Automatic Fan Controller

Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any type will do. I experimented with different models from 22K to 100K
and all worked fine after replacing the trimmer pot. The one used in the above circuit diagram was a 50K model. This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. Tolerance for my application (cooling a large powersupply coolrib) is 10%. Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase. I replaced my thermistor for a 60K hermetically
sealed glass type since the environment for my application may contain corrosive particles which may affect performance on a future date. P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit.

I used the 10-turn type for a bit finer adjustment but the regular type will work for your application.
R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers... :-)R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point. Depending on your application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until your satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me.

Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil. D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 amp.

However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp. If you like to make your own pcb, try the one above. The pcb is fitted with holes for the relay but may not fit your particular relay. It was designed for a Aromat HB1-DC12V type.

The variety and model of relays is just to great. How to mount it then? Well, I left ample space on the pcb to mount your relay. You can even mount it up-side-down and connect the wires individually. Use Silicon glue, cyanoacrylate ester (crazy glue), or double-sided tape to hold the relay in place. Works well. Note that the pcb and layout is not according to the circuit diagram in regards to the hookup of the fans. The PCB measures approximately 1.5 x 3 inches (4.8 x 7.6mm) If you print the pcb to an inkjet printer it is probably not to scale. Try to fit a 8-pin ic socket on the printed copy to make sure it fits before making the pcb...

Simple Digital Security System

You can use this simple and reliable
security system as a watchdog
by installing the sensing
loops around your building. You have
to stretch the loop wires two feet above
the ground to sense the unauthorised
entry into your premises.
Wire loops 1, 2 and 4 are connected
to the A, B and C inputs of 7-segment
decoder 4511 (IC1), respectively, while
the D input of IC1 is grounded permanently.
The loops are also connected to
a dual 3-input NOR gate and inverter
CD4000 (IC2) to activate the alarm.
Fig. 1 shows the circuit of the digital
security system, while Fig. 2 shows
the proposed wiring diagram for the
loops around the premises. Before using
this security system, make sure
that loops shown in Fig. 2 are con-
�� PHERDAUS ISLAM nected as shown in Fig. 1. If you don’t
want to use a buzzer, switch it off by
opening switch S2.
The circuit works off a 9V regulated
power supply. However, battery
back-up is recommended. A commoncathode,
7-segment display (LTS543)
is used for displaying whether the
loops are intact or not.
If loop 1 is broken, the display will
show ‘1’. If two or all the three loops
are broken, the display will show the
sum of the respective broken loop

Smoke Alarm Using 555

The smoke alarm circuit presented here is based on the readily available photon-coupled interrupter module and timer IC NE555. The photo interrupter module is used as the smoke detector, while timer 555 is wired in astable configuration as an AF oscillator for sounding alarm via a loudspeaker.

Circuit diagram of  Smoke Alarm Using 555: Click on image to enlarge

In the absence of any smoke, the gap of photo interrupter module is clear and the light from LED falls on the phototransistor through the slot. As a result, the collector of phototransistor is pulled towards ground. This causes reset pin 4 of IC 555 to go low. Accordingly, the timer is reset and hence the alarm does not sound.

However, when smoke is present in the gap of the photo interrupter module, the light beam from LED to the phototransistor is obstructed. As a result, the phototransistor stops conducting and pin 4 (reset) of IC 555 goes high to activate the alarm.

Note: The unit must be housed inside an enclosure with holes to allow entry of smoke.

Simple electronics Code Lock System

SIMPLE ELECTRONIC
CODE LOCK
The circuit diagram of a simple electronic
code lock is shown in figure.
A 9-digit code number is used
to operate the code lock.
When power supply to the circuit is
turned on, a positive pulse is applied to
the RESET pin (pin 15) through capacitor
C1. Thus, the first output terminal
Q1 (pin 3) of the decade counter IC (CD
4017) will be high and all other outputs
(Q2 to Q10) will be low. To shift the high
state from Q1 to Q2, a positive pulse must
be applied at the clock input terminal (pin
14) of IC1. This is possible only by pressing
the push-to-on switch S1 momentarily.
On pressing switch S1, the high state
shifts from Q1 to Q2.
Now, to change the high state from
Q2 to Q3, apply another positive pulse at
pin 14, which is possible only by pressing
switch S2. Similarly, the high state can
be shifted up to the tenth output (Q10)
by pressing the switches S1 through S9
sequentially in that order. When Q10 (pin
11) is high, transistor T1 conducts and
energises relay RL1. The relay can be
used to switch ‘on’ power to any electrical
appliance.
Diodes D1 through D9 are provided
to prevent damage/malfunctioning of the
IC when two switches corresponding to
‘high’ and ‘low’ output terminals are
pressed simultaneously. Capacitor C2 and
resistor R3 are provided to prevent noise
during switching action.
Switch S10 is used to reset the
circuit manually. Switches S1 to S10
can be mounted on a keyboard panel,
and any number or letter can be used to
mark them. Switch S10 is also placed
together with other switches so that any
stranger trying to operate the lock frequently
presses the switch S10, thereby
resetting the circuit many times. Thus,
he is never able to turn the relay ‘on’. If
necessary, two or three switches can
be connected in parallel with S10 and
placed on the keyboard panel for more
safety.
A 12V power supply is used for the
circuit. The circuit is very simple and can
be easily assembled on a general-purpose
PCB. The code number can be easily
changed by changing the connections to
switches (S1 to S9).

Electronics Jam

This jam circuit can be used in
quiz contests wherein any participant
who presses his button
(switch) before the other contestants,
gets the first chance to answer a question.
The circuit given here permits up
to eight contestants with each one allotted
a distinct number (1 to 8). The
display will show the number of the contestant
pressing his button before the
others. Simultaneously, a buzzer will
also sound. Both, the display as well as
the buzzer have to be reset manually
using a common reset switch.
Initially, when reset switch S9 is momentarily
pressed and released, all outputs
of 74LS373 (IC1) transparent latch
go ‘high’ since all the input data lines
are returned to Vcc via resistors R1
through R8. All eight outputs of IC1
are connected to inputs of priority encoder
74LS147 (IC2) as well as 8-input
NAND gate 74LS30 (IC3). The output
of IC3 thus becomes logic 0 which, after
inversion by NAND gate N2, is applied
to latch-enable pin 11 of IC1. With all
input pins of IC2 being logic 1, its BCD
output is 0000, which is applied to 7-
segment decoder/driver 74LS47 (IC6) after
inversion by hex inverter gates inside
74LS04 (IC5). Thus, on reset the
display shows 0.
When any one of the push-to-on
switches—S1 through S8—is pressed,
the corresponding output line of IC1 is
latched at logic 0 level and the display
indicates the number associated with
the specific switch. At the same time,
Electronic Jam
RAJESH K.P.
output pin 8 of IC3 becomes high, which
causes outputs of both gates N1 and
N2 to go to logic 0 state. Logic 0 output
of gate N2 inhibits IC1, and thus pressing
of any other switch S1 through S8
has no effect. Thus, the contestant who
presses his switch first, jams the display
to show only his number. In the
unlikely event of simultaneous pressing
(within few nano-seconds difference)
of more than one switch, the higher
priority number (switch no.) will be
displayed. Simultaneously, the logic 0
output of gate N1 drives the buzzer via
pnp transistor BC158 (T1). The buzzer
as well the display can be reset (to
show 0) by momentary pressing of reset
switch S9 so that next round may
start.
Lab Note: The original circuit sent
by the author has been modified as it
did not jam the display, and a higher
number switch (higher priority), even
when pressed later, was able to change
the displayed number.