Re: Looking for a Low Cost On Delay Timer

[unruh <unruh@xxxxxxxxxx>]

On 2013-08-22, Guv Bob <guvbob2003@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> "unruh" <unruh@xxxxxxxxxx> wrote in message news:q5hRt.261777$Dd5.106684@xxxxxxxxxxx
>> On 2013-08-21, Guv Bob <guvbob2003@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>> > "G. Morgan" <graham@xxxxxxxxxxxxxxxxx> wrote in message news:1jba199r3ropif879j9egfl40atou4jodi@xxxxxxxxxxxxxxxxxxxx
>> >> Bob La Londe Wrote:
>> >>
>> >> >Check the specs of an Altronix 6060
>> >>
>> >> Or the 6062.  I think they are about $30.
>> >>
>> >> http://www.altronix.com/products/product.php?name=6062
>> >>
>> >>
>> >>
>> >> --
>> >>
>> >> They who can give up essential liberty to obtain a little temporary safety,
>> >>  deserve neither liberty nor safety. - Ben Franklin
>> >
>> > Thanks guys.  Grainger has it for $30.   Before I head up there... this is not something that has to be done right away...   would putting together a circuit with this work?
>> >
>> > NTE955M Timer, (555 Type), 200 mA Iout, CMOS 8 DIP 1.20
>> > http://www.alliedelec.com/search/productdetail.aspx?sku=70215850
>> >
>>
>> A transistor /amplifier Ic, plus a capacitor, and a few resistors should
>> do it. What time scales you you need? How fast does it need to switch on
>> and switch off? What voltage tolerance do you need? -- ie if it is 11V
>> out is that OK? What supply voltage do you have? Can the input drop to
>> zero before 5 sec and what behaviour do you expect in that case?
>> How accurate must the 5 sec be? etc
>
> Hey unruh!
>
>> A transistor /amplifier Ic, plus a capacitor, and a few resistors should
>> do it. What time scales you you need?
>
> Initially input voltage is zero.  When input goes to 12VDC, output should energize immediately to 12VDC, stay eneregized for 4-6 seconds, then drop to zero again.  When input goes to zero, output stays off until starts over.

Immediately does not mean anything. on a time of 5 sec?1 sec 1
millisecond? 10^-100 sec? If you are looking at relays, they are of
order 1/10 sec. Is that good enough?


>> How fast does it need to switch on and switch off?
> Within 1/2 second is fine.

OK.

>
>> What voltage tolerance do you need? -- ie if it is 11V
>> out is that OK? What supply voltage do you have?
> Yes, 9-13 out is fine.  Supply is typically 11-12VDC.
>
>> Can the input drop to zero before 5 sec and what behaviour do you expect in that case?
> Yes it can.  Output should go on when input first goes on, then output should go off any time input is disconnected.
>
>> How accurate must the 5 sec be? etc
> 4-6 seconds is fine.  Not critical.
>

So a simple rc circuit opamp  should be fine.

The input goes through a resistor divider to the + input of an opamp,
the input also goes through a resistor (with a parallel reverse diode)) to a
capacitor to ground, and the junction of reistor and capacitor goes
to the - input of the opamp.
When input goes up, the output of the opamp will peg out positively,
until the capacitor charges up sufficiently at which point the ouput
will go to zero the resistor/capacitor is chosen so that this is 5 sec. If
the output goes to 0, the capacitor is discharged by the divider  and the
output stays at 0. If the resistor bridge low enough (say 1K ohm) and
the risistor/capacitor high enough (eg R=100K C= 2uF) -- I have not
checked these values -- they are at best orders of magnitude-- you would
not need the diode.

You could run a very high resistor from the cap to the positive
rail as well, to make sure that when the input is low, the capacitor
remains slightly charged, and the opamp is driven to low output.

Total cost ? < $1
Output current-- only limited by the opamp output current which could be
thousands of amps with suitable(very expensive) ouput transistors.
(for cheap opamps hundreds of mA.)


>