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High CMRR Instrumentation Amplifier (Schematic and Layout) design for biomedical applications

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Instrumentation amplifiers are intended to be used whenever acquisition of a useful signal is difficult. IA’s must have extremely high input impedances because source impedances may be high and/or unbalanced. bias and offset currents are low and relatively stable so that the source impedance need not be constant. Balanced differential inputs are provided so that the signal source may be referenced to any reasonable level independent of the IA output load reference. Common mode rejection, a measure of input balance, is very high so that noise pickup and ground drops, characteristic of remote sensor applications, are minimized.Care is taken to provide high, well characterized stability of critical parameters under varying conditions, such as changing temperatures and supply voltages. Finally, all components that are critical to the performance of the IA are internal to the device. The precision of an IA is provided at the expense of flexibility. By committing to the one specific task of
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Fridge door Alarm

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Beeps if you leave open the door over 20 seconds 3V battery operation, simple circuitry

 

 

R1____________10K   1/4W Resistor
R2___________Photo resistor (any type)
R3,R4________100K   1/4W Resistors

C1____________10nF  63V Polyester Capacitor
C2___________100µF  25V Electrolytic Capacitor

D1,D2_______1N4148  75V 150mA Diodes

IC1___________4060  14 stage ripple counter and oscillator IC

Q1___________BC337  45V 800mA NPN Transistor

BZ1__________Piezo sounder (incorporating 3KHz oscillator)

SW1__________Miniature SPST slide Switch

B1___________3V Battery (2 AA 1.5V Cells in series)
 
This circuit, enclosed into a small box, is placed in the fridge near 
the lamp (if any) or the opening. With the door closed the interior of 
the fridge is in the dark, the photo resistor R2 presents a high 
resistance (>200K) thus clamping IC1 by holding pin 12 high. When a 
beam of light enters from the opening, or the fridge lamp illuminates, 
the photo resistor lowers its resistance (<2K), pin 12 goes low, IC1 
starts counting and, after a preset delay (20 seconds in this case) the 
piezo sounder beeps for 20 sec. then stops for the same lapse of time 
and the cycle repeats until the fridge door closes. D2 connected to pin 6
 of IC1 allows the piezo sounder beeping 3 times per second.
 
 
  • Connecting D1 to pin 2 of IC1 will halve the delay time.
  • Delay time can be varied changing C1 and/or R3 values.
  • Any photo resistor type should work.
  • Quiescent current drawing is negligible, so SW1 can be omitted.
  • Place the circuit near the lamp and take it away when defrosting, to avoid circuit damage due to excessive moisture.
  • Do not put this device in the freezer.