Below temperature circuits with leds

[Xena E]

Check your connections and polaritiy of the diodes and red LED carefully.

Where are you getting negative votages from?

X

[Zero999]

Thanks, but no the leds don't change state when I adjust the voltage on PIn2. When the voltage on Pin2 is at 2.471V  D1 is off and D2 is on. When I increase the voltage to Pin2 , to 2.487V, D1 comes on, but D2 stays on, but not full brightness, as it seems to dim somewhat.
Also, as the temperature has increased by 0.6C, so does the output voltage from U3 decreased by about 0.25V

The current temperature is now 18.9C.

Update:
With the current temperature now at 19.5C, when I adjust the voltage so that D1 lights up and D2 is a little dimmer, the voltage on pin6 goes down from 7.47V to 2.63V

I would be interested to know now, at what temperature D1 comes on and D2 goes off.

The voltage drop on the LM741's output is higher than I thought,. Try adding a diode in series with D2.

The device you are using (741?) can't swing its output fully to the 0 volt rail whilst sinking current. If you want to have LEDs lit one at a time it won't be possible using that circuit.

Hint: Consider making use of the different forward voltages of the LEDs  to effect the switching.

X

I am using a LM741, but what other device do you recommend then. I have LM1458, LM324, LM358, LM4562, LM339 and a CA3130

Sorry, but how do I 'making use of the different forward voltages of the LEDs  to effect the switching'. Please explain to an amateur .

Thanks

Or use the LM358. That avoids having to mess around with lifting the LM35's ground.

[RobH]

Thanks for the new circuit, and I've now put it together.

Observations:
Both D1 and D2 are on all the time, and adjusting R2 does not turn either of them off
Turning R2 down to 0V gives a reading of 0.204V from  the LM35, which is just above what I would expect at a current temperature of 19C

The voltages on Pin1 of U3a is 5.28V and on Pin7 of U3b is 7.50V
The voltage at R1 and R2 is 0V, but can be adjusted.

I've tried x3 LM358's in case 1 was faulty, but they were not.

Again, thanks for your time and help.

[Xena E]

This is intriguing.
I spent a few minutes "breadboarding" the convoluted LED driver from 741 circuit I posted, and that worked as I expected.

Now we are on a different circuit?

OK, it's a wet horrible day outside I'm up with my chores, so I'll play along and do this one as well...

Question: what is the forward voltage of the two LEDs you're using? (Measure across them when they are on).

Can you post a photo of the latest circuit as assembled?

X

[Xena E]

OK.

Zero999 circuit with the 358 works exactly as intended with the caveat that at exactly the threshold point both LEDs are lit. That could be cured with a little hysteresis.

 

[RobH]

Well if the 'the convoluted LED driver from 741 circuit ' worked for you, I must have made some error somewhere but I have dismantled it now.

Yes it's also very wet and horrible here with about 50cms of snow lying.

The forward voltage of D1 is 2.71V  and D2 is 1.95V

Picture attached as requested. Please note that R5 was or is 100 ohm in the picture, but it is now 1K. I only realised when I looked at the picture close up!
The voltage readings are correct for the R5 resistor now.

Just saw your post about hysteresis, could you please advise on how little
Thanks

[Xena E]

Theres no connection to pin 4 of the 358 for a start...

X

[RobH]

Oops, no there isn't but now there is, my bad, doh!!

Now only D2 is lit. Also adjusting the pot doesn't change the voltage on Pin2 as it is constant at 0V.
I am getting 0.216V on Pin3 from the LM35.

If I use Pin1 for the output on zero999 circuit with a LM358, which pins are used for hysterisis. Or should I use Pin7.

Out of interest, which led , D1 or D2, is lit for the temperature I want it.

thanks

[Xena E]

Check your wiring around the pot.

D1 is lit when the LM35 is over set temperature, D2 is lit when the LM35 is is under set temperature.

Please don't ask how to swap which colour led is lit 

[RobH]

The wiring is ok round the pot, and it is because of the angle of the picture which makes it look like the wires are wrongly placed. Apologies for that.

