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| Electronic Tray Water Controller |
This device has a circuit that monitors the water level in water trays using two sensors one above other. When the water is below the lower sensor, a pump is turned on filling the tray. When the water reaches the second sensor the circuit turns the pump off. Don’t worry, it is very easy to assemble, and if you have any troubles simply send your question and I will glad to answer them! The pump and the solenoid valves may be any ordinary draining pumps and valves used in washing machines. They are sufficiently strong for this application and all components of the circuit are easily to find. You can buy these in an electronic parts store (for example Radio Shack) or by mail (ex. Jameco). For better security I have included an adjustable timer that automatically turns off the pump it if the upper sensor fails. I think you would not enjoy finding your greenhouse like a swamp if the pump doesn’t go off. It features:
The are two versions of the circuit: the one described here and one with inputs less likely to burn out. Circuit fundamentals: Perhaps you were thinking that the circuit is like a complicated puzzle without solution. Well, let me assure you that after reading the following descriptions, and with some imagination, you’ll easily understand how it works. As the circuit is based on electronic devices, some theory about how it works is needed. Let’s go! The first component to analyze is the AND logic gate. The designation “gate” comes from the fact that it can “leave out” some specific state depending on the states on its inputs. Gates compose a family of components. The more important are: AND Logical gate, OR Logical gate and NOT logical gate (or simply inverter). These names arise from the fact that they can perform the Boolean arithmetic functions with the same names. In the Boolean arithmetic we assume the existence of only two numbers (or logical states): 0 or 1. The AND function is just a multiplication of two or more states resulting on the logical state 1 if all the numbers are 1 like in ordinary arithmetic. Example: 0
AND 0 means 0 * 0 resulting 0 The last table is so called truth table of the AND gate with two inputs. If we let the letters A and B to inputs we have: Logical Symbol:
Truth table:
In electronics we can have two types of circuits: analogic and digital. On the analogical circuits the voltage can assume several values but in digital two possible levels of voltage are possible:
The AND gate may be represented as two series associated keys:
As you can see, only if the two keys are closed we have a logical 1 in the output. The OR function is the sum of the two or more states resulting in logical 1 if one or more inputs are 1. Example: 0
OR 0 means 0 + 0 resulting 0 Logical Symbol:
Truth table:
 Closing one key will result on a logical 1 at output. The last is the NOT function and it simply invert the logical state applied in its input. Example: NOT
0 results 1 Logical symbol:
Truth Table:
The NOT function may be represented by a small circle at inputs or output of another logical function. In the scheme you can see AND gates with small circles at the output meaning that the logical level at output was inverted. The truth table of this gate is the same as that for ordinary AND but whit the output inverted. In fact just an inverter was placed at its output so we have an INVERTER AND or NAND gate. Looking the scheme you can see that each NAND gates has one symbol inside. This is the hysteresis symbol meaning that they are SCHMITT TRIGGER NAND types. Hysteresis is the propriety that some circuits have to do not change your output state if the voltage applied doesn’t reach determined level. In case of the gates employed, this level varies with the power supply voltage and temperature. The following table shows some typical hysteresis values to the Motorola CD 4093 four logical NAND gates. The typical values for 25°C are:
Suppose that power supply is 15 Volts. Looking the table, only when voltage applied to inputs rises to 8.8 Volts does the output go low (zero state) and only when the voltage falls to 5.8 Volts does the output go high. Remember: this is an INVERTER AND! This behavior assures high noise immunity to circuit, so false triggering by noise will be more difficult. The next circuit is more interesting yet. Its name is Flip-flop; it can get a logical state applied on input and hold it at output memorizing this state. In fact the cache memory of your computer is formed by thousands of these circuits encapsulated on one single chip (one per bit). Surprised? The flip-flops used on the circuit are D type flip-flop. The “D” is for data meaning that data applied at input goes to output when a pulse is applied to the clock pin. A pulse is when voltage changes fast from one level to another and returning to initial level again after an elapsed time. Example:
The graphic shows a pulse rising from 0 Volts to 12 Volts, stays on this level for “t” seconds and falls again to 0 Volts. The rising edge of the pulse is called positive going edge and the fall edge called negative going edge. Flip-flops are classified according to the way that they memorize the states after the application of clock pulse. It can be Positive Logic Edge Clocked (on the rising edge of the pulse), Negative Edge Clocked (on the falling edge of the pulse) or Pulse Clocked (after pulse application). The D type flip-flop used on the circuit is positive logic edge clocked. Its logical symbol is:
The next component to analyze is the DIODE. The symbol, which you can see on the scheme, is an arrow pointing to a bar. These symbols indicate the behavior of the device: it only accepts current flow on the direction of the arrow (on the conventional sense). The arrow terminal is called ANODE represented by the letter A, and the bar is the CATHODE, represented by the letter K. When a voltage source is applied in the cathode (more than the voltage drop of the device) the diode presents a very low resistance, and electrical current can flow through the diode going to the load. But if we put the source in anode, it looks like a very high resistance like an open circuit and the current can’t flow to load. In the first case, the diode is called DIRECTLY POLARIZED, and in the second case REVERSE POLARIZED.
