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INTRODUCTION :
In this project we use a NTC thermistor to detect temperature change
& switch a relay On/Off according to a preset condition. NTC
thermistor is a temperature dependent resistor with Negative
Temperature Coefficient meaning its resistance changes inversely
with temperature. The higher the temperature, the lower its
resistance. A sample randomly selected from a batch of MF52E
thermistors gives the following characteristic chart
NOTE: the LED serves
as an indicator for the relay's activity. The LED turns on when the
relay is actuated. It is otherwise off
To test the operation of the
circuit, please follow these steps:
(1) Connect the ciruit to a stable DC 9V supply. Or you can
temporarily use a 9V battery for testing purpose
(2) At this moment, the LED should be on. If not, then adjust the
preset (trim pot) VR1 in anti-clockwise until the LED comes on
(3) Next, very slowly, turn the preset VR1 in the clockwise
direction until the LED just turns OFF. This is the trim pot setting
corresponding to the current ambient temperature
(4) Now, let the thermistor sense a temperature rise by placing
something hot near the thermistor (eg. a soldering iron, the flame
of a lighter,..). The LED should turn on instantly (relay actuated).
Remove the heat source and wait a minute or so and LED will be off
again
NOTA: LED
berfungsi sebagai penunjuk untuk aktiviti geganti. LED menyala
apabila geganti digerakkan.
Ia sebaliknya dimatikan
Untuk menguji pengendalian litar, sila ikuti langkah berikut:
(1) Sambungkan ciruit kepada bekalan DC 9V yang stabil. Atau anda
boleh menggunakan bateri 9V buat sementara waktu untuk tujuan ujian
(2) Pada masa ini, LED harus dihidupkan. Jika tidak, kemudian
laraskan pratetap (periuk trim) VR1 dalam lawan jam sehingga LED
menyala
(3) Seterusnya, perlahan-lahan, putar pratetap VR1 mengikut arah jam
sehingga LED hanya dimatikan Ini ialah tetapan periuk trim sepadan
dengan suhu ambien semasa
(4) Sekarang, biarkan termistor merasakan kenaikan suhu dengan
meletakkan sesuatu yang panas berhampiran termistor (cth. besi
pematerian, nyalaan pemetik api,..). LED harus dihidupkan
serta-merta (geganti digerakkan). Keluarkan sumber haba dan tunggu
sebentar atau lebih dan LED akan dimatikan
semula
 
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