The voltage  on Pin3 from the LM35 is 0.228V, which from what I read last week somewhere that the LM35  puts out? roughly 0.010V for each 1C, so 0.228V would mean its 22.8C, but it isn't. It is currently 20C

How do set a temperature for the LM35 , ie 21C, so D1 comes on and D2 goes off.

thanks

[Xena E]

The wiring is ok round the pot

The black wire to 0V and the white wire both go to connection strip 9. Therefore are shorted out? Thats why you have 0V at the wiper. it is the only explanation

The voltage  on Pin3 from the LM35 is 0.228V, which from what I read last week somewhere that the LM35  puts out? roughly 0.010V for each 1C, so 0.228V would mean its 22.8C, but it isn't. It is currently 20C

Tolerance of the device and your meter?

How do set a temperature for the LM35 , ie 21C, so D1 comes on and D2 goes off.

Already answered: get the circuit working and go from there.

X

[RobH]

I made some alterations with the circuit:
1st I swapped what I thought was a 3k pot , but  measured as a 5k, for a 1k then added a 1k resistor to give 2k.

Now the voltages are:
Pin2, between 2.38V and 2.6V
Pin3 reads 0.223V at a current temperature of 20.2C. The said temperature is measured on or from a Davies weather station, which reads internal and external temperatures. Using a normal indoor digital thermometer, it says 21C, so I'll live with that so to speak.

Also now, only D1 is lit and D2 is off, but I want to see what is lit when the temperature drops a few degrees.

Yes I found where you answered about toggling the comparator. Googling it says to change the input voltage , VIN, which varies between 2.38V and 2.6V. is it a case of lower the voltage, then lower the temperature threshold

Thanks

[Xena E]

I made some alterations with the circuit:
1st I swapped what I thought was a 3k pot , but  measured as a 5k, for a 1k then added a 1k resistor to give 2k.

If, you only have a 1k pot, just wire it as Zero999 diagram and change the R1 to 4k7.

Again.
The output LEDs will toggle from one to the other when the voltage at the pot wiper is matched by The LM35.
X

[RobH]

 If 'The output LEDs will toggle from one to the other when the voltage at the pot wiper is matched by The LM35.', then that would mean changing the resistor value which I added to R2 somewhat, or changing R2 for a 5K or even a 10K pot.

I say this because with a 1k resistor added to my R2, 1k,  the voltage at  pin3 is between 2.38V and 2.6V while the voltage on pin3 from the LM35 is just 0.202V with a current temperature of 19C.

I then changed the 1K resistor from 1K  to 2K, and the voltage at pin2 is now between 0.775v and 1.1V.

Does that sound right or am I off track somewhere.

Thanks

[SteveThackery]

@RobH: it's getting quite tricky to follow exactly where you are. Could you remind us which circuit you are using, and what the various component values are? Maybe repost the schematic, updated with the current values. Also, please put pin numbers on the LM358s. That will give is an easy reference point.

Second question: do you have an oscilloscope?

[Xena E]

Again

If, you only have a 1k pot, just wire it as Zero999 diagram and change the R1 to 4k7.


The output LEDs will toggle from one to the other when the voltage at the pot wiper is matched by The LM35.

X

[Xena E]

 

[RobH]

Yes, I tried with the 1k pot and a 4.7K resistor, but not quite as the circuit. I put the resistor to GND and adjustment was very very fine, so much so the figures were jumping up and down too much. I then tried a 10K pot, but adjustment , again, was all over the place. Anyway, I've now wired it as per the circuit and it is much better, adjustment wise.

The strange thing is , I've noticed today, is that while the LM35 is giving 0.209V and the current temperature is just 18C, if I adjust the input voltage to anything more than 0.184V D1 comes on and D2 goes off. So I have  the voltage in , VIN, to 0.184V and D2 is on, which is what I want.

thanks

[SteveThackery]

The strange thing is , I've noticed today, is that while the LM35 is giving 0.209V and the current temperature is just 18C, if I adjust the input voltage to anything more than 0.184V D1 comes on and D2 goes off. So I have  the voltage in , VIN, to 0.184V and D2 is on, which is what I want.

So does that mean it's all working now? Can you switch between the red and green LEDs just by adjusting the pot?

[Xena E]

.....
which is what I want.

thanks

TFFT.

Regards,
X

[RobH]

Yes it is working as I can switch between D1 and D2 with the pot.

Now I'm going to try other circuits which would? give the same result.

Thanks again