sensing circuit w/ out comparator

S

SklettTheNewb

Guest
I have created a simple light sensing circuit using 2 voltage dividers
and a comparator. Basically, high resistance and the output from
comparator is high.

I need to explore other options due to the fact the comparator needs
more power than I have available. I would like to make this circuit
with 1.5 volts and I've found a couple LED flash circuits that will
allow me to handle that end w/ 1.5v
(http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#15flash.gif)

but the comparator.... I dunno.

I thought I could use a voltage divider with a transistor, but when I
sat down to actually make this circuit, I realized I couldn't figure it
out.

I've researched op-amps as a comparator but that doesn't seem like a
good fit either.

I would be interested in any ideas that you guys might have.

Sorry to post so many questions here, I've been really active on this
project lately and I'm learning a lot here so it's hard to not come
back ;)

-Steve
 
Thanks Bob! I will try this out tonight, looks interesting.
The only component I don't recognize is the one in parallel to the LED
that says 100u, what is that?

THanks!
 
SklettTheNewb wrote:
I have created a simple light sensing circuit using 2 voltage
dividers
and a comparator. Basically, high resistance and the output from
comparator is high.

I need to explore other options due to the fact the comparator needs
more power than I have available. I would like to make this circuit
with 1.5 volts and I've found a couple LED flash circuits that will
allow me to handle that end w/ 1.5v

(http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#15flash.gif)

but the comparator.... I dunno.

I thought I could use a voltage divider with a transistor, but when I
sat down to actually make this circuit, I realized I couldn't figure
it
out.

I've researched op-amps as a comparator but that doesn't seem like a
good fit either.

I would be interested in any ideas that you guys might have.

Sorry to post so many questions here, I've been really active on this
project lately and I'm learning a lot here so it's hard to not come
back ;)

-Steve
Click to expand...
Hi, Steve. Sorry about the delay. Let's recap the bidding. You've
got a sensor that exhibits a variable resistance between 2K ohms and
64K ohms. When the sensor resistance is greater than 4K, you want an
LED to flash about once a second. The catch here is that you've got a
fresh Duracell AAA battery, and need to be able to have this thing
flash for 45 days with that battery as its only power source.

We talked before about your power requirements, and how a standard
oscillator won't do the job because of excess current which will eat up
the battery. The National Semiconductor LM3909 was made exactly for
flashing a standard LED off a single 1.5V cell with very low power
drain. Unfortunately, they're not making it anymore. However, there
are some parts still available if you look or scrounge, and there's
always the possibility of building one out of discrete components, as
linked in an earlier post.

National published an app note on the LM3909, called AN-154: "1.3V IC
Flasher, Oscillator, Trigger or Alarm"

http://www.fulcrum.ru/Read/CDROMs/NS/AN-154.pdf

On the second and third pages of the app note, it describes the circuit
operation for the basic 1.5V flasher circuit shown in Fig. 2. From
looking at their description and examining the feedback paths, it seems
that the flasher will not oscillate if only we can keep pin 2 (the +
end of the 300uF cap) from exceeding 1V. That shouldn't be too hard,
if we can find a comparator to do the rest of the job. Here's the
circuit (view in fixed font or M$ Notepad):


~~
~~
.--------o-----|<------. .---------.
| | RED LED | | |
| | | | |
| | | | |
| | | | |
| .--o------o------o------o--. |
| | 8 7 6 5 | |
| | | |
| | | +|
| | | ---
| | LM3909 | -
| | | 1.5V|
| | | |
| | | |
| | 1 2 3 4 | |
| '--o------o------o------o--' |
| | | | |
| | 330 | | |
| | uF | | |
| | ||+ | | |
'--------o--||--o '---------'
|| |
|
Mystery Signal |
From Control Ckt |
|
---------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Click to expand...

