Modulated Interlock Loop Safety Curcuit....

[hightechstuff2]

Hi Everyone!
I am looking for a schematic for this circuit but I cannot seem to find anything. What I am looking for is a simple circuit that it most likely 555 timer based.

What I want it to do is send a 5V square wave signal, maybe 10-100Hz, through a 12' to 200' max through one lead on a 25 conductor cable which then loops back through another lead at the end of the cable. From here the return signal is compared to the original signal. If the pulses are similar, it will then close a relay.

If anyone could supply me a schematic that is close to what I am looking for, that would be awesome!

Thanks!
Adam

 

[hightechstuff2]

Oh, sorry for the double post but I forgot to add something.

I would like the circuit to be able to ignore possible voltage drop and possible noise due to the length of the cable. I can't imagine these circumstances would greatly interfere as it is a shielded cable and the maximum loop length would be 400 feet.
Thanks Again!

 

[VEBill]

If you're just looping the signal back, then why not skip all the complications and just use DC? You can easily account for series resistance by using low current, and noise can be filtered and blocked.

If you insist on sending the LF series of pulses, then you should find yourself what is often called a cookbook of typical building blocks. A guy named Forrest M. Mims III made a whole series of such 555 and OpAmp Cookbooks that were once sold at Radio Shack. Also there are circuit libraries in oversized volumes at most libraries. And Google.

You have to define what you mean by "similar". Do you want the equipment to latch on open or closed circuits? There are "Missing Pulse Detector" circuits that will trigger if even one pulse is missing. And there are PLL-based tone detectors that will find the pulsed signal even if it is hardly there at all.

Similar test equipment used to test cables is available commercially (sometimes very cheap). They often have two circuits, one to trigger on intermittent opens and latch the fault, and another circuit to trigger on intermittent shorts and latch those faults as well.

 

[VEBill]

Looking at the title of your post, you're probably looking for the Missing Pulse Detector where the circuit is triggered if even one pulse is missing.

You would probably still have to shield for noise (obviously depending on the environment) and think about false triggers versus slow response.

Not to mention Built In Test if it is a safety system.

 

[hightechstuff2]

Hi VE1BLL,
Thanks for your post. My problem is that this system is designed to fit a DB-25 cable. I want an interlock that is virtually fail-safe. You see, if someone plugs this into a parallel port on a computer and the device receives a 5V signal,,,(if that is what it is looking for as you suggested)from that particular conductor, it possibly means serious injury for the operator. If the device is looking for a "coded" signal, their is much less of a chance in error.

 

[VEBill]

Modern PCs typically do not have parallel ports, nor DB-25 connectors. It might be cheaper to update all the PCs within the area to ensure there are none with such ports. ;-)

 

[hightechstuff2]

That is just not possible in my application. This device is being sold to hundreds of different clients.

 

[Henneshoe]

Maybe I am missing something, but why would you want to use a 555 timer circuit for this or almost any other circuit with the price of microcontrollers nowdays?

If I were you, I would use something like a Atmel AVR controller. An ATMEGA48 costs about $1.50 in your quantities. There is a free compiler available, and you can get a cheap debugger for $50. The only other components you would need are a few Resistors, Capacitors, and maybe diodes to add a LP filter for EM noise.

If you go with this route I would suggest using one of the microcontroller's timers to generate the square wave, and a ADC pin to read the loop-back. A GPIO pin and a mosfet could be used to trigger the relay.

 

[zappedagain]

This could be as simple as a 555 timer and a SR flip-flop (or equivalent). The pulse from the generator sets the flop-flop and the received pulse from the cable resets it. Or you could do this in a simple CPLD device and do it all in one chip (plus interface protection). Xilinx has tools that let you generate a schematic to program the device instead of having to learn a hardware definition language.

The 555 timer book and a digital logic book should get you started. The CPLD manufacturers have some good examples on-line too if you want to go that approach.


John D

 

[HSWSac]

If you wanted it super simple and wanted the 555, then generate your square wave with the 555, and pump the signal and loop back into the inputs of simple chip with an exclusive or gate (XOR). The output of the chip can trigger a relay or transistor if the signals don't match. (!XOR means they match, XOR means they don't match). If the loop was disconnected or if DC was on the return, the XOR would flip-flop, so you'd likely want to use some sort of latch (like a second 555) to keep the output on steady. If you're just illuminating an indicator with this signal, you could probably get away with the flip-flopping, or you could put a cap on the output to smooth it out.

 

[itsmoked]

Or just use DC but look at the parallel port description and and avoid using a pin that could send back 5V.

You could easily look at both a Centronics port and a 25pin RS232 and just pick a safe pin.

Or use a non-standard connector that doesn't plug in at all.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

Mims "Timer..." book has a simple "Missing Pulse Detector" circuit. It is a 555, a PNP transistor, and some bits and pieces.

 

[hightechstuff2]

Got it! Thanks a lot for your help! I designed a "Missing Pulse Detector" around a 556 timer and it looks like it should work great.
Thanks Again!
Adam

 

[hightechstuff2]

Okay guys, still stuck..... This missing pulse detector won't do the job either....

Is there a circuit, there has to be, that would look for a 100Hz or so square wave that will turn on when it sees this modulation and turn off when it does not? I'm pulling out my hair trying to find such a circuit and have had no luck.

Thanks!
Adam

 

[hightechstuff2]

Damn! I missed the above posts guys. I don't know how. I'm just not used to this forum format. Please ignore my above post.
Thanks a lot for all the help!
Adam

 

[waross]

I hope that the signals on the other conductors are immune to the cross talk from the square waves. Your square waves will contain components of much higher frequencies.

Bill
--------------------
"Why not the best?"
Jimmy Carter