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Low-cost circuit has many applications High or lo-w level fluid detector This simple circuit can detect high or low fluid levels in a tank and trigger a relay output accordingly. It's very easy to build and uses just two low-cost ICs, a relay and a handful of other parts. Design By PETER GRAY There are many applications for a fluid level detector such as the circuit presented here. Some of these applications include monitoring fluid levels in fish tanks, sumps, radiators and washing machines, controlling irrigation systems and pumps, and monitoring soil conductivity in greenhouses. Despite its overall simplicity, this circuit is very reliable. It's based on the LM1830 Fluid Detector IC from National Semiconductor and this feeds an AC signal through a pair of external probes. The circuit can easily be adjusted to detect a wide range of fluids and there's a changeover switch so that you can monitor for either high or low fluid level. The circuit for the fluid level detector is built on a small PC board that should only take a few minutes to assemble. The external switch allows either high or low fluid levels to be monitored. 52 SILICON CHIP Want to detect when the fluid in a tank rises above a preset level? Simple - just set the changeover switch to the HIGH position. If the switch is set to LOW, the circuit will detect when the fluid drops below the preset level. Because the circuit has a relay output, you can easily adapt it to suit your particular application. For example, you might use · the relay to activate an alarm if the fluid level in a tank falls below a certain level. Alternatively, you could use the relay contacts to automatically switch on a pump to top the tank up again. One obvious application is controlling a bilge pump in a boat. In this case, the unit is set so that it switches on the bilge pump when the water reaches a preset level. A small amount of hysteresis is provided by the circuit to prevent "hunting" at the critical level. On the automotive front, this device is suitable for monitoring fluid levels in radiator overflow tanks and in washer bottles. In outback regions particularly, it could save you from the traumas of a blown engine due to coolant loss. An option here is to delete the relay and substitute a piezo buzzer or LED to provide the low fluid level warning. A number of units could also be built to control mist sprays in a greenhouse or plant nursery. By using the probes to monitor soil conductivity, you could automatically switch on the mist sprays when the conductivity dropped below a certain level. An on-board trimpot lets you set the moisture level at which the circuit triggers. is able to turn on the output transistor. In Fig.l, the output transistor drives a small LED but it could also be used to drive a loudspeaker or a low-current relay. OK, that's basically how the chip works but there are one or two more wrinkles. One problem that can arise with the circuit of Fig.1 is that the impedance of the fluid we wish to detect is of a different order of magnitude to the internal reference resistor, RREF· This problem can be solved by coupling the oscillator output to the probe via an external reference resistor, instead of via the internal reference. Fig.2 shows the details. By selecting the value of this external reference resistor, the circuit can be made to work with fluids of virtually any conductivity. A filter capacitor can also be added to pin 9 of the LM1830 to filter the detector output. If this is done, the output transistor will switch on and remain on when the fluid level drops, instead of being pulsed on and off by the oscillator. Fig.3 shows the final circuit of the Fluid Level Sensor. In addition to the LM1830 (ICl), it also uses an LM393 comparator (IC2) and a BC547 transistor (Ql) to drive the relay. The circuitry around ICl is virtually identical to that shown in Fig.2. The .001µ,F capacitor bet- TABLE 1 Conductive Fluids lo1-COnductl11 Aulds City water Sea water All salt solutions All acids All alkaline solutions Household ammonia Water & glycol mixture Wet soil Coffee Distilled water Hydrocarbon fuels and solvents All mineral and vegetable oils Brake fluid Ethyl alcohol Methylated spirits Ethylene glycol Paraffin Dry soil DC blocking capacitor. An AC signal is applied to the probe to prevent plating and corrosion problems, as would occur with a DC source. Note that in Fig.1 we are assuming a metallic container (eg, a metal water tank). This container is simply