I have a basic heater,which has only two options-ON and OFF.
I want the heater to work only between two set temperatures,L and H.
I came across the age old Wheatstone bridge to measure unknown resistance.
Can a modification of this idea be used to get one, I am looking for?
Here is my thinking.Replace two 'arms' of the bridge by LM35,with the output being joined to the middle of the other two 'arms'. These two arms form a voltage divider. Adjust two resistances so that the output voltage matches with the voltage output of LM35 at ,say L .Depending upon the LM35 temperature, there will be current in one or other direction, when it becomes unbalanced. Use this to trigger, using NPN transistor, to start or shut off the heater. One would need to use 555 for latching.
I did try to get unbalance Wheatstone bridge with LM35 as two arms,using .220V=L and I could get voltage at the middle.But I couldn't get LED to glow.I am using BC547 and diagram is as follows:

Presently temperature is at 22c so I have adjusted R1 and R2 so that ratio is above 220. There is current toward LM35. My non knowledge of fixing R3 is probably(the charitable!) is reason.The other is --- it is not a workable sketch!
Please opine.
suresh paranjape

 

You don't have enough gain and the transistor bias is wrong in that circuit. What you need is a differential amplifier or an analog comparator circuit.

 

i would use a window comparator,set the upper and lower limits with the resistors, R1,R3, your Lm35 replaces the VR1, ......http://home.cogeco.ca/~rpaisley4/ComparatorWindow.GIF

 

Search "lm35 datasheet" I google. This device is made for use in thermometer and thermostat applications. There is an example circuit on the datasheet. If you don't have success there search on TI.com.

 

I'm good with op-amps, so this is how I do it. The direction and amount of "on" band is controlled with the 1N4001 and 680k on the right side.

http://forum.allaboutcircuits.com/blog/lm723-as-a-thermostat.532/

 

I want the heater to work only between two set temperatures,L and H.

Really? You want the heater to turn OFF when the temperature falls too low?

It's more normal to use a thermostat that is always on when the temperature is below the set point, and always off when above it. I've used a comparator with the LM35 for this purpose, and it works very well. The comparator controls a MOSFET switch, and from there you can control a relay or whatever your heater requires.

You can control the hysteresis to several degrees if you want, or reduce it to ±0.1°C or so if you add an op-amp to amplify the LM35 output a bit.

 

Thanks to all members for commenting on my simple project.
About Mr(MrChips) comment: I couldn't get /understand from the datasheet of LM35 whether I can get enough base current for the transistor to close.It appears to me from your comment that the difference between voltages at two ends wouldn't be enough to get 'sizable' current for transistor to operate.
About Mr(Dodgydave) comment:I came across the term 'a window comparator'. Thanks for the link. I had asked one website moderator to go over a circuit that I thought could work. Since I am just beginner, I sought his opnion.He opined that the circuit looks ok;however my requirement of controlling between 20c to 25 c may not be feasbile.I below give the circuit that I had done below.


May be I need to use another IC to amplify LM35 output before inputs to 741 above.
I do thank all others again for their help for a beginner like me.
I am sorry that my sentence "I want the heater to work only between two set temperatures,L and H." did convey the impression that I do not want heater on below L!. When I wrote that sentence, I had back in mind that the moment it goes below the heater would start.
It is very heartening to get such nice suggestions from unknown people! Long Live This Spirit !
suresh paranjape

 

your window comparator has no reference voltages on pins 2,3 of the 741 op amps, the presets are permanently at V+, they need resistors to pin 4.

 

That's more complicated than you need. Just use a comparator (I use LM339, which has 4 comparators in each package) with hysteresis. The data sheet will show you how. You don't need an op-amp unless you need precision better than ~0.5°C.

 

Hello,

You might want to have a look at the following page:
http://electronics-diy.com/electronic_schematic.php?id=1043

Bertus

 

Just use a comparator with hysteresis. You don't need an op-amp unless you need precision better than ~0.5°C.

That's about what I did with the LM723, except I used a thermistor instead of an LM35. Most of the local denizens could adapt that circuit to an LM35 in about 20 minutes, but a beginner might find it very difficult.