Specifications

CIRCUIT IDEAS
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
S.C. DWIVEDI
PRADEEP VASUDEVA
AUTOMATIC SPEED-CONTROLLER
FOR FANS AND COOLERS
The first two outputs of IC3 (Q0 and
Q1) are connected (ORed) via diodes D1
and D2 to the base of transistor T1. Ini-
tially output Q0 is high and therefore re-
lay RL1 is energised. It remains energised
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.
This continues until output Q4 is high.
When Q4 goes
low and Q5 goes
high, relay RL2 is
switched ‘off’ and
relay RL3 is acti-
vated. The fan
now runs at low
speed.
Throughout
the process, pin
11 of the IC 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 out-
puts 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
ters only to numerics, capital English let-
ters, and some symbols. You are at lib-
erty to store bit patterns for any other
data, in any other style, in the EPROM.
The input ‘BI’ indicating blanking in-
put (actually this is the OE signal of
EPROM) can be used for blanking the
display. You can also use this line for con-
verting it into a blinking display by con-
necting it to a suitable output pin of
counter CD4060.
You can also adapt the circuit for re-
sponding to ASCII input values by stor-
ing the character bit pattern in memory
pages, their address being equal to the
ASCII value of that character. Moreover,
it is possible to display characters of any
language and, if needed, the size of the
display can also be modified by using some
additional hardware.
after some time, and to slow later on.
After 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 generate 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.
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