shown connected to the circuit earth and forms the other probe input. What the circuit does is compare the resistance between the probe and the container with the internal reference RREF· If fluid is present, the probe resistance will be less than RREF and insufficient signal will be fed to the detector to turn on the output transistor. On the other hand, if the probe resistance increases above RREF (ie, if the fluid level drops below the probe level), a strong AC signal is coupled via the detector which then Table 1 lists some of the common conductive fluids which can be detected by the circuit. The nonconductive fluids listed in the table cannot be detected. How it works To understand how the circuit works, we first have to take a look at what goes on inside the LM1830 Fluid Detector. Fig.1 shows the internal workings of this chip. It contains an oscillator (to generate an AC signal for the probes), an internal reference resistor (RREF = 13k0), a detector, a driver stage and an open-collector output transistor. An external capacitor between pins 1 & 7 sets the oscillator frequency. As shown, the oscillator output is made directly available at pin 5 and is also applied to the probe via RREF and an external .05µF •cc •cc '10 O.IOf;,f TIMING CAP ~m " 8.IHl1 1,1 F ~ 12 •cc " LEO 12 •cc TfMIIB CAI'. OSCILLATOR DETECTOR 13 10 FILTER 9 GROUND 11 ':" Fig.1: basic circuit tor detecting low fluid levels. The oscillator generates an AC signal which is applied to the active probe. Fig.2: in this circuit, the oscillator output is coupled to the probe via an external reference resistor (RREF) instead of via the internal reference. By selecting this resistor, the circuit can be made to work with fluids of virtually any conductivity. SEPTEMBER 1989 53 1M' .001 I 1M RL 1 V • c:: : 14 12 PROBE INPUTS 100k TO CONTROLLED CIRCUIT B 11 .,. .i +12V ELJc 1M VIEWED FROM BELOW .,.. FLUID LEVEL SENSOR Fig.3: the final circuit of the fluid level sensor. The output signal from the LM1830 (IC1) is fed to IC2 where it is compared with a ½ Vee reference voltage. IC2 in turn drives Qt and the relay. ween pins 1 & 7 sets the oscillator frequency to about 7kHz, while the 22µF capacitor on pin 9 filters the detector output. Pin 5 is the oscillator output and this is coupled to one of the sensor probes via VRl and a .047µF capacitor. VRl functions as the external reference resistor (ie, the internal reference is not used). A trimpot has been used here so that the circuit can be adjusted to detect virtually any conductive fluid. When fluid is detected by the probes, the oscillator output is shunted to ground and ICla's output (pin 12) is high. Conversely, if the fluid level drops below the probes, the oscillator signal on pin 10 increases and this switches pin 12 low. The output signal on pin 12 is now coupled by DPDT switch S2 to the comparator stage (IC2). S2 simply reverses the voltages on, the comparator inputs to provide the high or low level warning functions. In the low warning mode, pin 12 of ICl is coupled to the inverting input of IC2 via a 100k0 resistor and Sla. At the same time, Slb switches IC2 's non-inverting input (pin 3) to ½ Vee (ie, half supply), as set by a voltage divider consisting of two lMO resistors. PARTS LIST 1 PC board, 82 x 44mm (available from Novocastrian Electronics) 1 DPDT miniature toggle switch 1 1 2V SPOT PC-mounting relay Semiconductors 1 LM1830N fluid sensor (IC1) 1 GL393 or LM393 voltage comparator (IC2) 1 BC54 7 NPN transistor (01) 1 1 N4002 diode (01) Capacitors 1 22µF 16VW PC electrolytic 1 .04 7 µF polyester 1 .001 µF polyester Resistors 3 1MO 1 1 OOkO 1 1 OkO 1 4 .7k0 1 1 OOkO trimpot Where to buy the parts A complete kit of parts for this project is available tram Novocastrian Electronic Supplies Pty Ltd, 24 Broadmeadow Rd (PO Box 87), Broadmeadow, NSW 2292. Telephone (049) 62 1358 or toll free on (008) 02 5942. The kit includes the PC board plus all on-board components but does not include the probes or power supply. The price is $19 .95 plus $3 .00 for postage and packing. Note: copyright of the PCB artwork associated with this project is owned by Novocastrian Electronics Pty Ltd. 54 SILICON CHIP This means that when pin 12 of ICl goes low (ie, the fluid level drops below the probes), pin 2 of IC2 is also pulled low via the 100k0 resistor. Thus, the comparator output is pulled high by the 4.7k0 pullup resistor on pin 1 and Ql turns on to activate the relay. Conversely, in the high warning mode, IC2's inverting input sits at ½ Vee and the non-inverting input