🚀 go-pugleaf

schuch@phx.mcd.mot.com (John Schuch) writes:

>In article <00965764.D96DDB80@Msu.oscs.montana.edu> uchrgrh@Msu.oscs.montana.edu writes:
>>I would like to build a solid state device that would turn off a water
>>pump in my aquarium if the water level drops  below a certain level.
>>I am willing to look at prefabricated stuff but would prefer to build it
>>my self.

      OK, check out the National Semiconductor LM1830 Fluid Detector,
which is designed specifically for turning things on and off based on
water level.  This unit uses a pair of conductive probes in the liquid,
and drives them with an AC signal at, typically, 6KHz, to avoid
electroplating crud onto the sensing probe.   Specifically recommended
for aquaria in the data sheet.  Will drive a 20mA load.  

      Also useful is the LM1801 Battery Operated Power Comparator, which
is also suitable for water level detection.  This part has provision for
hysteresis feedback, so that the pump doesn't turn on and off rapidly when
the level is near the threshold.  Will drive a 500mA load.  Designed for
battery operatio from a 9V alkaline battery for one year of standby 
(no output) operation.  Often used for leak alarms, etc.  Drives the
probes with DC, though, so it doesn't protect the probes from corrosion.

      Typically, you would drive an optically-isolated solid-state power
relay from one of these.  Something like the Digi-Key GH7018-ND.

					John Nagle