SADHAN CHANDRA DAS
RUPANJANA
(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 mu-nicipalities or corporations etc.
Display circuit. The basic elements of the circuit, as shown in Fig. 7, com-prise three quad 2-input XOR gates (IC1 through IC3) to get only the sum out-puts, a hardwired decimal-to-BCD con-verter (using diodes D1 through D16), and a 74LS47 BCD-to-7-segment
de-coder/driver (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).
W h e n the tips of sensors 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.
These lines (L-1 through L-10) rep-resent the decimal numbers 1 through
10. If line L-1 is at logic 1, BCD code 0001 is generated 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, corresponding to their BCD codes, are fed to the inputs of IC 74LS47 (7-segment decoder/driver) to drive 7-segment display DIS2. When line L-10 is high, display DIS3 is driven by transistor T1 (SL100) for decimal num-ber 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 resolu-tion.
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 Fig. 7: Unique liquid level indicator
Fig. 8: Audio alarm unit
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 respec-tively and a common re-sistor of 100-kilo-ohm.
When water touches the topmost sensor probe, transistor T2 con-ducts and transistor T3 is cut off. As a result 3.1V developed across zener ZD1 becomes available across pins 1 and 2 of melody genera-tor IC7 (UM66). The amplified musical alarm is heard from the speaker.
When the tank is nei-ther 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 lay-out as well as component layout (Figs 9 and 10 re-spectively) are included after integrating the power supply of Fig. 2 with liquid level indica-tor circuit of Fig. 7 and audio alarm unit of
Fig. 8. ❏
Fig. 9: Actual-size, single-sided PCB for the unique liquid level indicator
Fig. 10: Component layout for the above PCB
March
2001
Circuit Ideas
2001
S.C. DWIVEDI
sound from UM3561. Resistor R4 in se-ries with a 3V zener is used to provide the 3V supply to UM3561 when the re-SUKANT KUMAR BEHARA
AUTOMATIC HEAT DETECTOR
lay is in energised state. LED1, con-nected in series with 68-ohm resistor R1 across resistor R4, glows when the siren is on.To test the working of the cir-cuit, bring a burning matchstick close to transistor T1 (BC109), which causes the resistance of its emitter-collector junction to go low due to a rise in temperature and it starts conducting. Simultaneously, transistor T2 also conducts be-cause its base is connected to the collector of tran-sistor T1. As a result, relay RL1 energises and switches on the siren circuit to produce loud sound of a fire-brigade siren.
Lab note.
We have added a table to enable readers to obtain all pos-sible sound effects by returning pins 1 and 2 as suggested in the table.
Pin Designation Sound Effect
SEL1 SEL2
No Connection No Connection Police Siren +3V No Connection Fire Engine Siren Ground No Connection Ambulance Siren
Do not care +3V Machine Gun
T
his circuit uses a complementary pair comprising npn metallic transistor T1 (BC109) and pnp germanium transistor T2 (AC188) to de-tect heat (due to outbreak of fire, etc) in the vicinity and energise a siren. The collector of transistor T1 is connected to the base of transistor T2, while the collector of transistor T2 is connected to relay RL1.The second part of the circuit com-prises popular IC UM3561 (a siren and machine-gun sound generator IC), which can produce the sound of a fire-brigade siren. Pin numbers 5 and 6 of the IC are connected to the +3V supply when the relay is in energised state, whereas pin 2 is grounded. A resistor (R2) con-nected across pins 7 and 8 is used to fix the frequency of the inbuilt oscillator.
The output is available from pin 3.
Two transistors BC147 (T3) and BEL187 (T4) are connected in Darlington configuration to amplify the
S.C. DWIVEDI
H
ere is a musical call bell that can be operated by just bridg-ing the gap between the touch-plates with one’s fingertips. Thus there is no need for a mechanical ‘on’/‘off’switch because the touch-plates act as a switch. Other features include low cost and low power consumption. The bell can work on 1.5V or 3V, using one or two pencil cells, and can be used in homes and offices.
Two transistors are used for sens-ing the fsens-inger touch and switchsens-ing on a melody IC. Transistor BC148 is npn type while transistor BC558 is pnp type.
The emitter of transistor BC148 is shorted to the ground, while that of transistor BC558 is connected to the positive terminal. The collector of tran-sistor BC148 is connected to the base of BC558. The base of BC148 is connected to the washer (as shown in the figure).
The collector of BC558 is connected to pin 2 of musical IC UM66, and pin 3 of IC UM66 is shorted to the ground. The output from pin 1 is connected to a tran-sistor amplifier comprising BEL187 transistor for feeding the loudspeaker.
One end of 2.2-mega-ohm resistor R1 is connected to the positive rail and the
other to a screw (as shown in the fig-ure). The complete circuit is connected to a single pencil cell of 1.5V.
When the touch-plate gap is bridged with a finger, the emitter-collector junc-tion of transistor BC148 starts conduct-SUKANT KUMAR BEHARA
ing. Simultaneously, the emitter-baser junction of transistor BC558 also starts conducting. As a result, the collector of transistor BC558 is pulled towards the positive rail, which thus activates melody generator IC1 (UM66). The output of IC1 is amplified by transistor BEL187 and fed to the speaker. So we hear a musical note just by touching the touch points.
The washer’s inner diameter should be 1 to 2 mm greater than that of the screwhead. The washer could be fixed in
the position by using an adhesive, while the screw can be easily driven in a wooden piece used for mounting the touch-plate. The use of brass washer and screw is recommended for easy solder-ability.