Re: 555 timer questions

From: Rubicon (no email)
Date: 06/09/04 

Date: Wed, 09 Jun 2004 05:23:26 GMT

John,

Thankyou for the detailed reply.

Just a couple more questions if you have time.

Why the 100K resistor on the first ASCII sensor/timer circuit and not
the others and also what does the K by the VREF on the non-inverting
comparator with hysteresis mean? Am I missing something obvious there?

Thanks,

Andrew.

On Mon, 07 Jun 2004 15:20:10 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Tue, 08 Jun 2004 05:55:55 GMT, (Rubicon) wrote:
>
>
>>John,
>>
>>Sorry for the lack of information.
>>
>>Three triggering circuits to the 555, all operating off a 9V battery.
>>The first is an RF switch circuit with an NPN transistor and resistor
>>between V+ and the collector for the negative "pulse".
>>
>>The second a simple sensor circuit again with a transistor/resistor
>>for the negative "pulse".
>>
>>The third is a low voltage circuit. I have found an opamp/zener diode
>>based low voltage alarm circuit which might do if I eliminate the LED
>>and its resistor, replace the LM339 with an LM393 and the buzzer with
>>the same transistor/resistor method of activation.
>>http://www.electronics-lab.com/projects/sensors/023/
>>
>>I understand that the 555 needs a negative pulse and that all three
>>circuits only provide the negative and not the quick return to
>>positive. Unfortunately I'm not sure of any other relatively easy way
>>to do this.
>>I think I'll need two timers here, the first for the RF circuit which
>>could be a 555 (or one half of the 4538) in monostable mode to allow
>>for retriggering after its timed out. The second timer though would
>>need to be a "perpetual input pulse holdoff after the first output
>>pulse" type to prevent the sensor and low voltage circuits from
>>constantly re-activating it.
>>
>>Battery conservation is a factor but I haven't yet looked into methods
>>of doing this.
>
>---
>OK.
>
>The first thing is that 'retriggerable' refers to being able to extend
>the timeout of the one-shot while the output is still hot. That means
>that if you've triggered a one-shot with a 1 second timeout and it's
>only halfway through its timeout and you hit it with another trigger
>pulse its timeout will be extended for 1 second after the application
>of the new trigger without the output changing state. So, for those
>two input pulses you'll get an output pulse which is 1-1/2 seconds
>long. If it's _not_ retriggerable, then hitting it with additional
>input pulses will have no effect on the timing until the output times
>out, and then the first input after that that will trigger it again
>for 1 second. 555's aren't capable of being retriggered without the
>use of extra hardware, but '123s and 4538s are, with just wiring
>changes.
>
>The second thing is that by "perpetual input pulse holdoff after the
>first output pulse" I meant that you'd get a single output pulse from
>the timer no matter what happened on the input during or after it
>timed out, and that after that you'd have to manually reset the
>circuit to get another output pulse.
>
>However, from your description, above, it sounds like you'd like one
>output pulse whenever the input voltage goes below a certain level,
>and the LM393 circuitry shown below should do that for you.
>
>As far as sorting out your input pulse problem goes, you could do
>something like this:
> .
> .
>SENSORS . TIMER CIRCUITRY
> .
> .
> +9V . +9V +9V
> | . | |
> [R] . [10k] [100K] +-----+
> | . | | 3|__ |
> +--> .>---[0.1µF]---+-[<1N4148]---+-----------------+--O|TR |
> | . | | |
> C . | +-----+
>--B . | 7555
> E . |
> | . |
> GND . |
> . |
> . |
> . |
> +9V . +9V |
> | . | |
> [R] . [10K] |
> | . | |
> +--> .>---[0.1µF]--+--[<1N4148]---------------------+
> | . |
> C . |
>--B . |
> E . |
> | . |
> GND . |
> . |
> . |
> . |
> . |
>That low voltage sensor you linked to is a real POS |
>in that it has no hysteresis and will probably |
>chatter like crazy when it gets close to the |
>threshold, but you can fix it by doing something |
>like this: |
> . |
> . . . . . . . . . . . |
> . |
> +9V +9V . |
> | | . |
> [R3] [R4] . |
> | | . +9V |
> | +--[R2]--+ . | |
> | | | . [10K] |
>VIN>-[R1]--|--+--|+\ | . | |
> | | >--+--> .>--[0.1µF]--+--[<1N4148]--+
> +-----|-/ .
> | 1/2 LM393 .
> |K .
> [VREF] .
> | .
> GND .
> .
>
>Make sure you short all the inputs and the output of the unused
>comparator to ground.
>
>The resistor values in the places I left unspecified will determine
>the drain on your battery, and you'll need to choose them with that in
>mind as well as with what the highest impedances you can use and still
>have the circuit operate reliably will be.
>
>Also, if you're trying to sense when the 9V goes low you'll need to
>connect the VIN end of R1 to ground and arrange for the voltage
>divider R1 R2 R4 to cause the voltage on the comparator's
>+ input to go lower than the voltage on the - input when the battery
>voltage falls to the voltage you've chosen for the alarm point.
>
>A pretty good choice for a reference would be something like a common
>LM 185/285/385, where you could get down to around a 10µA constant
>drain on the battery to keep the reference's output voltage constant,
>but just make sure that that 10µA is flowing when the battery gets
>down to the low voltage you want to sense!
>
>--
>John Fields

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