Download Ahmed Tarek Reda sec(1)

Cairo University
Faculty of Engineering
Biomedical Engineering DPT
Mohamed Ashref Oransa
sec(2)
Ahmed Magdy Ahmed
sec(1)
Ayman Mohamed Anwar
sec(1)
Ahmed Tarek Reda
sec(1)
Description.
The circuit is as simple and straight forward so that, it can
be easily implemented. The thermistor offers a low
resistance at high temperature and high resistance at low
temperature. This phenomenon is employed here for
sensing the fire.
The IC1 (NE555) is configured as a free running oscillator at
audio frequency. The transistors T1 and T2 drive IC1. The
output (pin 3) of IC1 is couples to base of transistor T3
(SL100), which drives the speaker to generate alarm sound.
The frequency of NE555 depends on the values of
resistances R5 and R6 and capacitance C2. When
thermistor becomes hot, it gives a low-resistance path for
the positive voltage to the base of transistor T1 through
diode D1 and resistance R2. Capacitor C1 charges up to the
positive supply voltage and increases the the time for which
the alarm is ON. The larger the value of C1, the larger the
positive bias applied to the base of transistor T1 (BC548). As
the collector of T1 is coupled to the base of transistor T2, the
transistor T2 provides a positive voltage to pin 4 (reset) of
IC1 (NE555). Resistor R4 is selected s0 that NE555 keeps
inactive in the absence of the positive voltage. Diode D1
stops discharging of capacitor C1 when the thermistor is in
connection with the positive supply voltage cools out and
provides a high resistance path. It also inhibits the forward
biasing of transistor T1.
Circuit diagram with Parts list.
Notes.


The circuit can be powered from a 6V battery or a
6V power supply.
The thermistor can be mounted on a heat resistant
material like mica to prevent it from damage due to
excessive heat.

The LED acts as an indication when the power supply is
switched ON.
Components:
1. Ic1 NE555
2. Kit-v-NTC10k
3. BC 548
4. BC 558
5. R 470 Ω ¼ W
6. R 33 K Ω ¼ W
7. R 2.2 K Ω ¼ W
8. R 560 Ω ¼ W
9. R 47 K Ω ¼ W
10. C-10 UF -50 V
11. C-00.01 UF -50 V
12. C-0.047 UF-100 V
13. DI-1N4005
14. LED 5M RED
15. Speaker 8 Ω 0.5 W
16.
BD 135
Application:
It can be used as a Fire alarm
Data sheet :
Ic1 NE555:
Rev. 1.0.2
Features:
• High Current Drive Capability (200mA)
• Adjustable Duty Cycle
• Temperature Stability of 0.005%/C
• Timing From Sec to Hours
• Turn off Time Less Than 2Sec
Applications:
• Precision Timing
• Pulse Generation
• Time Delay Generation
• Sequential Timing
Description:
The LM555/NE555/SA555 is a highly stable controller
capable of producing accurate timing pulses. With
monostable operation, the time delay is controlled by
one
external resistor and one capacitor. With astable
operation,
the frequency and duty cycle are accurately controlled
with
two external resistors and one capacitor.
555 is a very commonly used IC for
generating accurate timing pulses. It
is an 8pin timer IC and has mainly
two modes of operation: monostable
and
astable.
In
monostable
mode
time delay of the pulses can be
precisely
controlled
by
an
external
resistor and a capacitor whereas in
astable mode the frequency & duty
cycle are controlled by two external
resistors
and
a
capacitor.
555
is
very
commonly
used
for generating
time delays and pulses.
Pin Description:
:
Pin
No
1
2
3
4
Function
Name
Ground (0V)
Voltage below 1/3 Vcc to
trigger the pulse
Pulsating output
Active low; interrupts the
timing interval at Output
Ground
Trigger
Output
Reset
5
6
7
8
Provides access to the
internal voltage divider;
default 2/3 Vcc
The pulse ends when the
voltage is greater than
Control
Open collector output; to
discharge the capacitor
Supply voltage; 5V (4.5V 16 V)
Control
Voltage
Threshold
Discharge
Vcc