I made a digital temperature sensor based on this circuit. Components are as follows:

• LM358 IC
• Resistance Temperature Detector (unknown resistance but it's a small glass-type thermistor from a device that heated up to 250C/482F)
• 10K Pot
• LED
• 330 Ohm Resistor
• 10K Resistor
• 18650 battery (4.2v fully charged according to my multimeter)

The diagram below hopefully represents the instructions given at the aforementioned link:

I have heat source that I want to run off the same 18650. When the heat source is plugged in the temperature sensor does not work (the voltage going to the temperature sensor circuit drops to ~2v). Everything works fine if I use two individual power sources.

Questions:
1. To get the sensor and the heat source to run from the same 18650 battery, is it simply a matter of using different resistors? If so, what resistor values are required?
2. If the resistors aren't the issue, what options are there to get both the temperature sensor and the heat source to run off the same battery?
3. Is not knowing the rating of the thermistor a major issue (trying to keep wastage and costs down)?

 

Welcome to AAC
Post the schematic please.
What is the heat source?

 

The heater is too large a load for that battery. You cannot solve that except to use a different heater or more (or separate) power.

 

Everything works fine if I use two individual power sources.

What is the separate power source for the heater? That source should be able to power the Temp Sensor as well.
This is the schematic for the Temp Sensor according to your link, max current draw around 12 ma.

 

What is the separate power source for the heater? That source should be able to power the Temp Sensor as well.
This is the schematic for the Temp Sensor according to your link, max current draw around 12 ma.
View attachment 230642

I ran the heat source off the battery and the sensor off a 4.5v/1.5a dc adaptor. The adaptor didn't have enough amps to drive the heat source. Thanks for the accurate schmatic, does it matter where the heat source is connected? Thanks again

 

The heater is too large a load for that battery. You cannot solve that except to use a different heater or more (or separate) power.

The orignal sensor and the heat source came out of the same device. The battery ran both just fine.

 

If the battery can power the heat source it certainly can power both unless you have something connected incorrectly.

 

Welcome to AAC
Post the schematic please.
What is the heat source?

Schematic attached, thanks to sghioto
The heat source is a small ceramic element type thing out of small dc heater

 

How are you connecting the heater element to the sensor?

 

If the battery can power the heat source it certainly can power both unless you have something connected incorrectly.

I had wired it incorrectly as I couldn't tell from the photos. Your schematic made it clear, thanks again.

 

How are you connecting the heater element to the sensor?

It wasn't the heater element connection causing the issue, I had the resistors around the wrong way (330 where the 10k should have been and vice versa) and the sensor wires weren't making consistent contacts. All working from one power source now. Sorry for the inconvenience, my inexperience made it hard to interpret the correct schematic from those low-res images on that Instructables page.

 

I saw LM358 IC and battery 4.2v and my first though is what is the working voltage range of that opamp?

Checking the datasheet, I see the common mode input range for 5 volt operation (close to 4.2) is 0 to V+ -2 volts.
So your opamp inputs need to be between zero and 2.2 volts.