On Feb 1, 5:22 pm,
martinj...@yahoo.com wrote:
hi all,
could you pls help with a small fluid level sensor that keeps the pump
on all the time see below
link aprox 1/2way down page, i have built it and tested all parts but
the pump wont go off once level is reached. many thanks for any help
martinhttp://waterpoweredcar.com/pdf.files/D9.pdf
Hi, Martin. Sorry for your troubles. If you're talking about the
circuit on page 12, it's definitely less than ideal. First off, even
if everything's working OK, the voltage at the base of Q3 won't go
below 4V. That means your "turnoff" voltage at your relay coil could
be as much as 3.3V, which just might not turn it off, anyway. And if
you're using an automotive electrical system (nominal 13.8V or so),
"turnoff" voltage could be as high as 4.6V or more. Not good, because
a relay requires a lot less voltage to stay pulled in once it's
there. If you were to ask if a 12V relay, once energized, would stay
closed if 4.6V were applied, I'd have to say, "probably".
Second, I didn't look exhaustively at the article, so I'm not sure
what relay you're using. But I can tell you that the poor Tr3 is
probably overstressed unless it's a very high resistance relay coil.
It might have smoked, and since transistors frequently fail shorted,
that might also be responsible for your miseries.
Just for grins and giggles, I'd start by replacing Q3 with a beefier
TIP31 (can handle 3 amps load current), and try replacing R4 with a
180 ohm resistor instead of 1.8K. That will allow Q3 to turn off for
real. I'd also make sure I wasn't using a relay that required more
than 1/2A or so when fully energized.
Actually, poking around with this simple switching circuit is good
practice in basic electronics, but this should be a cautionary note
about the level of technical expertise in the whole article. I'm not
sure you want to go down this primrose path, sir. Note that the autor
specifically says in the first paragraph that he is not recommending
you actually build anything he's describing. I'd suggest that might
be good advice from him, even if nothing else is.
Good luck
Chris
