Re: Comparator for sensor
On Tue, 31 Jan 2006 15:24:24 -0500, Manimozhi Baskaran
<baskaran@xxxxxxxxxxxxxxxxxx> wrote:
>John Fields wrote:
>
>>On Tue, 31 Jan 2006 10:20:28 -0500, Manimozhi Baskaran
>><baskaran@xxxxxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>>>Hi,
>>>
>>>I am building an application that needs to beep when the voltage output
>>>
>>>
>>>from the sensor device is less than 0.8 Volts. I am not sure what would
>>
>>
>>>be the right design that I can use for the buzzer to beep. The voltage
>>>for the sensor is 5 V DC supply. Should I consider using 741/LM series?
>>>
>>>
>>
>>---
>>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.
>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.
---
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
.
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