Comparator for sensor

The dual comparator, LM393 has inputs that work all the way to the negative supply rail, so it should detect .8 volts just fine. These comparators have an open collector output that turns on a connection to th negative rail when the - input is more positive than the + input.

--- No. Consider using a real comparator, like John Popelish suggested.

However, an LM393 may not be able to give you enough output current to run your beeper, so if you could post what its voltage and current requirements are we can suggest an appropriate means of driving it. Also, if you could tell us what you're using for a sensor we'll be able to suggest how to set up a proper "front end" and provide any hysteresis the circuit may require.

-- John Fields Professional Circuit Designer

My Infra Red sensor will detect the temperature range of -40 degree to

+60 degree centigrade variation with:

-40 degree to output a typical voltage of < 0.21 V -20 degree to output a typical voltage of 0.21V,

-15 degree to a typical voltage of 0.3 V etc.,

I like my sensor to detect only the temperature range -40 degree to -5 degree range which would typically vary between 0. 2 V to 0.8 Volts. The typical supply voltage to the sensor will be 5V and its supply current will be 2 mA. The output of the sensor will be anywhere between 0.25 Volts to 4.75 Volts. The output current will be less than 2mA. What type of circuit design should I use in order to detect the range so that it can beep when the voltage is less than 0.8 V which I believe is what I should use as my reference voltage.

Any assistance will be appreciated.

Thanks Mani

John Fields wrote:

--- Please bottom-post.

I'm confused.

In your first post you said that you wanted the circuit to detect when the sensor's output voltage was less than 0.8V, but now you say you want the circuit to detect only the range between -40C to -5C, which corresponds to the voltage range of 0.2V to 0.8V.

I take that to mean that if the sensor's output voltage is less than

0.2V or greater than 0.8V you don't want the beeper to beep.

You also say that the sensor's output will vary from 0.25V to 4.75V, which means that it will never be able to get to 0.2V.

If, since you can't get to 0.2V, all you need is something which can detect an output voltage of less than 0.8V from the sensor, you could do something like this:

+5V>---------+---------------+--------+----------+ | | | | [1K2] | [10K] | | | | | | +---|--[1M]--+ | Rin | | | Rfb | | VIN>--[10K]--------------+--|+\\ | E | | >------+--[1K]--B 2N4403 +--[1690]---+--|-/ LM393 C |K | | |+ [LM385-2.5] [806] | [BEEPER] | | | | GND>---------+-----------+---+-------------------+

Rfb is used to provide positive feedback ("hysteresis", in this case) around the switching point, which will keep the output of the comparator from chattering if the input rise and fall times are slow. With the values given for Rin and Rfb, that hysteresis will correspond to about 1% of 0.8V, or 8mv, which should be enough if there's not a lot of noise on the input signal.

If there is a lot of noise on the input signal, try bypassing VIN to GND with an "appropriately" sized capacitor.

-- John Fields Professional Circuit Designer

at that voltage, a simple drive to the base of an NPN via a resistor would work to clamp down on a PNP base driven circuit.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

741's won't run off a single sided 5V supply, but you could do it using 1/2 a LM324 qaud op-amp to drive a piezo speaker.

There's a dual version of the 324 too, but I forget it's number, and anyway, 324's are dirt cheap. possibly something like this: ---+-------+----------+----------- +5 | | | | | 100K | | | | | +-----[22K]-------+ 560K | | 1/4 | | |\\| | |\\ 324 | (A)+-----|+\\ +------|+\\ | in | | >--. | | >-----+-+--|| -------|-----|-/ `-|

--
Unfortunately, there\'s no guarantee that would switch at precisely
0.8V

--
Using the 5% (carbon film?) resistors you\'ve chosen for the
reference divider will result in an uncertainty of the switching
threshold with the low end being at 0.755V and the high end being at
0.892V.

Switching the 560K to 4220 +/- 1% and the 110K to 806 +/- 1% , both
metal film, will tighten up the error band by bringing up the lower
end to 0.788V, the high end to 0.815V, and lowering the tempco.

In actuality, a small pot would be a better choice for tempco, since
with a resistive element more likely to be isothermal than two
resistors, the drifts would track (in the pot) and the ratio between
the top and bottom resistors woulod stay the same, causing the
reference voltage to remain the same as long as the supply stayed
constant.

Hi John,

I believe that I missed your original msg on the thread. I am unable to determine which is the right comparator chip to use for my specifications. I started with 741 then look at LM393 suggested by John P. Also investigated on to LM324 (as Jasen had suggested). But I believe that I missed John F. idea on the thread? What would be the right option to go?

Thanks,

John Fields wrote:

--- The circuit you should use depends on how accurately you want to detect the -5C point (0.8V) and how much current your beeper needs. Can you supply that data?

Here's a copy of my earlier post:

"Please bottom-post.

I'm confused.

In your first post you said that you wanted the circuit to detect when the sensor's output voltage was less than 0.8V, but now you say you want the circuit to detect only the range between -40C to -5C, which corresponds to the voltage range of 0.2V to 0.8V.

I take that to mean that if the sensor's output voltage is less than

0.2V or greater than 0.8V you don't want the beeper to beep.

You also say that the sensor's output will vary from 0.25V to 4.75V, which means that it will never be able to get to 0.2V.

If, since you can't get to 0.2V, all you need is something which can detect an output voltage of less than 0.8V from the sensor, you could do something like this:

+5V>---------+---------------+--------+----------+ | | | | [1K2] | [10K] | | | | | | +---|--[1M]--+ | Rin | | | Rfb | | VIN>--[10K]--------------+--|+\\ | E | | >------+--[1K]--B 2N4403 +--[1690]---+--|-/ LM393 C |K | | |+ [LM385-2.5] [806] | [BEEPER] | | | | GND>---------+-----------+---+-------------------+

Rfb is used to provide positive feedback ("hysteresis", in this case) around the switching point, which will keep the output of the comparator from chattering if the input rise and fall times are slow. With the values given for Rin and Rfb, that hysteresis will correspond to about 1% of 0.8V, or 8mv, which should be enough if there's not a lot of noise on the input signal.

If there is a lot of noise on the input signal, try bypassing VIN to GND with an "appropriately" sized capacitor."

-- John Fields Professional Circuit Designer

Thanks John for reposting this.

Can you please clarify this?

1690 - is this 1690 ohms across [LM385-2.5]? Is LM385 a diode to keep the noise/power more stable? 806 - is that 806 ohms across the - ve and ground on LM393?

Regards, Mani

John Fields wrote:

--- I keep asking you to bottom post, as is the custom here, but you persist in top posting, and I keep asking for the accuracy you want when detecting the -5C point and the current required for your beeper to work but, for some reason, you refuse to supply that data.

If you will reformulate your post with your questions at the bottom of the post you're replying to, and answer my questions, then I will answer yours.

Otherwise, perhaps someone with more patience than I have at this point will do you the courtesy.

-- John Fields Professional Circuit Designer