The source applied on the anode must be higher than 0,3 Volts (germanium type) or 0,7 Volts (silicon type) to overcome the potential barrier of the diode. On the circuit some diodes are used to make OR logical gates with several inputs. Description of circuit: The modularity concept was thought out when the circuit was designed enabling the user assemble independent units as needed. With the ICs used (the 4013 and the 4093) two independent circuits can be assembled on the same board. The scheme presents two equal circuits one above the other using the four NAND Schmitt trigger gates of the 4093 (U1A to U1D) and the two D positive edge triggered flip-flops of the 4013 IC (U2A and U2B) so only the upper circuit will be described here. The circuit has two sensors named SENSOR 1-A and SENSOR 2-A.SENSOR 1-A is the low level sensor and SENSOR 2-B is the high level sensor. The circuit verifies if water level is below the SENSOR - 2A and if so turns on the pump and the solenoid valve 1. When the water reaches SENSOR 2-A the pump and the valve 1 are turned off. The ICs used are one 4093 and one 4013. The 4013 have two type D positive edge triggered flip-flops and the 4093 four NAND Schmitt trigger gates. If the tray was empty or water level was below both sensors when the circuit is turned on, U1C and U1D NAND gate outputs will be at High level. This high state at U1C output is applied in a resistor-capacitor-diode network (R5, C1 and D1) which generates a negative going edge pulse to U2A flip-flop Set pin. As described in the above text, with this applied pulse the U2B Q output goes to High level. This high level is applied by means of D7, R12 network to transistor Q2 base closing RL2 contacts turning the pump ON, and by D6, R9 network to transistor Q1 base closing RL1 contacts turning the valve 1 on. Also U2B Q output is connected to a resistor-capacitor-diode (TP2, C4 and D4) network. This network is the adjustable timer. When a voltage is applied to series resistor-capacitor network the capacitor charge exponentially; it means that the voltage at its terminals increase exponentially with the time. The resistor value (in Ohms), times the capacitance value (in Farads), equals the RC time constant (in seconds). The time required to the voltage at the capacitor terminals reach the applied voltage is 5 RC time constants. For example, suppose that the adjustable resistor TP2 was set at the middle value: 160kΩ. The capacitor C4 value is 220μF meaning that the RC time constant will be: 160 000 Ω * 0.00022 F = 35.2 seconds, so the needed time for the capacitor voltage reach the applied voltage will be 5 * 35.2 = 176 seconds (2 min. and 56 sec.). Looking the scheme, the capacitor voltage is applied to U2B RESET pin by D3. When the voltage at its terminal reaches 12 Volts (the voltage from U2B Q output) the U2B Q output will go to low level turning the pump and valve 1 off. The D4 diode connected in parallel with TP2 serve to quickly discharge C4. By the time water reaches the second sensor, U1D output goes Low and the network composed by R6, C2 and D2 generates a clock pulse to U2B flip-flop. Because the D input is connected to ground (Low level) this level is transferred to Q output turning the off the pump, valve 1 and discharging C4. If sensor 2 is damaged this clock pulse cannot be generated. In this case when the voltage on the timer capacitor reaches the power source voltage the U2B the flip flop will be restarted: the Q output will go low turning off the pump, valve 1 and discharging C4. As described later, the circuit features manual reset and manual control of the pump and valves. It was made by using two pushbuttons type keys:
As can be seen on the scheme the diodes D15, D5 from U2A, U2B and diodes D10, D7 from S5, S4 are connected to a point named EXPANSION POINT. If the user wants assemble more units controlling the same pump all the circuits must be connected on this point as described below:
The scheme show the two circuits connected as described. Assembling: This circuit can be assembled on a universal prototyping circuit board, with a wire-wrap technique, or in a PCB designed for it. For beginners, the first manner is the most easily done because the components can be arranged as desired. Suggestion: Simply distribute the components on universal board in similar manner that showed on scheme.
