Tech@electronics india

Thursday, 29 October 2015

Automatic night light using 555 timer

Description
A cheap and simple automatic night light using few transistors and NE555 timer is shown here . The circuit will automatically switch on the AC lamp when night falls and the lamp will automatically switched off after a preset time.

The working of this night light circuit very simple. An LDR is used as the sensor here. At day time the resistance of the LDR will be low and so do the voltage drop across it, the transistor Q1 will be in conducting mode . When darkness falls the resistance of LDR increases and so do the voltage across it. This makes the transistor Q1 OFF. Base of Q2 is connected to the emitter of Q1 and so Q2 is biased on which in turn power IC1.

NE 555 is wired as monostable multivibrator that is automatically triggered at power ON . This automatically triggering is achieved with the help of capacitor C2. The output of IC1 remains high for a time determined by resistor R5 and capacitor C4 . When output of IC1 goes high transistor Q3 is switched ON which triggers triac T1 and the lamp glows . A 9V battery is included in the circuit in the order to power the timer circuit during power failures . Resistor R1 and diode D1 , capacitor C1 and Zener D3 forms the power supply section of the circuit . R7 and R8 are current limiting resistors.

Notes:

The circuit can be assembled on a very board.
Preset R2 can used to adjust sensitivity of the circuit.
Preset R5 can be used to adjust the ON time of the lamp.
The wattage of L1 must nor exceed 200W
Heat sink is recommended for BT136.

for more information about 555 timer IC

more information about 555 IC

Sunday, 25 October 2015

Silicon Controlled Rectifier

What is SCR ?

It is Silicon Controlled Rectifier , use in power electronic device. It use as a switching devices in many high voltages area and high temperature .

figure shows physical diagram and symbol of SCR.
SCR is well known member of thyristor family, although there are many different members are available in thyristor family but SCR are so widely used that as if thyristor and SCR become synonymous.

If we see from the constructional and operation point of view , it is a four layer (PNPN) three terminals (anode,cathode, gate) semiconductor device. This device has two states ON and OFF. We can turn it ON by sending gate current signal between second P layer and cathode. But we cannot turn OFF by control signal. That means we have control upon its turn ON, once it goes to conduction mode , we lose control over it.

It can block both forward and reverse voltage but can conduct only in one direction.It use in high power application like AC -DC converter etc.