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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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Digital Step-Km Counter

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Max. range: 9,950 meters with two digits

Slip it in pants' pocket for walking and jogging

 

Parts:

R1,R3____22K   1/4W Resistor
R2________2M2  1/4W Resistor
R4________1M   1/4W Resistor
R5,R7,R8__4K7  1/4W Resistor
R6_______47R   1/4W Resistor
R9________1K   1/4W Resistor

C1_______47nF   63V Polyester Capacitor
C2______100nF   63V Polyester Capacitor
C3_______10nF   63V Polyester Capacitor
C4_______10µF   25V Electrolytic Capacitor

D1_______Common-cathode 7-segment LED mini-display (Hundreds meters)
D2_______Common-cathode 7-segment LED mini-display (Kilometers)

IC1______4093   Quad 2 input Schmitt NAND Gate IC
IC2______4024   7 stage ripple counter IC
IC3,IC4__4026   Decade counter with decoded 7-segment display outputs IC

Q1,Q2___BC327   45V 800mA PNP Transistors

P1_______SPST Pushbutton (Reset)
P2_______SPST Pushbutton (Display)

SW1______SPST Mercury Switch, called also Tilt Switch
SW2______SPST Slider Switch (Sound on-off)
SW3______SPST Slider Switch (Power on-off)

BZ_______Piezo sounder 

B1_______3V Battery (2 AA 1.5V Cells in series) 
 
 

Device purpose:

This circuit measures the distance covered during a walk. Hardware is located in a small box slipped in pants' pocket and the display is conceived in the following manner: the leftmost display D2 (the most significant digit) shows 0 to 9 Km. and its dot is always on to separate Km. from hm. The rightmost display D1 (the least significant digit) shows hundreds meters and its dot illuminates after every 50 meters of walking. A beeper (excludable), signals each count unit, occurring every two steps.
A normal step was calculated to span around 78 centimeters, thus the LED signaling 50 meters illuminates after 64 steps (or 32 operations of the mercury switch), the display indicates 100 meters after 128 steps and so on. For low battery consumption the display illuminates only on request, pushing on P2. Accidental reset of the counters is avoided because to reset the circuit both pushbuttons must be operated together.
Obviously, this is not a precision meter, but its approximation degree was found good for this kind of device. In any case, the most critical thing to do is the correct placement of the mercury switch inside of the box and the setting of its sloping degree.

Circuit operation:

IC1A & IC1B form a monostable multivibrator providing some degree of freedom from excessive bouncing of the mercury switch. Therefore a clean square pulse enters IC2 that divides by 64. Q2 drives the LED dot-segment of D1 every 32 pulses counted by IC2. Either IC3 & IC4 divide by 10 and drive the displays. P1 resets the counters and P2 enables the displays. IC1C generates an audio frequency square wave that is enabled for a short time at each monostable count. Q1 drives the piezo sounder and SW2 allows to disable the beep.

Notes:

  • Experiment with placement and sloping degree of mercury switch inside the box: this is very critical.
  • Try to obtain a pulse every two walking steps. Listening to the beeper is extremely useful during setup.
  • Trim R6 value to change beeper sound power.
  • Push P1 and P2 to reset.
  • This circuit is primarily intended for walking purposes. For jogging, further great care must be used with mercury switch placement to avoid undesired counts.
  • When the display is disabled current consumption is negligible, therefore SW3 can be omitted.