John R. Amend and Kenneth Emerson Montana State University Bozeman. Montana 59715
An Inexpensive Electronic Flood Detection Device for Chemistry Laboratories
Flooding caused by broken condenser hoses or malfunctioning apparatus in an unattended lahoratory at night is a continuing problem for any active research group. Control devices containing shutoff solenoids can be purchased, but thev are auite exnensive. Moreover. the ones familiar to these authors are acti;ated by a change in water pressure, or by a float of some kind in an overflow tube. Manv kinds of a m a ratus are not easily adapted for these kinds of detection. w e have designed a simple electronic control circuit (Fig. 1) which is connected to a printed circuit moisture detector. The detector has essentially infmite resistance as long as it is dry, but a minute amount of water on its surface reduces the resistance drastically and triggers complete shutdown of the apparatus. In our case the system is connected to a water still; the condenser water is run through a normallv closed solenoid valve made for use in an ordinary home dishwasher. I t is not a continuous duty solenoid, and if run a t full line voltage for extended periods becomes quite wann. The circuit is arranged so that only about 80% of the line voltage is applied to the solenoid, which reduces its operating temperature to a reasonable value. The device has been in use for more than 16 months, and often has operated continuously for two or three days. Several times malfunctions have occurred which resulted in water on the floor. In every case the detector system has functioned properly to shut off the apparatus and water flow. The circuit uses inexpensive parts-a 741 operational amplifier, a small 12-V ac transformer, and small dc relays. The bnly expensive part of the circuit is the power relay. ~e have two stills operating from the detector, and the total current when both-are in operation is about25 A. This requires a power relay capable of handling that load continuously. The circuit of the flood detector is quite straightforward. Transformer TI,diodes Dl and Dz, and capacitors C1and Cz provide a simple f 10 V dc power supply to operate the 741 operational amplifier. This amplifier is operated in the comparator mode, with its noninverting (+) input referenced to ground. The inverting (-) input is held near the negative wwer SUDD~Vvoltage bv the one meeohm resistor R,. causine the out;$ of the imGlifier to remain a t its limit. (When drv. .. the ~. r i n t e dcircuit moisture detector has ~ r a c t i cally infinite resistance, thus effectively isolating the operational amplifier input from the positive Dower SUDD~V.) .. . . Diode D3conduks whenihe amplifierbutput & positive, causing the small dc relay Rlyl to close. Closure of this relay arms the
circuit and permits operation of the 110-V power relay Rlys At this point the push-button switch Sw, is momentarilv closed, causing theil0-v power relay tocl&.e, turning on t h e heater and apparatus outlets and activating the solenoid valve through dropping resistor R ~The . lower set of contacts on the power relay Rlys latch the relay "on" until its power is removed; it then requires momentary closure of Swl to again close the relay and start the system. This prevents the circuit from restartine itself after sensine a flood and shuttine down. ~ C ~rectifier b4,and The second dcrelay circuit ( R ~ Y Zbridge resistor R -& .~ l a.va two-fold role. Rs. in series with the solenoid. drops the power applied to the sojbnoid from 110 V to about 85 V. This is sufficient to operate the solenoid. but limits its heat dissipation to a level permitting continuous operation. I t also protects the system from solenoid failure. The voltage drop across RQis rectified by bridge rectifier Dq, filtered by CL and used to power dc relay Rlyz. If the solenoid winding burns out, stopping the water flow, Rlyz will open, thus turning off the power relay and the heating elements and other equipment it controls.
Figwe 2. The pinted circul moishne detector is etchsd on a singlkhided b -n ~ QiaSS-BDOxv . . .DrintBd circuit board. me comer . . is immersion-tin oiated ~-~ &inp to poten it h m canosfon.The detma 43 placed clrcu t side oam m the flwr.Drops of solder placed on me donuts in lhe corners hold !he corcu,t board tar enough horn me floor to allow water to seep under F~
mate
Part
G DI,Ds 0, D4
R1 R2 Ra
Ct. C2. C3 Rly,. Rly2 RIY~
Typical U n l 741 Operational Amplifier 100 PIV diode rectifiers 100 PIV diode bridge rectifier
Resistor. 1 megohm. % W Resistor. 100 K, '1, W Resistor 470 ohm. 10 W Capacitor. 500 ufd, 25 V Sigma 65F1-12DC sensitive dc relay 110-V power relay. DPST. 25-A
Wice $0.60
0.30 0.90
0.08 0.08 0.60 0.50
3.50 12.00
C0"tBCtS ~
SWI.Sw2
~
Normally open push-bunon switch. Alco MSP-103 T, 12.6-V bansformer,center-tapped Solenoid valve. normally Hotpoint dishwasher valve, W D l 5 X 5093 closed ac receptacles, grounding
0.90 6.00
15.00 0.75
1. Circuit diagram of the flooddetector. The solenoid valve p i u a into mket &. Sockets S. and S3power heaters, stirring motors. etc. Figure
Volume 55, Number 4, April 1978 1 271
The printed circuit moisture detector is simply a 2 X 3-in. piece of siugle-sided glass-epoxy printed circuit hoard on which a grid pattern has been etched (see Fig. 2). The copper remaining on the circuit hoard is immersion-tin plated prior to use to protect it from corrosion. When water bridges the grid pattern on the printed circuit moisture detector, the resistance of this detector falls from practically iniiiity to less than 100K ohms, thus changing the voltage at the inverting input of the operational amplifier from about -10 V to about +8 V.This causes the amplifier output to switch from its positive limit to its negative limit, diode D3 stops conducting, and relay Rlyl opens removing the power
272 1 Jownal of ChemicalEducation
to the 110-Vac power relay. Latching contacts on the power relay prevent it from starting again until the push-button switch Sw, is pressed. Any number of moisture detectors may he connected in parafiel-we are operating two stills from the device and have three detectors placed strategically in low spots around the room. Acknowledgment
This research was funded in part by the US.Environmental Protection *gency, Duluths Minnesots, Research Grant No R803950.
