Sadhan Chandra daS
Fig. 7: Unique liquid level indicator
a
separate alternative circuit of a unique liquid level indicator to provide a display in terms of the percentage of full-scale level in OHT is shown in Fig. 7. It can either be used to replace the digital display circuit included in Fig. 1 (by simply connecting the 10% and 100% sensor probes of Fig. 7, additionally, to points marked ‘A’ and ‘B’respectively in Fig. 1, apart from connec-tion of +5V and +12V supplies and ground points) or it can be used in conjunction with an audio alarm unit shown in Fig. 8
and the power supply circuit in Fig. 2 inde-pendantly.
The latter configuration can be used when you do not desire to h a v e a u t o -matic control for switching
the pump mo-Fig. 8: Audio alarm unit
tor on and off but need only to be warned when water reaches 100% and also when its level drops to 10% so that you may manually switch the pump motor on or off, as the case may be.
This level indicator can show the dis-crete levels in percentage from 0 to 100%
with 10% resolution. An audio alarm circuit has been incorporated to generate audio alarm when the tank level reaches 100% and also when the level drops to 10%. The input to the audio alarm circuit
Fig. 9: Actual-size, single-sided PCB for the unique liquid level indicator
Fig. 10: Component layout for the above PCB
(Fig. 8) is tapped from line-1 and line-10 representing 10% and 100% levels respec-tively in Fig. 7.
If, in place of displaying the liquid lev-el in percentage, one wants to display only the digits 0 through 10, then 7-segment display DIS1 and LEDs (LED1 through LED4) for ‘%’ symbol can be removed. This circuit can be used for premises which have overhead tanks and the water supply is provided by municipalities or corpora-tions etc.
Display circuit. The basic elements of the cir-cuit, as shown in Fig.
7, comprise three quad 2-input XOR gates (IC1 through IC3) to get only the sum outputs, a hardwired decimal-to-BCD converter (using diodes D1 through D16), and a 74LS47 BCD-to-7-segment decoder/driv-er (IC4). When the tip of sensor-1 is in touch with the water, the line (L-1) connected to pin 3 of IC1 (CD 4030) goes to logic 1 state (+5V).
When the tips of sen-sors 1 and 2 both touch the water, pin 3 of IC1 goes to logic 0 (0V), while line L-2 connected to pin 4 of IC1 becomes high (+5V). Thus which one of the lines (L-1 through L-10) will be at logic 1 would depend on which last sensor (counted from bottom of the tank) is in touch with the water. If the tank is totally empty, all the lines, L-1 through L-10, would be at logic 0.
T h e s e l i n e s ( L - 1 through L-10) represent the decimal numbers 1 through 10. If line L-1 is at logic 1, BCD code 0001 is gener-ated due to conduction of diode D9 only. Similarly, if line L-3 is at logic 1, BCD code 0011 is generated due to conduction of diodes D6 and D16.
The voltages, corre-sponding to their BCD codes, are fed to the in-puts of IC 74LS47 (7-seg-ment decoder/driver) to drive 7-segment display DIS2. When line L-10 is high, display DIS3 is driven by transistor T1 (SL100) for decimal number 1.
Since all the time the unit place digit of the percentage display is 0, the cath-odes of corresponding segments of DIS1 have been permanently connected to 0V (ground) through current-limiting resistors of 330 ohms each. In this way the circuit displays 0 to 100 per cent of liquid level with 10 per cent resolution.
One may or may not use diode D1. In
this circuit the resistors of 56-kilo-ohm are connected across the inputs of XOR gates and ground, while resistors from R2 to R5 have been used for passive pull-down action.
Audio alarm unit. Fig. 8 shows the circuit for audio alarm. The base of transistor T2 (BC108) is connected to the terminals of lines L-10 and L-1 via diodes D21 and D22 respectively and a common resistor of 100-kilo-ohm.
When water touches the topmost sen-sor probe, transistor T2 conducts and tran-sistor T3 is cut off. As a result 3.1V devel-oped across zener ZD1 becomes available across pins 1 and 2 of melody generator IC7 (UM66). The amplified musical alarm is heard from the speaker.
When the tank is neither 100% full nor it is above 10% (but less than 20%), transistor T2 cuts off while transistor T3 is saturated to make the voltage across pins 1 and 2 of IC7 at almost 0V, and hence no sound is produced by the unit.
