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?
---
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."