Simple Lie Detector with Digital Panel Meter

[Markan]

Hi

I am looking for a simple lie detector cicruit for my son's science
project. We have a Digital Panel Meter with a 0-200Mv range. The resistance
we would like to measure would be about 0.2 - 2Mohms.

This does not need to be accurate. Help would be appreciated on how to find
a suitable circuit diagram.

Thanks

Markan

 

[Robert Monsen]

Markan wrote:

Hi

I am looking for a simple lie detector cicruit for my son's science
project. We have a Digital Panel Meter with a 0-200Mv range. The resistance
we would like to measure would be about 0.2 - 2Mohms.

This does not need to be accurate. Help would be appreciated on how to find
a suitable circuit diagram.

Thanks

Markan

If you put a 10M resistor in series, and use a 1.2V source (actually, a
1.5V battery would work fine), you'll get 200mV when the resistance is
2M, and about 23mV when it's at 200k ohms. That sounds about right.

However, your digital panel meter may have a low resistance itself, thus
messing with the reading. Hopefully, it will have a 10M or higher
resistance, so it won't interfere too much. If it interferes too much,
come back and somebody will help you build a buffer to measure the
voltage with.

HOwever, I think this circuit will be fine. Use courier font, or other
monospaced font to view.

10MEG
___ +
.--|___|---o------o Probe1
| |
| |
| |
| / \ Meter
1.5V --- (_/_)
- \_/
| | -
'----------o------o Probe2

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

If you need to power the meter, use separate batteries.

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[Markan]

Robert Monsen <rcsurname@comcast.net> wrote in
news:XNqdnVWZ2Yr2K23cRVn-vg@comcast.com:

Markan wrote:
Hi

I am looking for a simple lie detector cicruit for my son's science
project. We have a Digital Panel Meter with a 0-200Mv range. The
resistance we would like to measure would be about 0.2 - 2Mohms.

This does not need to be accurate. Help would be appreciated on how
to find a suitable circuit diagram.

Thanks

Markan

If you put a 10M resistor in series, and use a 1.2V source (actually,
a 1.5V battery would work fine), you'll get 200mV when the resistance
is 2M, and about 23mV when it's at 200k ohms. That sounds about right.

However, your digital panel meter may have a low resistance itself,
thus messing with the reading. Hopefully, it will have a 10M or higher
resistance, so it won't interfere too much. If it interferes too much,
come back and somebody will help you build a buffer to measure the
voltage with.

HOwever, I think this circuit will be fine. Use courier font, or other
monospaced font to view.

10MEG
___ +
.--|___|---o------o Probe1
| |
| |
| |
| / \ Meter
1.5V --- (_/_)
- \_/
| | -
'----------o------o Probe2

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

If you need to power the meter, use separate batteries.

Robert

Thank you this works great!

Mark