Thermostat with lm35: help finding out the error

[joerack]

Hello all

I'm trying to build a thermostat with 2 opamps, the first amplifies the analog voltage and the second as a comparator. the comparator should give me a v output whenever the ref (coming from the voltage divider) is higher than what's coming out from my Lm358 configured as hysteresis comp

so the first part of the circuit works, but measuring from the lm393 I'm always getting -9v

please help

https://www.dropbox.com/s/utah1nbrmgwtfxd/IMG_20160614_092224.jpg?dl=0

 

[(*steve*)]

can you post the schematic here? I can see it on dropbox but I can't zoom in on it to see what's going on.

 

[joerack]

done

 

[joerack]

I inverted a polarity (duh!)

so I'm getting a constant +4/5v from the first opamp, and a constant 8.45 from the second (obs: I'm not using a 1m pot as in the scheme, but a 100k)
I need to make the comparation work so that when the temp is lower than the voltage divider, nothing comes out of the lm393

 

[(*steve*)]

Remember that the LM393 has an open collector output. You will need to add a 2.2k resistor (or similar) from the output to the +ve supply to enable the output to go high.

 

[hevans1944]

Expanding on Steve's comment in post #5: The LM393 has an open-collector output that will sink (typically) 16 mA when the output is "low" and will present an open-circuit when the output is "high". To drive the relay coil, you connect one side of the coil to +V and the other side to the LM393 output. A reverse-biased diode across the relay coil is recommended to absorb back-emf produced when the coil is turned off, i.e., LM393 output "high". Make sure your relay coil will operate on 9 V DC at less than 16 mA actuation current!

 

[AnalogKid]

As noted, a 393 has an open collector output that can only sink current, not source it.

1. IF (big if) the relay coil current is less than 50% of the 393 max rated output current, then it is safe to have the 393 drive the relay coil directly (with an added suppression diode across the coil. IF NOT, then an external driver transistor is needed, and this will change the polarity of the input connections. So, figure out the output first and post an updated schematic, then the input can be straightened out.

ak