Hello,

I am working on project that takes the body's temperature, but the output needs to be in the form of frequency.

I am looking at using a thermopile, LM358 op amp and a AD7740 voltage to frequency converter. Using the datasheet for the AD7740 and this link for the thermopile to LM358, http://www.instructables.com/id/Thermopile-Sensor/step3/Op-Amp/

I have created the schematic below, but I have some doubts if this will work. I have concerns in that the AD7740 needs a power in, so where does the battery get grounded? Also, Is there another temperature sensor that can be used to simplify this circuit?

I do apologize if I am asking redundant questions or if I am way off. I have been removed from electronics for the past 15 yrs or so and I am trying to get back in and reading everythnig that I can.

 

Where are the power and GND connections for the LM358? That would be a good place to connect the battery ground. Are you sure the LM358 will operate from a single supply at such a low voltage?

Oh..., and the LM358 and the voltage to frequency converter should also probably have the same ground.

 

Sorry about that PH1, is actually the thermopile (I was not able to find a thermopile in Eagle). The thermopile produces millivolts. Is that enough to power the LM?

 

I would look into using an LM34 and an LM131 as shown in the attachment taken from page 8 of the TI LM34 datasheet.

 

Thanks for the help.

Trace, I looked at that but I am looking to design something using a 3 volt lithium button battery.

Taking Papa's help I tweaked my schematic to the following:

Thanks

 

Does the sensor have to be a thermopile?
How about a thermistor in a 555 timer circuit?

 

With a 2032 battery, a 555 would have to be CMOS.

 

You could try a MAX6577. It provides a frequency output based on temperature. Oh, wait a minute, it's not very accurate so it may not suit your purpose.

 

Sorry about that PH1, is actually the thermopile (I was not able to find a thermopile in Eagle). The thermopile produces millivolts. Is that enough to power the LM?

No. The LM358 needs at least a positive supply of at leas several volts. This is not the same thing as the input signal which is measured in milivolts. RTFDS! That means Read the ****ing Data Sheet.

OOOPS...I see you found the datasheet. OK then.