LM35 opamp LM358 relay temp CTRL´d for INCUBATOR

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
You need to feed the Lm35 into pin 2 of the Lm358, and the temperature reference on pin 3, using an npn transistor on pin 1, then as the temperature rises above the level, the output will turn off the relay, and cool down to the lower level, then switch on, voltage levels are 375mV on 380mV off,

the hysteresis is only 5mV.

thats. 37.5C to 38C
Yes Sir that´s it, no more than 1 degree up/down, thanks i´ll work on that know.
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71

wayneh

Joined Sep 9, 2010
17,496
Awesome Wayneh, truly i need to understand and manage better the Ohms law since i´m not a professional but a hobbyist or enthusiast but using what i´m trying to do, this is not for playing games, i´ll try to find a MOSFET now to test what you advice, now wich pin config you think i should use from the sensor to opamp (LM358) in my case, cause most project use LM741 or any other, i´ve allreqady have this one and don´t wanna waste it, any other advice wi´ll be welcome.
A common and widely available MOSFET is the IRF540 or IRF 510. The latter was carried at Radio Shack, if you happen to be near one that is still open. These are both old and more advanced ones are available, so consider them just an example. Either would be overkill for switching 100mA.

Speaking of old, the LM741 is about the oldest and least advance op amp you can find. Forget about it and move on. The LM358 is slightly less ancient but will be fine for this application. It's worth noting, though, that you are using it as a comparator (output is on or off, not in between) and there are purpose-built comparators such as LM339 (also widely available) which are superior for this function. I wouldn't say you should switch away from the LM358, since you already have it, but you should be aware of the situation.
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
A common and widely available MOSFET is the IRF540 or IRF 510. The latter was carried at Radio Shack, if you happen to be near one that is still open. These are both old and more advanced ones are available, so consider them just an example. Either would be overkill for switching 100mA.

Speaking of old, the LM741 is about the oldest and least advance op amp you can find. Forget about it and move on. The LM358 is slightly less ancient but will be fine for this application. It's worth noting, though, that you are using it as a comparator (output is on or off, not in between) and there are purpose-built comparators such as LM339 (also widely available) which are superior for this function. I wouldn't say you should switch away from the LM358, since you already have it, but you should be aware of the situation.
Thnkas a lot man for your advice, i´ll take in count all of this ahead.
Ricky, there is a dedicated chip for heating applications using a triac and thermistor its from Temic the U217b, accurate to 0.5C .
Man this U217b is simply awesome as your recomendation, i will be not afraid to move in another direction less complicated and still efficient yet, my only concern is if i will find this at the shop near me, since i live outside the US or EUROPE, so the first thing i have to ensure is to find this ic the rest is common stuff so i will finish with the components i have allready and the rest i will see then, many thanks for your attention friend. i´m taking note of everything.
 

wayneh

Joined Sep 9, 2010
17,496
There are indeed a lot of ways to skin the cat. My food dehydrator is similar to an incubator and it contains a very simple circuit for controlling an AC powered heating element. The circuit is based on Motorola's CA3059 IC, which unfortunately is obsolete but has been replaced by the Atmel T2117 which is very similar to the U217b.

Anyway, it's designed for direct (through a resistor) connection to the mains and controls a triac based on detecting zero-voltage crossings. It makes its own DC supply. A thermistor senses temp and turns the heater on or off. All of this is handled by the IC, so the PCB is quite simple. There's a thermal fuse that will trigger if the temp rises to about 20°F above normal range. (I think that would be a good addition to your incubator. You might want a cutout at just a few degrees above target, to avoid cooking your chicks.)

Thermostat and a lightbulb is the time-tested solution. Anything more is just showing off.
 