now monitors pin 12 of ICl. Normally, the fluid level will be low and so pin 12 of ICl will hold pin 3 of IC2 below the voltage on the inverting input at pin 2. Thus, pin 1 of IC2 will be low and Ql and the relay will be off. Now, when the fluid level rises above the probes, the output transistor inside ICl turns off. Pin 3 of IC2 is now pulled high by the remaining lMO resistor which means that the voltage on the noninverting input is now greater than the voltage on the inverting input. Thus, IC2's output is again pulled high by the 4.7k0 pullup resistor and Ql and the relay turn on as before. Power for the circuit can be derived from any suitable + 12V source; eg, a plugpack supply. Although we have specified a nominal + 12V rail, this can be varied over the range 5-15V with no changes to component values except to the relay coil rating. Construction Fig.4 shows how all the parts are mounted on the PC board. There's nothing tricky here; you can mount the parts in any order you wish Vee OSCILLATOR OUTPUT OSCILLATOR OUTPUT (RREf) OPTIONAL DETECTOR FILTER INPUT CAPACITOR OUTPUT 12 Cl Fig.4: parts layout for the PC board. We used tinned copper wire for our probes but serious applications will require stainless steel probes to minimise corrosion problems. GNO Cl Fig.5: inside the LM1830 fluid detector. The two transistors at left form the oscillator. When the fluid level drops, the oscillator signal is fed to the base of the detector transistor which then pulses the driver and output transistors. RESISTORS No. □ □ □ □ 3 Value 1MO 1 1 1 100k0 10k0 4.7k0 although we suggest that you leave the relay until last. Be sure to install the two ICs and the diode the right way around. Pin 1 of each IC is adjacent to a small dot or notch at one end of the moulded plastic body of the device. The probes for the prototype were nothing fancier than a couple of short lengths of tinned copper wire. These were connected to the PC board using light-duty figure-8 flex. For most applications though, stainless steel probes will be required to minimise corrosion. If you are monitoring the fluid level in a metallic container, the earthed probe input can simply be connected directly to the container as shown in Fig.2. The active probe is then set to the trigger level. Testing To test the unit, set VRl to about mid-range, connect the power supply and introduce the probes to a glass of water. If S1 is set to HIGH, the relay should turn on the moment the probes touch the water and release as soon as they are removed. 4-Band Code brown brown brown yellow black black black violet 5-Band Code green gold yellow gold orange gold red gold brown brown brown yellow Now switch S1 to low - the relay should initially be on with the probes out of the water and then switch off when they contact the water. If the unit fails to work correctly, try adjusting VRl. If VRl is set too low, pin 12 will remain low regardless of the probe resistance, and the relay will remain either on or off (depending on the setting of S1}. Troubleshooting What if it doesn't work? There's not much to go wrong so troubleshooting is easy. First, go over your work carefully and check the parts placement and all the values. Check that the ICs are the right way around, that the resistor values are all correct and that the switch wiring is correct. If this doesn't reveal anything, switch your multimeter to the 20V range and use it to monitor the voltage on pin 12 of ICl. You should get a reading of close to OV with the probes out of the water and a reading of about + 11V with the probes immersed in water. Check black black black violet black black black black yellow brown orange brown red brown brown brown POLYESTER CAPACITORS No. □ 1 □ 1 Value IEC EIA .047µF .001µF 47n 1n 473K 102K the circuitry around ICl if you don't get the correct results here. If ICl checks out, remove the probes from the water and check the voltages on pin 2 of IC2. You should get a reading of a bout + 1V for one position of S1 and + 6V (halfsupply) for the other position of S1. · The same voltages should appear on pin 3 but with the switch positions reversed. If you don't get the correct readings here, the wiring to S 1 is probably incorrect. IC2 can be checked by monitoring its pin 1 output. This should give a reading close to OV for one position of S1 and about + 8.5V for the other. Finally, you can check the operation of Ql by measuring its baseemitter voltage. This should be OV when pin 1 of IC2 is low and about 0.65V when pin 1 switches high. ~ SEPTEMBER1989 55