# Is it possible?

Discussion in 'Electronic Components' started by CH, Jan 28, 2004.

1. ### CHGuest

Hello,

I am having problem finding out if this is possible:

A battery dependent device (5V) containing a microprocessor
and a pyroelectric sensor, in one mode of operation is to use
close to zero (idealy zero) current untill light hits the sensor, at
which point, the device will activate. The problem is that the
voltage from the sensor at about 40 microVolt is to switch on
a 5V circuit.
Are there any transistors/thyristors or FETs in combination of amplifiers
that can do this??

Sincerely
Christian Hovden

2. ### Tim WescottGuest

You say "pyroelectric" and "light" in the same sentence -- do you mean a
pyroelectric sensor detecting far-infrared, or do you mean photoelectric
sensor detecting visible or near-infrared light?

If you're detecting visible or near-infrared you should be able to use a
photocell, and you're done. Even if you have to add an auxilliary sensor
this may be worthwhile.

If you're truely bound to using a sensor that only detects 40uV then I don't
think you can do it with zero current drain. As long as you're willing to
accept some current drain and response-time limitations then you can do this
no problem. Here's how:

Use a circuit with a very low-drain clock (think wristwatch), and a fairly
standard amplifier that can detect your 40uV. Design your amplifier to use
the minimum amount of energy (not power, but energy) in powering up,
stabilizing, and checking for energy on your sensor. Use the clock to keep
everything turned off, then turn things on once in a while to check. Your
average current drain will be the current drain of the clock (which will be
vanishingly small) plus the charge consumed by each measurement divided by
the time interval (in seconds) between measurements.

You have two main tradeoffs: first, how slowly can you afford to operate,
because the less you wake up and look the less average current you'll
consume, and second, you'll find a charge vs. accuracy trade-off, where
decreasing the current consumed by your detector, or the amount of time it's
on, will decrease it's reliability.

I suspect that you now have everything you need, but contact me if you want
the circuit designed: