Re: 555 timer questions

From: Rubicon (no email)
Date: 06/08/04 

Date: Tue, 08 Jun 2004 05:55:55 GMT

On Sun, 06 Jun 2004 13:57:00 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Mon, 07 Jun 2004 03:50:45 GMT, (Rubicon) wrote:
>
>>Hello,
>>
>>If more than one circuits negative pulse output is going to a 555
>>monostable timers trigger input should there be something on each
>>circuits output to isolate them from each other? Perhaps a small
>>signal diode?
>>
>>I also require a timer circuit for "once only" activation with no
>>retriggering. A search of old posts resulted in a 2000 post suggesting
>>the use of a 74HC4538 and to forget the 555 with its limitations. I
>>have yet to research the 74HC4538. Any suggestions on how to achieve
>>such a timer?
>
>---
>You really haven't included enough information to get a good answer.
>
>For instance, you don't say anything about the multiple input pulses
>which are going to the single 555. That is, their amplitude and
>duration (whether they last longer than the output pulse and where
>they're coming from. Caps? TTL? A mechanical switch of some sort?
>
>For your 4538 query, it would be nice to know whether by "once only"
>you mean perpetual input pulse holdoff after the first output pulse or
>whether you mean no retriggering before the chip has timed out.
>
>--
>John Fields

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.

Thankyou,

Andrew.

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