Im trying to tigger a 555 timer with a comparator. Im using the 555 timer in a monostable configuration with the supply voltage at 7 volts. When the input to the 555 comes via the high output of the comparator at 7 volts, the output of the 555 timer is high instead of low. But when the input to the 555 comes via the power supply at 7 volts, the output of the 555 timer is low as expected until the input is changed from 7 volts to ground. Is there two little available output current from a 317 comparator? It seems I had it working properly at one time, but maybe I was using a CMOS 555, Im note sure.............would it matter?
Trace out a circuit diagram from what you actually have running (not what you think you have running) - perhaps that will help you find the fault. If not post the diagram somewhere and post the link here and someone will help.
Thanks for your input, but I did actually take the output across the pull up resistor attached to the output of the comparator and th 7 volt rail supply.
First off, a 317 is a voltage regulator, not a comparator. But maybe that was a wording mistake?
It would be nice to know what kind of comparator you are actually using ?
Most comparators pull to the common with an open collector NPN and have no pull up R. You may want to employ a pull R on that line..
The Trigger input requires a pull to common of 1/3 of VCC or less to activate it, it uses a PNP logic on the input.. Having your comparator in the low state is simply turning on the trigger and setting the output to a High state.
Having the (Vcc) rail connected to it, is actually not triggering anything, so it remains where it is and when the threshold is reached, it'll turn off the output, in case the output isn't yet off from some prior trigger or when ever the threshold reaches the 2/3 Vcc level.
Only a pull to common and 1/3 or less (VCC) on the trigger will actually make the trigger do it's thing! (+) volts has no effect other than pulling it up to remove stray signals.
The Threshold on the other hand, works the other way around.. It does nothing when it gets pulled to common and operates when it sees at least
2/3 or more of the Vcc supply.. That will put the timer in the off state, as long as the trigger isn't being held in it's operating region.
And remember this, if you hold the trigger in its operating region longer than the one shot calculations on the threshold, the output will remain on until you get that trigger out of the on region, at which point, the internal Flip Flop will then flip to the next step to turn off the output and discharge the cap. That is, if the Threshold is still at 2/3 or more of Vcc at that time.
If you have digested what I have explain to you, I think this should open up some shadows on how the 555 works and things it can do.
In short, you need to reverse of your logic on the comparator and use a cap to isolate the trigger if the trigger may stay on longer than the one shot calculations. That is up to you..
Yes I was mistaken, the comparator is a 319. I do understand the operation of the 555 as you explained. What I cant understand is why the output of the 555 is on when VCC is applied to the input of the timer. The input to the 555 comes from the output of the 319 which is connected to a 1K resistor connected to VCC.
Hmm. Well, that seems to be a little different than what you said before?
Any way, if you're using a simulator, start your DC voltage at 0, do this in the simulation set up. Applying the VCC to the trigger when it has been establish that the output is off, should not turn on the output after that fact.
You could be experiencing a problem of the trigger getting an initial sink pulse on power up.
To fix your problem, you could use a 100ohm R from the VCC to the RESET pin and a .1uf from the RESET pin to common.. This will force a short reset of the timer on initial power of Vcc. It will force it in the off state. You most likely have an issue with some capacitance that is causing a short SINK pulse on the trigger during initiation. You need to make sure the RESET input stays lower during the initiation state.
I tried that, and unfortunately it didnt fix the problem. Still, when the trigger input to the 555 comes from the output of the comparator (or Vcc) the output of the 555 is high, but when I switch the trigger input to Vcc directly, the output goes low (and stays that way untill I decrease the input to 1/3 Vcc)
--- Sound to me like what's happening is that there's a low-going transient from the comparator triggering the 555 on power up.
Since the 555's RESET voltage is 1.2V max, at any supply voltage, then what you need to do is hold the reset pin lower than 1.2V for as long as it takes the transient to die away.
Assuming a 5V supply, 100 ohms and 0.1µF gives a time constant of
2.7µs from 0V to 1.2V, (the 555's RESET release voltage) which is way too fast if what you're trying to do is keep a turn-on transient from the comparator from triggering the 555.
Here's a circuit for a 555 one-shot with a passive power-on reset: (View in Courier)
What we have to do is figure out the values of R4 and C4 which will keep U1-4 below 1.2V for the time required to let the transient on U1-3 die away.
Just for grins, lets say 10ms will do it and were running a 12V supply.
Then we can write:
Vcc t = RC ln ----------- = KRC Vcc - Vth
Where T is the time required, in seconds, R is the resistance required, in ohms, C is the capacitance required, in farads, Vcc is the supply voltage in Volts, Vth is the threshold voltage, in Volts, and ln is log to the base e.
If you need more time you can make either R4 or C4 bigger, and C3 is a _must_ if you're using a bipolar 555 and should be connected directly across pins 1 and 8. If you're using a CMOS 555 then it isn't needed.
I will try your suggestions and post the results. For now, I simply buffered the input to the 555's trigger from the comparator, and it is now working as it is supposed to. If it is a low-going transient on the output of the comparator which is causing the problem, wouldn't it die out on its own given enough time?
I learned a valuable lesson on the importance of using de-coupling capacitors. Things worked just fine as soon as I put the 100nF cap between pin 1 and pin 8. Should I get in the habit of always using them on the Vcc pin on ICs?
Unless Vcc and Gnd are both on planes, yes. If they're on planes, and wiring to those planes is *short*, you can get away without decoupling caps on each part. You should have some larger caps somewhere on the board, too.
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