How To drive a Relay from an Exit Pin of 8-bit Shift Register IC

snipped-for-privacy@gmail.com (Actorindp) wrote in news:171dfda7.0410010829.4f8ff324 @posting.google.com:

While it has been a long time since I fooled with this type of thing, I do not believe you have enough current output to drive a relay coil. You could maybe switch an higher current capable SCR through an opto-isolator (to protect the circuit from high current).

from the

Take the signal from the register to an IC with an isolated amplifier circuit that drives its own relay.... pick up a 12v source elsewhere and run one leg of that though the IC board relay contacts to the relay you wish to operate.

The contact rating on the IC card relay contacts must be of course greater than the coil on the relay you want to pick up...Id suggest at least 3 times greater to get any kind of reliability.

I wouldnt suggest you try to do it with straight solid state electronics... that puts a lot of potential stray voltage sources too close together for my taste at least.

You build or buy the IC card to be driven by the exit pin voltage and current rating...say 2vdc, at 5ma. or whatever. You can buy switching that operates in any of those ranges.

You can buy voltage relays adjustable to drop out say at 1 volt and pick up at 5 volts on a variable voltage signal if thats the case.... that will give you some tolerance for voltage variables in your circuit (the voltage may vary with termperature or for other reasons and give you false signals)....or you can use a device to accept current flow of say 5 to 20 milliamps...if thats the variable coming off the pin.... energizing a relay at whatever preset you wish...say to close contacts a 8 ma. or more... but thats not a usual application... usually milliamp signals are used to drive analog devices such as temperature and pressure read outs as current flow varies through thermisters (temp sensing elements).

To discover the spec requirements for the IC card you may need to meter the pin output. voltage or ma and select your controlled device accordingly.

The problems with these systems are anomalies in system resistance due to temperature changes, the effects of stray curents or electro magnetic generated interferences, bonding variables, loose or oxidized connections...and aging components...and the sheer number of components...so that for instance a system such as failed at the denver air port (baggage handling) even a 1/100th of 1% failure rate amounted to one failure a day or more...that sent the entire system to slinging baggage though the walls shutting the entire air port down. With jury rigs and no integrated engineering, you can be making bad guesses on how your addition will influence the rest of the network. .. how touchy the thing is etc...

These are control cascade effects that I try to eliminate in my systems... I have one actual physical function. say product coming off of a converyor belt... initiate the other needed functions. ..and by easily verifiable means...no analysis needed. I do not have one controller dependent on other controllers as the sole control, figuring out these cascades will not be the forte of todays new crop of low riders and rappers as the skilled baby boomers retire (50 million of em over the next 4 years)... over half of the most qualified work force.

Simplicity will work though. Its always best.

If your system is function controlled... then one error or failure simply stops that part of the system...say one take off conveyor for example.. the rest of the system remains functional... just product flow on that single aspect part dies. I try not to arrange cascades when I can and instead parallel systems.

Complexity is not a wonderful thang.

then after you build it, and 5 years go by, or two weeks or whatever...and you are gone.. the new guy gets to try to fathom what the hell is going on with the hair ball....with only God knowing which wire is loose.... the owner gets to buy all new stuff... if its like some plants.... they just move the entire mess to china.. so the complexity is not even job insurance.

Simplicity and viability are job insurance over the long term.

(just killing time on this end... trying to put off figuring out piping on a job with no walk through allowed.... and a quote needed tomorrow.)

Phil Scott

Whatever else you do, make sure you put a diode across the relay coil, which conducts in the opposite direction to the actuating current. Otherwise you'll blow the output of whatever you use to drive the coil, from the emf generated when you cut the drive current. The diode must be rated to carry the required drive current.

Sylvia.