# hey, suggestion for circuit design

#### rzr85

Joined Feb 20, 2007
2
hey guys/gals...
i need help, and thats why im here hopefully i can get some useful info so i can work such things out on my own. i have a hw to design a temperature sensor, but it has its limitations. elements that i am allowed to use:
fixed resistors
thermistors ntc
9v batt (1,2, or max 3..no more)
any led

what i cannot use: opamps, capacitors, amps, diodes...the whole lot

requirements for the sensor design: to act as a monitor for SID (sudden infant death syndrome) where 34C and 39C are signs that the child will die. (normal temp is acceptable 37C)

okay...so this is what i have so far:
a tri-color led with specifications of: max A ~ 30mA; red (2v) green (2.2v)
i am using two fixed resistors (235ohm and 210ohm) and one thermistor (210ohm @ 37C, B=3500).

i have attached the circuit layout for normal body temperature~39C and according to my setup - it should give a red indicator (red) @ 2.022v

FIRST - is that circuit appropriate? assuming that i did the calculations correctly...the current through each resistor is 10mA and voltage across the led is 2.022v ...which gives a red light

SECOND - now as i decrease the temp on the thermistor to 34C, the resistance goes to 233ohms...i calculate the current division, and find out that the current through the thermistor is 9.48mA and through the fixed resistor is 10.52mA...assuming this is right...the volt will now be at 2.2v as current passes through path of least resistance and the led should glow green

THIRDLY - as temp remains the same, to 37C, the resistance is 210ohms and doing the same..the voltage turns out to be 2.1v and the led glows yellow

thanks for staying in this thread please do let me know if what i have based the design is appropriate or is it fundamentally flawed.

~rzr

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