Light sensor?

Posted by on November 24, 2008, 9:48 am

I have a tiny little robot which is sold by Radio Shack. It looks like a
bug, and has two sensors which make it turn when it contacts a vertical
surface.

It lives in my cupboard, but the battery is dead, presumably because one
of the kids left the power switch on.

Is there any hope of my devising a subminiature light sensor and switch
that I can insert into the circuit path, so that when the cabinet door
closes, the power in the robot will go off?

Any suggestions?

--
The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts.
-- Bertrand Russel

Posted by GregS on November 24, 2008, 10:20 am

>I have a tiny little robot which is sold by Radio Shack. It looks like a
>bug, and has two sensors which make it turn when it contacts a vertical
>surface.
>It lives in my cupboard, but the battery is dead, presumably because one
>of the kids left the power switch on.
>Is there any hope of my devising a subminiature light sensor and switch
>that I can insert into the circuit path, so that when the cabinet door
>closes, the power in the robot will go off?

A very sensitive relay, perhaps unavailable, could run off a large photodiode
or solar cell. Most trip circuits require power. Just put on an LED
to indicate power on.

greg

Posted by Uniden on November 24, 2008, 11:52 am

zekfrivo@zekfrivolous.com says...
wrote:

> >I have a tiny little robot which is sold by Radio Shack. It looks like a
> >bug, and has two sensors which make it turn when it contacts a vertical
> >surface.
> >It lives in my cupboard, but the battery is dead, presumably because one
> >of the kids left the power switch on.
> >Is there any hope of my devising a subminiature light sensor and switch
> >that I can insert into the circuit path, so that when the cabinet door
> >closes, the power in the robot will go off?

>
> A very sensitive relay, perhaps unavailable, could run off a large photodiode
> or solar cell. Most trip circuits require power. Just put on an LED
> to indicate power on.

Put an IR LED on the robot modulated at 40 kHz (perhaps us a 555 timer).
The LED is on when the power is on. Put an IR receiver module (like
used with remotes) in the cabinet and when that receiver detects an
"on" robot signal (the modulated IR) the receiver drives a loud piezo
alarm.

:-)

Posted by Eddy Wright on November 27, 2008, 11:27 am

On Nov 24, 8:48 am, EskWI...@spamblock.panix.com wrote:

> I have a tiny little robot which is sold by Radio Shack. It looks like a
> bug, and has two sensors which make it turn when it contacts a vertical
> surface.
> It lives in my cupboard, but the battery is dead, presumably because one
> of the kids left the power switch on.
> Is there any hope of my devising a subminiature light sensor and switch
> that I can insert into the circuit path, so that when the cabinet door
> closes, the power in the robot will go off?
> Any suggestions?
> --
> The whole problem with the world is that fools and fanatics are always so
> certain of themselves, but wiser people so full of doubts.
> -- Bertrand Russel

Here is a simple circuit using a small relay that will disconnect the
circuit when no light hits the photocell -
http://www.wrighthobbies.net/examples/lightswitchformouse.jpg

The photocell and R1 act as a voltage divider controlling the NPN
transistor. When light shines on the photocell, the transistor
energizes the relay and the circuit is powered. When the mouse is
placed in a dark place like the cabinet, the relay is no longer
energized, cutting off the power. The relay needs to work with your
battery voltage. I used an Axicom FP2D3009 5v non-latching relay. Its
about the size of a 10-pin DIP chip. If you have 3 battery cells a 5V
relay should work.

The light switch consumes 25mA when active and around .02mA in
darkness. In total darkness, the current draw was too small to measure
(ohm's law - 5V / 5M Resistance = .0001mA). If there is partial
light, the circuit will consume more in the off state.

I'm sure there are many other ways to do this but this was the least
part count I could come up with.

Eddy Wright
Wright Hobbies Robotics

The mouse will shut off in any dark spot so if it scurries under the
couch, it will shut off. You can adjust R1 to change the darkness
shutoff threshold. Increasing R1 reduces the darkness sensitivity. I
use 10k and 100k values and both worked fine.

Parts:
Relay - Axicom FP2D3009 5V or similar
PhotoCell - 2M in Darkness, 1K in full light
Q1 - 2N3904 or equivalent
R1 - 10K to 100K or higher

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