Simple timer circuit needed

[BobG]

I need a sinple circuit that will be on for 2 secs and off for 4 secs
and then repeat indefinetely.I built one with 555 and pots, but it is
too sensistive to humid condition I am using it for: the timing is not
constant.
May an opamp?. Many thanks in advance, Bob.

 

[Terry Pinnell]

"BobG" <bobgagno@videotron.ca> wrote:

I need a sinple circuit that will be on for 2 secs and off for 4 secs
and then repeat indefinetely.I built one with 555 and pots, but it is
too sensistive to humid condition I am using it for: the timing is not
constant.
May an opamp?. Many thanks in advance, Bob.

What level of precision do you want? With decent components

(especially the timing capacitor), you can expect the 555 itself (over
a wide temperature range) to give 1% accuracy for the standard type,
2% for the CMOS type. My 555 data sheet doesn't mention humidity, but
then I wouldn't expect it to, as that's going to affect *most*
components, not just the IC. I'd guess that the extra stray resistance
between leads, pins and circuit tracks might be the most significant
effect. An interesting experiment would be to run a simple astable
(any sort) first i a dry kitchen and then suspended over your morning
hot bath.

Can't the circuit be positioned away from the worst of the humidity?
Or compensated with a draught of fresh air?

BTW, don't forget that the very *first* period of a 555 astable is
significantly longer (maybe 40%) than all subsequent ones.

--
Terry Pinnell
Hobbyist, West Sussex, UK

 

[petrus bitbyter]

"BobG" <bobgagno@videotron.ca> schreef in bericht
news:Wn_8d.180437$ji3.1888353@weber.videotron.net...

I need a sinple circuit that will be on for 2 secs and off for 4 secs
and then repeat indefinetely.I built one with 555 and pots, but it is
too sensistive to humid condition I am using it for: the timing is not
constant.
May an opamp?. Many thanks in advance, Bob.

Humidity influences neither the 555 chip nor most other components directly.
It is the leads, the PCB traces or the coaltrace of the pots. You can fight
humidity by laquer like nail polisher and using hermeticaly sealed pots. The
right way these days is using some logic. Microchip has eight pins PICs and
even six pin chips that can do the job very easely without the need of
external timing components. AVR has some tiny eight pins chips as well.

petrus bitbyter


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[John Popelish]

BobG wrote:

I need a sinple circuit that will be on for 2 secs and off for 4 secs
and then repeat indefinetely.I built one with 555 and pots, but it is
too sensistive to humid condition I am using it for: the timing is not
constant.
May an opamp?. Many thanks in advance, Bob.

What timing values did you use with the 555?
What capacitor type are you using?
Are you using the CMOS or junction transistor version of the 555?

Humidity affects the timing components, almost independently of what
chip is watching their action, so you have to solve the humidity
problem some other way.

Using an opamp as an active guard driver might help. This involves
copying the timing voltage with a follower amplifier and surrounding
the timing node with this copy voltage, so that there is no voltage
across the board from the timing node. It doesn't help with leakage
across the bodies of the components, though.
Potting in epoxy may be the answer.

--
John Popelish

 

[Lord Garth]

"BobG" <bobgagno@videotron.ca> wrote in message
news:Wn_8d.180437$ji3.1888353@weber.videotron.net...

I need a sinple circuit that will be on for 2 secs and off for 4 secs
and then repeat indefinetely.I built one with 555 and pots, but it is
too sensistive to humid condition I am using it for: the timing is not
constant.
May an opamp?. Many thanks in advance, Bob.

Use a crystal oscillator and divide that by an approiate amount to derive a
1 Hz pulse.
Use that pulse to clock a divide by 6 counter. The MSB from the counter
will give
you the cadence you desire.

count 4 2 1
0 0 0 0 off
1 0 0 1 off
2 0 1 0 off
3 0 1 1 off
4 1 0 0 on
5 1 0 1 on