OBJETIVOS ESPECÍFICOS
1. MARCO TEÓRICO
1.1 GENERALIDADES DE LA ZANAHORIA CHANTENAY
D
uring summer nights, the tem- perature is initially quite high. As time passes, the temperature starts dropping. Also, after a person falls asleep, the metabolic rate of one’s body decreases. Thus, initially the fan/ cooler needs to be run at full speed. As time passes, one has to get up again and again to adjust the speed of the fan or the cooler.The device presented here makes the fan run at full speed
for a predetermined time. The speed is decreased to medium after some time, and to slow later on. Af- ter a period of about eight hours, the fan/ cooler is switched off.
Fig. 1 shows the circuit diagram of the system. IC1 (555) is used as an astable multivibrator to gen- erate clock pulses. The pulses are fed to decade dividers/ counters formed by IC2 and IC3. These
ICs act as divide-by-10 and divide-by-9 counters, respectively. The values of ca- pacitor C1 and resistors R1 and R2 are so adjusted that the final output of IC3 goes high after about eight hours.
The first two outputs of IC3 (Q0 and Q1) are connected (ORed) via diodes D1 and D2 to the base of transistor T1. Initially output Q0 is high and therefore relay RL1 is energised. It remains ener-
gised when Q1 becomes high. The method of connecting the gadget to the fan/cooler is given in Figs 3 and 4.
It can be seen that initially the fan shall get AC supply directly, and so it shall run at top speed. When output Q2 becomes high and Q1 becomes low, relay RL1 is turned ‘off’ and relay RL2 is switched ‘on’. The fan gets AC through a resistance and its speed drops to medium value. This
continues until output Q4 is high. When Q4 goes low and Q5 goes high, relay RL2 is switched ‘off’ and relay RL3 is activated. The fan now runs at low speed.
Throughout the process, pin 11 of the IC3 is low, so T4 is cut off, thus keeping T5 in saturation and RL4 ‘on’. At the end of the cycle, when pin 11 (Q9) becomes high, T4 gets saturated and T5 is cut off. RL4 is switched ‘off’, thus switching ‘off’ the fan/cooler.
Using the circuit described above, the fan shall run at high speed for a com- paratively lesser time when either of Q0
or Q1 output is high. At medium speed, it will run for a moderate time period when any of three outputs Q2 through Q4 is high, while at low speed, it will run for a much longer time period when any of the four outputs Q5 through Q8 is high.
If one wishes, one can make the fan run at the three speeds for an equal amount of time by connecting three decimal decoded outputs of IC3 to each of the transistors T1 to T3. One can also get more than three speeds by using an additional relay, transistor, and
associated components, and connecting one or more outputs of IC3 to it.
In the motors used in certain coolers there are separate windings for separate speeds. Such coolers do not use a rheostat type speed regulator. The method of connection of this device to such coolers is given in Fig. 4.
The resistors in Figs 2 and 3 are the tapped resistors, similar to those used in manually controlled fan- speed regulators. Alternatively wire- wound resistors of suitable wattage and resistance can be used.
blown fuse inDicaTor
aShutoSh KuMar Sinha
G
enerally, when an equipment in- dicates no power, the cause may be just a blown fuse. Here is a circuit that shows the condition of fuse through LEDs. This compact circuit is very useful and reliable. It uses very few components, which makes it inexpensive too.Under normal conditions (when fuse is alright), voltage drop in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in second arm it is only 2V. So current flows
trigger the siren. When the fuse blows, red LED glows. Simultaneously it switches ‘on’ the siren.
In place of a bicolour LED, two LEDs of red and green colour can be used. Similarly, only one diode in place of D1 and D2 may be used. Two diodes are used to increase the voltage drop, since the two LEDs may produce different volt- age drops.
through the second arm, i.e. through the green LED, causing it to glow; whereas the red LED remains off.
When the fuse blows off, the supply to green LED gets blocked, and because only one LED is in the circuit, the red LED glows. In case of power failure, both LEDs remain ‘off’.
This circuit can be easily modified to produce a siren in fuse-blown condi- tion (see Fig. 2). An
oVer-/unDer-VolTage cuT-off
wiTh on-Time Delay
H
ere is an inexpensive auto cut- off circuit, which is fabricated using transistors and other dis- crete components. It can be used to protect loads such as refrigerator, TV, and VCR from undesirable over and under line voltages, as well as surges caused due to sudden failure/resumption of mains power supply. This cir-cuit can be used directly as a standalone circuit between the mains sup- ply and the load, or it may be inserted between an existing automatic/ manual stabiliser and the load.
