How to design a sensor detection circuit ?

[sush]

sensor detection

Dear friends

i want to design a circuit which can detect whether a sensor is present or not..the signal voltage can vary from 0-100mV from the sensor. i just have to detect the sensors presence logically whether its 0 or 1.that means whether the sensor is present or not in the circuit. if its not present display "OPEN" on the screen..

waiting for help
thanks

 

[BABESH]

Re: sensor detection

hello
What u need to do is to add one analog comparator which changes state from low to high when signal is not in (0-100)mV range.
You can use single supply 5V op-amp for this purpose.
all the best thanks

 

[newelltech]

Re: sensor detection

If your sensor is has low output impedance, you could add a pullup resistor network to the input so that when the sensor is absent, the voltage goes to say 150 mV. Then whenever you get a reading of more that 100 mV you know the sensor is not present. You can select the resistor network so that the resistance is high enough to not significantly affect the sensor voltage when present, yet low enough that the leakage into the ADC guarantees an open circuit voltage of more that 100 mV.

 

[Jepeto]

Re: sensor detection

Connect any CMOS logic gate to the sensor output
If the sensor presents the gate fill “0’’ in its input/s
If the sensor not presents the CMOS gate will oscillate
To get constant logic level the gate output can be connected to one shoot

 

[newelltech]

Re: sensor detection

Connect any CMOS logic gate to the sensor output
If the sensor presents the gate fill “0’’ in its input/s
If the sensor not presents the CMOS gate will oscillate
To get constant logic level the gate output can be connected to one shoot

This reply makes absolutely no sense. A CMOS logic gate triggers at about Vcc/2 so it won't ever change and won't oscillate.

 

[Jepeto]

Re: sensor detection

Connect any CMOS logic gate to the sensor output
If the sensor presents the gate fill “0’’ in its input/s
If the sensor not presents the CMOS gate will oscillate
To get constant logic level the gate output can be connected to one shoot

This reply makes absolutely no sense. A CMOS logic gate triggers at about Vcc/2 so it won't ever change and won't oscillate.

Perhaps I don’t understand the question
If the sensor present and its output 0v – 150mv logic gate like OR will see always “0” at the inputs and its output always “0”
If the sensor not present, and what I understand is that there is no sensor, the OR gate inputs floating, unconnected inputs cause CMOS gate out to oscillate, connecting the output to retriggerable one-shoot will give you steady “1”

 

[newelltech]

Re: sensor detection

It is very bad practice to leave a CMOS input floating - it could be 0 or 1 or noise. It's totally unpredictable. That's why I suggested adding a pullup resistor so that when the sensor is not there the input goes a little over 150 mV. This way you don't need to add a logic gate, the ADC senses that the input is over 150 mV and thus the sensor is not connected. The pullup resistor network must not add too much to the sensor output or it will introduce an undesirable offset.

For the TTL logic family, unconnected inputs will be at 0, but this is also bad practice and wouldn't work here because the gate would load the sensor.

 

[sush]

sensor detection

thanks newelltech. your method really worked out. Thanks a lot.

 

[megzhuy]

sensor detection

I don't understand how the sensor works in your circuit.
First, there are several kinds of architecture sensor in COMS.Second, there is dark current in sensor if it is based on CMOS.
Maybe you can look for some papers about optical mouse or some chips about piture. I will find how the sytem works. It's not simple.