfectly fitted to the application that I'd use it anyway. It was designed to drive the hammer solenoids in impact printers which are exactly the kind of load you want to control.

If you are familiar with processors I think you could build, code, debug and have working prototype in one decent work day. Even leaving room for the inevitable interruptions.

 

[Brandnew]

Thanks I will look into these.

I got sidetracked and wasn't able to put any more time into this project. I'm not extremely familar with these micro-controllers.

Do they need a power and ground wire hooked up as well?? Do you know of a good place to purchase these MC's and the programming kits?? This would be my first one so any assistance you could provide would be appreciated.

As to the relay, I haven't selected one yet. I'm waiting to see what MC I choose. I would hope I could get a MC that would run the relay. The relay would be set-up to run a circuit in the normally-closed position (no signal from the MC). Then when the MC is activated it would switch the relay to the open position and inturupt the circuit.

I know this set-up would be very simple for many of you, but as you can see by my speciatly this is not my strong point. BTW, this would not run anything critical so if it malfuntions it have virtual no impact.

Again, any additional information you can provide would be greatly apprecaited. Sorry for my lack of knowledge in this area.

 

[buzzp]

PIC microcontrollers can be purchased through digi-key or future electronics. The programmer (basic) is $199 or you can build one if you have the time. The new flash chip is cheap and is part number 12F675. This chip is not required for your application but it will work. It has onboard A/d so you could implement a pot to adjust the frequency trip point. Or use a digital pot or dip switches (if limited trip points are ok). The cheapest might be the 12c508 chip. Not sure they have a flash version of this.

I would feed the signal into the microcontroller (amplitude <5V) on a DC reference of 2.5VDC so you can see the positive and negative with the microcontroller. The signal should go through some current limiting impedance (likely not mandatory but safe). You can generate an interruppt internal when the pin crosses zero (rising or falling edge). The signal is already a square wave so the rising or falling edge interrupt will mean a zero cross. Count the time in between interrupts. Depending on how you write the code to interrupt, this time will be a complete cycle if you do not change the interrupt from falling to rising, say. Invert and then you have the frequency. You can use a transistor to drive a relay coil to take whatever action you desire.
Good luck.

 

[Madcow]

I do believe with the PIC you could use the capture and compare module to determine frequency.
If your on a limited budget you can start programming the PIC12f675 with the pickit1 for $36.

http://www.microchip.com/1010/pline/tools/picmicro/demo/pickit/index.htm

Hope this helps

 

[buzzp]

Never knew they had this programmer madcow. Thanks for link. (I see this is flash only programmer). What do you mean by using the capture and compare module? Are you referring to the comparator? Please explain.

 

[sdmays]

I would like to second Madcow's suggestion. Brandnew, if you are interested in using a microcontroller, the Pickit is an EXCELLENT idea!

It is inexpensive,
It can easily program Microchip's flash microcontrollers,
You have everything you need to 'play' with the microcontroller,
It has great examples illustrating useful microcontoller concepts (including 'timers' and 'interrupts' both of which will make your project MUCH easier).

All things considered, it would probably be less expensive to design and construct this circuit using analog circuitry, but I believe the investment in this Microchip kit will pay off - especially if you ever encounter such a project again. By that I mean, you will not only have both the hardware and experience you need to repeat this exercise, but once you have written and debugged a successful program, all you will need to do in the future is change/add lines of code to realize the next circuit of this nature.

Brandnew, to answer your question, the microcontroller will require 5VDC and ground. Everything you need to use your microcontroller will be shown in the literature that accompanies the Pickit - of course, practically any microcontroller or manufacturer of microcontroller you choose will give great notes on the internet on how to use their microcontroller (opportunity for you to do a 'little' research before deciding, etc...)

 

[Brandnew]

Thanks for the help. I'll have to try madcow's suggestion out. The price is right for this project.

Thanks again everyone!!

 

[MustangDan]

Brandnew,

Not sure what type of project you're working on, but in an industrial setting, I know I prefer to choose off-the-shelf components that can readily be replaced (without fabrication or programming on your part).

My suggestion is to use a Redlion Controls IFMR (Integrated Frequency Modulated Relay). I have used this product personally, and have found it to be very easy to use and reliable. It's programmable by dip switch settings on the side, so once you buy the relay, you're done; no writing a program!

You just set the frequency where you want the relay to actuate, give it some power, and VIOLA! ...and you can set the frequency from 0.1Hz to 25KHz, so tweak away!

Check out:

http://www.redlion-controls.com/products/doc/10007.pdf

Best of Luck!

 

[Brandnew]

Thanks Dan,

This is actually something I'd like to add to a car circuit. That piece you show seems a little big to squeeze into a car (4x3x1). I could do it; but it's large.

It really just seems like overkill for my small application. Do you have any idea what that costs??

 

[sdmays]

The Carlton-Bates Company (

http://www.carlton-bates.com)

sells the Red Lion control relay suggested by MustangDan. I will let you find out on your own how much it costs. Per the spec sheet, the model number is IFMR0036 for the 6VDC to 36VDC relay, so you can use that number to search the Carlton-Bates web-site. Bear in mind, this particular relay is meant to mounted on a din-rail, even though I suppose if you are creative, you can find an alternate way to mount it. You might be able to find the same relay in a panel mount package, which might change it's physical size.

I still prefer the microcontroller idea...

 

[Brandnew]

Ouch!! I like the MC idea too!

Thanks all!

 

[Madcow]

Buzzp,

Some of the PIC's have a capture/compare module.
It has a counter that starts counting when a trigger event occurs.
In capture mode it counts until it receives the next trig.
then moves the result to a register.This is nice for simple
period counting.
In compare mode you load a register and it sets an interupt or pin when the counter reachs the count in the compare register. I never used this mode. Might be useful for duty
cycle detect.
I dont think the PICKIT micro has the capture and compare,
but I think it supports ones that do. You should be able to
get samples of those from their site.

 

[sdmays]

I'm looking at the PicKit User's Guide and the last tutorial it describes/explains is for &quot;Frequency Counting with Timer1 Gate.&quot; I haven't read through this particular Tutorial yet, but I plan to do so.

 

[buzzp]

Thanks for the additional info madcow. Never had the need to use this part of a PIC. I did freq the old fasion way by simply generating an interrupt on rising or falling edge and using the internal timer. Sounds like I might be able to eliminate some code using capture/compare.