Hello again,
I am presently working on a alarm system for my pole barn and looking for any critique on my design. This is a window mounted break beam circuit. The outputs will be sent into a microcontroller inputs measuring voltage levels. I borrowed it from the internet, and slightly modified it.
Right now, the voltage was set at 10V, just random, and will likely be dropped to 9V for convenience and resistors adjusted accordingly. I will be keeping the led current at around 75mA unless someone suggests differently. They will be always on, and I figured the lower current will add to their lifespan.
At approx. 30", I am seeing 7 volts at the output, dropping to .3 when the beam is interrupted. The IR leds are 1.2vf and Imax is 100mA. I will be shielding the phototransistor in a tube, and bumping the 100 ohm resistor to 2 watts.
I thought of motion detectors, but the cold temperatures will render them inop.... I looked at the 38.5 kHz modulated circuits, but don't have the time for the complexity or the patience to work through the bugs. Any advice would be appreciated... Thanks, Mike

 

I don't really understand the physical orientation you are planning. Are the LEDs and the phototransistor 30" apart, as on either side of a passageway or what? Are you planning to have several of these connected to a μC? What does the μC do?

I also don't understand why you think IR is not an option. My experience with IR detectors is that they work better in cold weather because of the greater differential between body heat and ambient temperature. But I have never tried to use them in extremely cold temps.

 

Correct, the led(s) are 30" away from the phototransistor. They are on opposite sides of the window. The voltage was taken from the output at that distance.
I am using a uC because it allows me to utilize a different type of alarm on the door, and set a shunt time to enter and exit the building easily. It also is easy to program it to re-sample the inputs after (x) time and reset the alarm. The inputs are simply reading a voltage level and alarming if one of the zones drops out.

The PIR units I looked at are limited to temps around 20 degrees F We frequently go below that, and the chance of damaging the unit isn't worth the headache.