I posted a thread recently asking how I could best detect the AC magnetic field of an open frame relay circa 1960's. I received many great suggestions but what I have settled on is using a hall effect sensor in very close proximity to the relay coil. The problem I pose here is that I need a circuit that can convert this signal:

to this signal:

The relay in question pulses for about 88ms but other relays being monitored may activate for as long as 2000ms. The plan is to send the conditioned pulse to an MC interrupt line so it can take action on the raise and/or fall (relay ON/relay OFF) of the signal. See block diagram below:

Any ideas would be greatly appreciated.
Thanks in advance,
Scott

 

It looks like all you need is a monostable multivibrator (one-shot), one of the standard 555 timer IC circuits. Once triggered, the output remains fixed for a given time period and subsequent trigger pulses are ignored until that period ends and the timer resets.

 

This chip http://www.linear.com/product/LTC6993-1 should work for you.

 

The 555 already mention often used this way as a watch dog timer .
Or directly intro the MC if it is a Pic a capture/compare mode should do it.
Max.

 

The relay in question pulses for about 88ms but other relays being monitored may activate for as long as 2000ms. The plan is to send the conditioned pulse to an MC interrupt line so it can take action on the raise and/or fall (relay ON/relay OFF) of the signal.

If you need to know how long the relay is on then you would need a retriggerable one-shot with a >20ms timeout period.

Here's the LTspice simulation of a 555 configured as a retriggerable one-shot, which should do what you want.

 

The relay in question pulses for about 88ms but other relays being monitored may activate for as long as 2000ms. The plan is to send the conditioned pulse to an MC interrupt line so it can take action

If inputting to a interupt already I would investigate doing it all in the MC and set up the watch dog in it directly.
Max.

 

To allow for pulse train timing tolerances, you could measure the period of the first pulse (between the first and second pulse rise) with the MC and then, when there are no more pulses after that period of time, the pulse train has terminated.
So you will know the pulse train has terminated within one pulse period of time.

 

See

 

A charge pump and a schmitt trigger can also do the job.

6 channels from one hex buffer is possible.

 

See
View attachment 130736

Based on the description from the MAX6816 datasheet, I don't think this will help with a continuously oscillating signal:

These switch debouncers remove bounce when a switch opens or closes by requiring that sequentially clocked inputs remain in the same state for a number of sampling periods. The output does not change until the input is stable for a duration of 40ms.
​

If the hall effect sensor output is oscillating in relation to the AC coil frequency, and assuming that AC is derived from the 50-60Hz mains (which is just a guess, but seems likely,) then the signal will NEVER be stable high for 40ms, and therefore the output will never switch high.

I have no prior experience with this chip, so please correct me if I've misinterpreted the datasheet.

 

I posted a thread recently asking how I could best detect the AC magnetic field of an open frame relay circa 1960's. I received many great suggestions but what I have settled on is using a hall effect sensor in very close proximity to the relay coil. The problem I pose here is that I need a circuit that can convert this signal:
View attachment 130702
to this signal:
View attachment 130703
The relay in question pulses for about 88ms but other relays being monitored may activate for as long as 2000ms. The plan is to send the conditioned pulse to an MC interrupt line so it can take action on the raise and/or fall (relay ON/relay OFF) of the signal. See block diagram below:
View attachment 130704
Any ideas would be greatly appreciated.
Thanks in advance,
Scott

Just curious, what hall effect sensor have you decided to use? Have you tested it yet to see what kind of field strength/output voltage you'll be working with?

As to your main question in this thread, I would probably handle it in code, unless you're severely restricted by your mcu in terms of memory use, code size, etc. Code is free and can be used over and over again for every input. An electronic solution means extra expense and more soldering for each additional input. Don't get me wrong, there are times it's better to have the electronics ease the load on the mcu, but if it's a toss-up, I'd do it in code.

 

Just curious, what hall effect sensor have you decided to use? Have you tested it yet to see what kind of field strength/output voltage you'll be working with?

I'm using this little guy. It does detect when it is positioned properly but a more sensitive sensor would be better.
https://github.com/bento-electronics/sensorpack/blob/master/Hall-Effect-Switch-A3144/image.png

 

Thanks to all who responed. I'm going to go with Sensacell's charge pump suggestion. As seen below it provides what I am looking for.
Scott