Dodgydave

Joined Jun 22, 2012
11,285
Hey Wayneh, that chip is the same as the Temic U217b i quoted in post #25 ,

prob made in China and sold to major chip companies....
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
Ricky, the U217b is on Ebay
Yes i just checked on that and then called the shop near my house they don even have that code in theyr system hahaha i couldn spect more.

ok so far i put everything togheter with the lm358 and the npn transistor and the relay is working fine but i have chatter, the circuit does lik a dimming while it goes off/on stage, i guess need some hysteresis or something to prevent from that it will damage the relay, and now the opamp is getting hot. i´m working 12v 1.5A supply.
 
Last edited:

Dodgydave

Joined Jun 22, 2012
11,285
The ic is warm because of the rapid pulsing of the output, caused by no hysteresis resistor, also make sure you have a diode across the relay coil to prevent back Emf.

Did you put the sensor to pin 2 as i said, also can you redraw the circuit and post it to see what you have done?
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
There are indeed a lot of ways to skin the cat. My food dehydrator is similar to an incubator and it contains a very simple circuit for controlling an AC powered heating element. The circuit is based on Motorola's CA3059 IC, which unfortunately is obsolete but has been replaced by the Atmel T2117 which is very similar to the U217b.

Anyway, it's designed for direct (through a resistor) connection to the mains and controls a triac based on detecting zero-voltage crossings. It makes its own DC supply. A thermistor senses temp and turns the heater on or off. All of this is handled by the IC, so the PCB is quite simple. There's a thermal fuse that will trigger if the temp rises to about 20°F above normal range. (I think that would be a good addition to your incubator. You might want a cutout at just a few degrees above target, to avoid cooking your chicks.)

Thermostat and a lightbulb is the time-tested solution. Anything more is just showing off.
Very nice Wayneh, i´m just there to get this done, few fixings needed to finish. i´m getting some chatt in teh relay as the ceircuit goes off and back on, thats the only thing need to be fix the previous versions i did dind´t do that so i almost know what´s the solution, anyway your advice will be welcome, thanks.
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
The ic is warm because of the rapid pulsing of the output, caused by no hysteresis resistor, also make sure you have a diode across the relay coil to prevent back Emf.

Did you put the sensor to pin 2 as i said, also can you redraw the circuit and post it to see what you have done?
Yes i have allready a diode peralleled to the relay i read enough about it, very important and thatnks for remark me about it, ok i´ll try the resistor indeed i got that in previous version of the circuit, i´ll get back with that and check, almost done, cheers.
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
The ic is warm because of the rapid pulsing of the output, caused by no hysteresis resistor, also make sure you have a diode across the relay coil to prevent back Emf.

Did you put the sensor to pin 2 as i said, also can you redraw the circuit and post it to see what you have done?
Yes i did connect everything as you said my friend. attached is the new circuit
TempSenRelay2.JPG
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
The ic is warm because of the rapid pulsing of the output, caused by no hysteresis resistor, also make sure you have a diode across the relay coil to prevent back Emf.

Did you put the sensor to pin 2 as i said, also can you redraw the circuit and post it to see what you have done?
I´ts not warming anymore is fine, now just need to fix the dimming efect cause its make chatter the relay.
 

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
Hysteresis, put a 470K to 1 M resistor across pins 1,3.
ok man, i did test with 500K, 1 and 4.7 M and less than before still doing chatt, addtional the transistor is getting hot now remember it goes straigh to the relay and this one draw 100mA do i need a resistor there?.
 

Dodgydave

Joined Jun 22, 2012
11,285
remove R4, and make R1 a 100K, this will give you upto 2 volts on pin 3, also make the hysteresis resistor 500K, that should give you an hysteresis voltage of 7mV.
 
Last edited:

Thread Starter

ricky diaz

Joined Feb 16, 2015
71
remove R4, and make R1 a 100K, this will give you upto 2 volts on pin 3, also make the hysteresis resistor 500K, that should give you an hysteresis voltage of 7mV.
My friend so far this is what i have, replaced all components as u toll me, once tested i´ve found 2.5M on R1 is the most suitable value for my necesities (almost 1 deg), but relay still chattering, this task is almost done just this little issue must be fixed, any advice as always welcome, attached the new draw.
tempsenrelay3.JPG
 
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