I'd like to build a circuit to turn off
some of the equipment on my fish tank for
maybe 10 minutes during feeding time.
I don't know how much of a load you are talking about.
Is there a low-parts-count circuit I can
build to shut off AC power to a socket
for that amount of time when I push a
button?
I've been scribbling with 555's,
comparators, relays, power supplies and
stuff, but I'm wondering if I'm
overlooking something simpler.
All suggestions welcome.
Since you've been looking at all that, I won't feel sheepish about
posting a schematic I saw in sci.electronics.design a while back.
:
: +9V +9V +9V +9V
: | | | |
: | | | |
: | \ | | Pushbutton
: | / R1 | O S1
: | \ 33k | \
: | / | \
: | | | \
: D2 | | | O
: 1N4148 _|_ | |<e Q1 |
: /_\ +--------| 2N3906 |
: | | |\c PNP |
: | | | |
: | \ | |
: | / R2 +----------------+
: | \ 3.3k | |
: | / --- C1 |
: | | --- 100uF \
: | | | / R4
: | | | \ 470
: | | | /
: +---------------+ |
: | | +------, |
: | | | | D1 |
: | | | | 1N4148 ))|
: | ||' M1 \ --- ))| R5
: | ||< BSS145 / / \ ))| Relay
: '--'|, \ --- ))| 500 ohms, nom.
: | R3 / | | 200VAC contacts
: | 5.6M | | | normally closed
: | | | | e.g., SRC devices
: | | | | PRMA 1B05
: gnd gnd gnd gnd
http://www.srcdevices.com/pdf/dip14ser_src1.pdf
You'd need a 9VDC source, though. Most folks will recommend that the
9V be isolated from the AC line via a transformer, so there is that,
too. (You might be tempted to develop the 9V more directly from the
AC line to save the transformer, but keep in mind that pushbutton.)
Since the operating current is about 10mA during that 10 minutes you
want, a 9V battery (nominally, they are spec'd at 25mA) will run for
about 100 hours down to about 5V, which may not be enough to operate
the relay (the one I mention there is available at Digikey for $5.56
in ones, they have stock right now, and it specs a minimum of 3.75V to
operate) for all that time. But if you get even 20-50 hours out of
it, that may be enough for you. So a 9V battery would be an option, I
think.
You can use a lower voltage, or higher, but you will need to adjust C1
and R3 to set the timing. The duration is something like:
-ln( Vth of M1 / Vsupply ) * R3 * C1
I think the Vth is about 3V for the BSS145, but I haven't checked it.
With Vsupply=9, R3=5.6M, and C1=100uF, I get about 615 seconds delay,
which is close to 10 minutes.
Jon