Begin by putting the ICs sockets in the board and soldering them. After that, put the diodes and transistors. When you are soldering diodes and transistors do this quickly or the excessive heat can damage them! At last, add the resistors and capacitors. Cut the excessive leads after soldering components. They help to dissipate excessive heat. The next step is making the connections with wire. The best wire is the wire designed for wire wrapping assemblies. Removing the wire isolation cap of this wire type is very easy: just rest the solder iron to melt it. Wrap few turns in the leads before soldering assure a best contact and, prevents that the wires from coming loose when soldering. Verify if the appearances of all solders are brilliant. If some are opaque or rough, it indicates a cold soldering, and may cause bad contact. Remove this whit a dessoldering pump and solder again. Make the wires to the sensors the more short possible. This prevents false triggering by electrical noise. Brushes of electrical motors, wall interrupters and relays contacts generate electrical noise. If your greenhouse is near of industries with electrical engines, you must enclose the circuit on a metal box.
Assembling the sensors The sensors are simply to make: Two screws (per sensor) disposed horizontally side by side and separated 5 mm or less compose them. The screws may be rust-less type, but it will not be possible to solder the wire on them, so wire terminals should be used. Solder the wire on the terminal and fix it on the screw sensor between two nuts. The sensors can be fixed directly on the tray or in a small rectangular piece of an insulating material like Plexiglas. It allows adjusting the water levels at your choice by only moving the sensors height. If you decide put it directly on the tray, use silicon rubber glue covering the nuts on the outer side of sensors to avoid water leakage. If your trays are metal or made of other conductive material, you must insulate this material between the screws and the tray. You can assemble the power source in the same PCB or in other PCB. It is not critical. Calibration and use: When you turn on the circuit with the water level below both sensors, the pump automatically goes on, but if no you can press the manual start button or fill manually by pressing the manual control button. If the required time to reach the second sensor was greater than the adjusted on the timer, the pump and valve will go off before water reaches the second sensor. In this case adjust the timer for a slightly long time. Some trials will be need made before to adjust the correct time. It is desirable adjust the time a bit long more that the time needed to fill the tray allowing the water turn off first the pump and valve. Bill of materials: Resistors R1, R3, R10, R11, R14, R16, R23 – 1k Ω R9, R12, R22 – 2,2k Ω R8, R13, R21 – 10k Ω R2, R4, R14, R16 – 27k Ω R5, R6, R7, R18 – 100k Ω R7, R19 – 2,2M Ω TP1, TP2 – 470k Ω Trim-pot Capacitors C1, C2, C5, C6 – 100nF C3, C8 - 100μF/16V electrolytic C4, C7 - 220μF/16V electrolytic Diodes D1 a D17 – 1N4148 LED 1, LED2, LED3 – Red Light Emitter Diodes Transistors Q1, Q2, Q3 – 2N2222 Integrated Circuits U1 – 4093 U2 - 4013 Relays and keys RL1, RL2, RL3 – Relay single pole, one inverter contact type, 12 A/125 V (FRS10C-S12 or other) S1 –1 pole 2 positions switch S2, S3, S4, S5, S6, S7 – Pushbutton normally open Power supply: Integrated circuit U1 – LM7812 Voltage regulator Semiconductors D1, D2 – 1N4004 Capacitors C1 – 100nF C2 –2200uF/25V electrolytic F1 – 300mA Fuse T1 – 8+8V/500 mA (with center tap) Transformer
Miscellaneous: An metal or plastic box to accommodate the circuit, solder iron, solder pump, rust-less screws and nuts for sensors, silicon rubber glue, IC sockets, led supports, fuse support, wire (wire-wrap type), wire terminals, prototype board (Radio Shack 276-150 or equivalent). THE CIRCUIT HAS PARTS CONNECTED TO AC POWER LINE. BE CAREFULL WHEN HANDLING THE AC POWER LINE. IT CAN BE HAZARDOUS TO YOUR HEALTH!! |
This page is maintained by John Brittnacher, |