The only question left is figuring out a way to turn the LM3909 (or the
discrete equivalent) on and off depending on the sensor resistance.
National Semiconductor came to your rescue again with the LM10 (which
is still in production).

http://cache.national.com/ds/LM/LM10.pdf

It's a low power combination op amp and 200mV reference that will work
well on power supplies down to 1.1V. Life is good. Here's the mystery
control circuit:

___
.----|___|---o----------------------------|<------->
| 330K | .-----------------. 1N4001 To
| | | | Flasher
| | | |
| .--------o------o------------. |
| | |6 |8 7| |
| |OpAmp.--|------|-----o------o----|--------.
| | | | | | | | |
| |2 |\| | | |\| Ref | | |
.--|---o---|-\ | '---|-\ Amp 1| |200mV +|
| | | | >-' LM10 | >-----o----o ---
| o---o---|+/ .---|+/ | | -
| | |3 |/| +| |/| | | |1.5V
| | | | --- | | | |
| | | | 200mV - | | | |
| | | | Ref | | | | |
| | | '---------o-----o------o----|--------'
| | | 4| o----.
| | '----------------------------' | |
| | .-. |
| | 10K| | |
| | | | |
| | '-' |20K
| | | .-.
| '-------------------------------------o | |<---.
| | | | |
| .-. '-' |
| Sensor| | | |
| 2K to 64K| | | |
| '-' | |
| | | |
| GND GND |
'---------------------------------------------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


And here's how it works. The 200mV reference part of the LM10 provides
a reference voltage for a voltage divider between the 10K resistor and
your sensor. When your sensor goes above 4K, the VDVR goes above about
57mV. That's the value you've set with the 20K pot. Both of these
voltages are compared by the op amp section of the LM10. The output of
the LM10 goes high when that happens, which means it isn't interfering
with the charging of the 300uF cap, and the flasher does its thing.
When the sensor gets below 4K, the output of the op amp goes low. With
the series diode drop, it will keep the voltage at pin 2 of the LM3909
at around 0.8V, which will prevent oscillation/flashing. The 330K
resistor is added as positive feedback at the non-inverting input to
provide hysteresis, which prevents the in-betweens" from messing things
up.

I guess you're about set here. Even though both ICs are "micropower"
(at least by early 80s standards), you're going to be cutting it a
little close with the 45 day part on one AAA battery. I suppose it
could be tweaked a bit, but I think it will probably be good enough.
As long as you use a Duracell alkaline, you should make it.

Good luck with your project, Steve.
Chris
 
I'm speechless, that is the most awesome experience I've had on the web
w/ maps.google.com following in a close second! :)

Thanks a million, Chris, I will go off this weekend and get everything
I need to prototype this guy.

I'm thinking I will make the LM3909 w/ discreet parts on a separate
breadboard, then make the sensing on another. That way I can get each
one working separatly and then tie them together for the grand finale!

Thanks again, I really appreciate the time you spent on this.

Have a great weekend,
Steve
 
SklettTheNewb wrote:
I'm speechless, that is the most awesome experience I've had on the
web
w/ maps.google.com following in a close second! :)

Thanks a million, Chris, I will go off this weekend and get
everything
I need to prototype this guy.

I'm thinking I will make the LM3909 w/ discreet parts on a separate
breadboard, then make the sensing on another. That way I can get
each
one working separatly and then tie them together for the grand
finale!

Thanks again, I really appreciate the time you spent on this.

Have a great weekend,
Steve
Click to expand...
Slight error -- you need a series resistor with the diode to avoid
overcurrent at the LM10 output:

___ ___
.----|___|---o-|___|----------------------|<------->
| 330K | 100 .-----------------. 1N4001 To
| | | | Flasher
| | | |
| .--------o------o------------. |
| | |6 |8 7| |
| |OpAmp.--|------|-----o------o----|--------.
| | | | | | | | |
| |2 |\| | | |\| Ref | | |
.--|---o---|-\ | '---|-\ Amp 1| |200mV +|
| | | | >-' LM10 | >-----o----o ---
| o---o---|+/ .---|+/ | | -
| | |3 |/| +| |/| | | |1.5V
| | | | --- | | | |
| | | | 200mV - | | | |
| | | | Ref | | | | |
| | | '---------o-----o------o----|--------'
| | | 4| o----.
| | '----------------------------' | |
| | .-. |
| | 10K| | |
| | | | |
| | '-' |20K
| | | .-.
| '-------------------------------------o | |<---.
| | | | |
| .-. '-' |
| Sensor| | | |
| 2K to 64K| | | |
| '-' | |
| | | |
| GND GND |
'---------------------------------------------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Good luck
Chris
 
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