Lots of noise in this thread. That's weird since JWHassler nailed this in the very first reply.
look for a "retriggerable one-shot" or "missing-pulse detector"
look for a "retriggerable one-shot" or "missing-pulse detector"
I have to disagree with this fact, in digital domain the decision whether the pulse is the correct length will take no longer than 1.1s in the 0.9-1.1s case, and no longer than 1.01s in a 0.99-1.01s case, so narrow band doesnt make things worse at all.The narrower the bandwidth, the longer (more input cycles) it will take to decide. Whatever method you use - analogue or digital. It's a fact of life!
From the OP's description, the options are: high, low or 1 Hz. He is not looking for 1.1 Hz, or 0.997 Hz. From an engineering point of view, the goal is to solve his problem to a degree that is system works. The goal is not to split hairs or wax poetic.From an engineering point of view, it's usually not acceptable to sample a single cycle to determine if something has a valid frequency. Multiple data has to be sampled and averaged (and sometimes variance comes into play too) for it to be considered valid. I speak from personal experience in this case... but of course, in the end it all depends on the application's requirements.
Yeah... I have to admit that I like a little poetry here and there every once in a while, and maybe I broke the KISS rule with my suggestion ... but then again our friend hasn't told us what his application is for...From the OP's description, the options are: high, low or 1 Hz. He is not looking for 1.1 Hz, or 0.997 Hz. From an engineering point of view, the goal is to solve his problem to a degree that is system works. The goal is not to split hairs or wax poetic.
Frequency is the inverse of the period. And the period only happens once.OK, I know what you are saying. You could measure the time the square wave is high and low and get an answer in a single cycle. But is this measuring frequency or the time of a pulse? Surely it's only a "frequency" if you have a repeating waveform (with a spectrum). Any mathematicians in the house?
Or Watch-dog, set the time for 1hz period.How about the dumbest charge pump solution?
I think this is the way to go, but I don't have these kind of chips available. I will try this with a 555 timer. (which I have a lot handy)look for a "retriggerable one-shot" or "missing-pulse detector"
I have tried a simple 555 missing pulse detector (schematic here, page 13), it won't work if the input stay low. And I have tried feeding the signal via a input capacitor, still no luck. Maybe I am missing something here??With a 555 missing pulse detector, (watch-dog timer) if the input is via a input capacitor, only the pulses are received by the 555, any permanent high or low the timer times out.
Pulses keep it in reset.
Max.
Inspired by your circuit, combine with a 555 missing pulse detector, I think I got a solution. Please see above comment and attachment.I guess you cannot expect a very fast response time between status changes.
How about a simple capacitor coupled inverting op amp amplifier with rail-to - rail input/output and a low reference voltage.
Here is a filtered, capacitor-coupled amplifier. It will turn on for about 0.5 seconds when signal goes high, but no divice will be able to tell the difference between a high and a 1 Hz signal for the fist half second.
View attachment 76149
And one short pulse when "high"
View attachment 76150
I would like to try something like this, but I don't have this chip available. But thanks for your input, as always!Try a retriggerable monostable such as MC14528 or MC14538:
Hi Nandu,Hi Bug13,
Here's your circuit.
Regards,
Nandu.
Ah I see, I am sorry to confuse you guys.
The circuit I want is the output stay high when the input is 1hz square wave.
The output is low when the input is always high or alway low.
This should clear up the confusion. Sorry about that.
To review.
Input is low, output is low.
Input is high, output is low.
Input is 1 Hz square wave, output is high.
Hi Nandu,
My simulation shows:
Input high -> output high
Input low -> output low
Input 1Hz -> output 1Hz
But that behavior is not what Bug13 is looking for, as the other quotes document. Am I missing something?Hi Bug13,
That's correct. It ought to behave that way with the 'trigger' & 'reset' shorted.
Short answer, no.So I continue to wonder why this isn't being done with a microcontroller... This can be done very easily with a single very small microcontroller... is this a exercise in masochism?
Sorry, I didn't know what that meant.Hi Djsfantasi,
You missed my last sentence 'I'm sorry for my bloomer'!
Regards,
Nandu.