A separate parts list and actual-size PCB layout as well as component layout (Figs 9 and 10 respectively) are included after integrating the power supply of Fig. 2 with liquid level indicator circuit of Fig. 7 and audio alarm unit of Fig. 8. ❏ IC5 - UM66 melody generator DIS1-DIS3 - LTS 542 common anode
7-segment display T1, T3, T4 - SL100 npn transistor T2 - BC 108 npn transistor D1-D16,
D21, D22 - 1N4001 rectifier diode ZD1 - 3.1 volt zener diode LED1-LED4 - Red LED
Resistors (all ¼-watt, ±5% carbon, unless stated otherwise):
R1 - 3.3-kilo-ohm R2-R5 - 1.5-kilo-ohm R6-R24 - 330-ohm R25-R34 - 56-kilo-ohm R35-R44 - 33-kilo-ohm R45 - 100-kilo-ohm R46 - 2.7-kilo-ohm R47, R48 - 680-ohm Capacitor:
C1 - 100µF, 25V electrolytic Miscellaneous:
LS - 8-ohms speaker 7.5 cm dia - SS 304, 5 mm dia and 3mm
dia stainless steel rods of appropriate length for anode and cathodes respectively.
- Multi-core feed wire
Readers’ comments:
Q1. I have noticed, when the water level reaches the probe No 4, the C' segment LED of DIS 2 (LT542) does not glow.
The same is the case even when the water level reaches probe No. 5 and probe No 6. Kindly suggest the correc-tive actions.
M. Raja Bangalore Q2. I have constructed the circuit which is working perfectly. Instead of eleven roads, I want to use a stainless-steel hollow pipe that is sealed at one end and contains ten normally-open type
magnetic reed switches on the inside and one floating magnet on the outside as the sensor. Could you suggest me the changes required in the circuit if this type of sensor is used?
J.P. Thakkar con-nected to the sensors. If you find these alright, replace resistors R2 through R5 by 5.6k in the circuit before you switch on the power. If you wish to construct the circuit Intelligent Water Level Controller' published in February issue of EFY, the same replacement may be
done for resistors R19, R20, and R21.
The anode voltage should be +12V to +15V, which may or may not be regulated.
You may also follow the modifications shown in Fig. 1 (component numbers shown for Intelligent Water-Level Control-ler) to correctively display the water-level.
It is due to the fact that the output of the CMOS ICs (4030) are loaded by 1.5k resistors.
EFY: The circuit works satisfactorily with-out resistors R2 to R5.
a2. Sensors using a stainless steel hollow pipe cannot be used in this circuit. Imag-ine the float is in between 5th and 6th
reed micro switches, then no switch will be closed and the display will show only 0, although the water level is in between 5th and 6th sensors. Moreover, the number of wires from the reed switches remains the same.
Nowadays for non-conductive liq-uid, coaxial or parallel-plate type ca-pacitors are used with a suitable circuit that employs a frequency-to-voltage converter and displays the level of non-conductive liquid. But for non-conductive liquid like water, it is not advisable be-cause due to electrolysis the sensors get damaged soon. The value of resistors R2 through R5 is 1.5k. Use 4.7k or 5.6k re-sistors instead to avoid loading effect of ICs 4030. ❏ Fig. 1: Modification to level controller
Interface Your PrInter wIth 8085 MIcroProcessor
Shaila Ghanti
I
t is very convenient to interface a printer to print 8085 programs. Here a simple hardware interface circuit with its driver software is described that would enable student to take printout of the 8085 programs in hexadecimal codes along with their memory locations in the format: xxxx DD, where XXXX is the 4-bit hexadecimal address and DD is 2-bit hexa-decimal data.For most types of printers, the data to be printed is sent to the printer as
ASCII characters on eight parallel lines. The printer receives the characters to be printed and stores them in an inter-nal buffer. When the printer detects a car-riage return (odH), it prints out the first row of characters from the printer buffer.
When the printer detects a second car-riage return, it prints out the second row of characters. The process continues until the desired characters are printed.
Transfer of ASCII codes from the microprocessor to a printer needs to be done on a handshake basis because the microprocessor can send characters much faster than the printer can print them.
The printer must in some way let the
mi-croprocessor know that its buffer is full, and it cannot accept any more characters until it prints out some of the already stored characters. A common standard for interfacing with parallel printers is the