Freeze Protector Circuit

This is the simple freeze protector circuit diagram. It is also called timing circuit. It is just hobby project for beginners. When you need to operate automatically any device in the fixed time later after coming AC power supply, you can use this circuit. 

The voltage amplitude is high when AC power come. Suddenly high voltage destroys the electronics device. For this reason, in the time of load shedding we need to switched off electronics device (Audio/Video player, CD/DVD Player, TV, Freeze or any electronics device). This circuit solve this problem.

Hobby Electronics helps the beginners to know the timing circuit specially using 555 IC timer. NE555 IC is the very simple well known parts. Its configuration is very simple. You can use this timer any simple circuit. 

In this circuit use 12volt power supplies. 300k variable resistor is used for timing control. Timing resistor and capacitor are 300k and 1000uF. Only change the value of the variable resistor for your required timing. 

The circuit output pin 3 is connected with resistor (10k) and output 12 volt relay. Relay is connected as figured with the device. Making cost is too low. Anyone can make this circuit easily.

First-Order Band-Pass Filter

          Application of first order band pass filter to find out the output voltage gain magnitude of specific frequency. A specific range of frequency can pass through the amplifier which has a specific bandwidth of this band pass filter.

A band pass filter is a frequency selector. It allows one to select or pass only one particular band of frequencies from all other frequencies that may be present in a circuit. This type of filter has a maximum gain at a resonant frequency.

A band pass filter is the combination of high pass and low pass filter combination. It has a pass band between two cut off frequency f_H and f_L such that f_H > f_L. Any input frequency outside this pass band is attenuated. There are two types of band pass filters wide band pass and narrow band pass. If the quality factor Q < 10 and Q > 10 then it would be wide band pass and narrow band pass filter.

          The relationship between Q, the 3-dB bandwidth and the center frequency f_c is given by,

             
       According to Figure 01 of band pass filter circuit, there are two sections. One is first order high pass section and other is low pass section.

 For first order high pass section the output voltage equation is,

For first order low pass section the output voltage equation is,

Putting the value of first order high pass section output voltage  from equation (1),

So the final voltage equation of first order band pass filter is,

        
         The voltage gain magnitude of the band pass filter is equal to the product of the voltage gain magnitudes of the high pass and low pass filters.

Therefore the equation (2) is,

Where,  = Total band pass gain

f = frequency of the input signal (Hz)

 = low cut off frequency (Hz)

= high cut off frequency (Hz)

The above first order low pass band width filter was designed by taking following precautions,

a.       
 Limited magnitude of input voltage was applied at the input, so that the op-amp must not be driven to saturation.

b.                 Only selected frequency can pass through the filter.

c.                         Overall gain of the first order band pass filter is the multiplication of  high pass filter gain and low pass filter gain.

A Small project on Battery level Indicator

Battery level indicator is basically a design of light column voltmeter. It displays a column of light which height is proportional to the voltage. In the practical sense of view battery level indicator is used to show the voltage level of battery.

Working principle of this circuit is battery voltage compared with the reference voltage. This circuit is constructed by using 741 op-amp ICs. Op-amp is differential amplifier which output will be positive or negative, depends on the difference of input voltages.

When the input voltage difference is positive with respect to reference voltage and battery voltage, then the output is positive and the LED will emit light. If negative LED cannot emit light. It is the main principle of battery level indicator.

Voltage level indicated by the column of LEDs. For easy of reading LED rearranged in a vertical array. The top four LEDs indicates the battery voltage is high. Sequentially the lower level indicates the battery voltage is low and bottom four LEDs indicates very low voltage of the battery.

So, if the battery voltage continues to decline then LEDs will die in sequence from top to bottom.

In this circuit 1mA current is adjusted for reference voltage. V_cc is divided by the total number of resistor for finding the value of resistor R.

       

On the other hand, the resistor value of R₁ is calculated by the equation,          

                   

Component:

1. Op-amp IC 741

2. R = 3kΩ

3. R₁ =220kΩ

4.  Diode 

Operation:

The voltage drop against the each resistor is 3 volt.

So when the V_in = 0v or less than 3v the output of all op-amp are at

–V_sat . The silicon diodes protect the light emitting diodes against excessive reverse bias voltage.

When V_in is increased to a value 3v to 6v, only the output of op-amp#1 goes positive  to light which indicate “Very Low”.

When V_in is increased to a value 3v and 6v, the output of op-amp#1 and  op-amp#2  are at +V_sat  and goes positive to light which indicate “Low”.

When V_in is increased to a value 6v and 9v, the output of op-amp#1, op-amp#2 and op-amp#3 are at +V_sat  and goes positive to light which indicate “Medium”.

Similarly, when V_in increased to 12v or higher, the output of all op-amps are at +V_sat  and all op-amps output goes positive to emit light of all LEDs which indicate “Full” .