The on-time delay circuit not only protects
the load from switching surges but also from quick changeover (off and on) effect of over-/under-voltage relay, in case the mains voltage starts fluctuating in the vicinity of under- or over-voltage preset points. When the mains supply goes out
of preset (over- or under-voltage) limits, the relay/load is turned ‘off’ immediately, and it is turned ‘on’ only when AC mains voltage settles within the preset limits for a period equal to the ‘on’ time delay period. The on-time delay period is presetable for 5 seconds to 2 minutes duration, using presets VR3 and VR4. For electronic loads
such as TV and VCR, the on-time delay may be set for 10 seconds to 20 seconds. For refrigerators, the delay should be pre- set for about 2 minutes duration, to protect the compressor motor from frequently turning ‘on’ and ‘off’.
In this circuit, the on-time and off- time delays depend on charging and
discharging time of ca- pacitor C1. Here the dis- charge time of capacitor C1 is quite less to suit our requirement. We want that on switching ‘off’ of the supply to the load, the circuit should be immediately ready to provide the required on-time delay when AC mains resumes after a brief interruption, or when mains AC voltage is interrupted for a short period due to over-/under- voltage cut-off operation. This circuit is also useful against frequent power supply interruptions resulting from loose electri-
cal connections; be it at the pole or switch or relay contacts, or due to any other reason.
Here supply for the over- and under- voltage sampling part of the circuit [marked +12V(B)] and that required for the rest of the circuit [marked +12V(A)] are derived separately from lower half and upper half respectively of centre-tapped
secondary of step-down transformer X1, as shown in Fig. 1. If we use common 12V DC supply for both parts of the circuit, then during relay ‘on’ operation, 12V DC to this circuit would fall below preset low cut-off voltage and thus affect the proper opera- tion of the sampling circuit. The value of filtering capacitor C4 is so chosen that a fall in mains voltage may quickly activate under-voltage sensing circuit, should the mains voltage reach the low cut-off limit.
In the sampling part of the circuit, wired around transistor T1, presets VR1 and VR2 are used for presetting over- or under-voltage cut-off limits, respectively. The limits are set according to load volt- age requirement, as per manufacturer’s specifications.
Once the limits have been set, zener D1 will conduct if upper limit has been exceeded, resulting in cut-off of tran- sistor T2. The same condition can also result when mains voltage falls below the under-voltage setting, as zener D2 stops conducting. Thus, in either case, transistor T2 is cut-off and tran- sistor T3 is for- ward biased via resistor R3. This causes LED1 to be ‘on’. Simulta- neously, capacitor C2 quickly dis- charges via diode D5 and transistor T3. As collector of transistor T3 is pulled low, tran- sistors T4 and T5 are both cut- off, as also transistor T5. Thus, LED2 and LED3 are ‘off’ and the relay is de- energised.
Now, when the mains voltage comes within the acceptable range, transistor T2 conducts to cut-off transistor T3. LED1 goes ‘off’. Transistor T5 gets forward bi- ased and LED2 becomes ‘on’. However,
Fig. 1: Power supply
Fig. 2: Schematic diagram of over-/under-voltage cut-off with on-time delay
TABLE I
Showing State of LEDs for Various Circuit Conditions
Circuit condition LED1 LED2 LED3 Relay/Load
Over or under voltage ON OFF OFF OFF cut-off in operation
On-time delay in operation OFF ON OFF OFF AC voltage normal
after on-time delay OFF OFF ON ON
transistors T4 and T5 are still ‘off’, since base of T4 via zener D4 is connected to capacitor C1, which was in discharged condition. Thus, LED3 and relay RL1 or load remain ‘off’.
Capacitor C1 starts charging slowly towards +12V(A) rail via resistors R6 and R7, and presets VR3 and VR4. When the
potential across capacitor C1 reaches 6.8V (after a delay termed as on- time delay) to breakdown zener D4, transistor T4, as also transistor T5, gets forward biased, to switch ‘on’ LED3 and relay RL1 or load, while LED2 goes ‘off’. Should the mains supply go out of preset limits before completion of the on-time delay, capacitor C1 will immediately discharge because of conduction of transistor T3, and the cycle will repeat until mains supply stablises within preset limits for the on-time delay period.
The on-time delay is selected by ad- justing presets VR3 and VR4, and resis- tor R6. Zener diode D3 is used to obtain regulated 9.1 volts for timing capacitor C1, so that preset on-time delay is more or less independent of variation in input DC voltage to this circuit (which would vary according to the mains AC voltage). To switch ‘off’ the relay/load rapidly during undesired mains condition, the timing ca- pacitor C1 is discharged rapidly to provide complete control over turning ‘on’ or ‘off’ of relay RL1 (or the load). The functioning of the LEDs and relay, depending on the cir- cuit condition, is summarised in Table I.