Photoconductive cells used in alarm circuits. Suppose that the photocell in the
ID: 2083156 • Letter: P
Question
Photoconductive cells used in alarm circuits. Suppose that the photocell in the circuit of Fig. 12-11, which operates from an infrared beam, becomes damaged and has to be replaced. The specs on the original unit where 1 K-ohm at 2 fc, 1 M-ohm dark. You can't get an exact replacement, but you have a choice of three possible cells. Here are the specs: (a) Cell 1: 0.6 K-ohms at 2 fc, 1.5 M-ohms dark, and peak response at 5150 Angstroms. (b) Cell 2: 100 K-ohms at 2 fc, 20 M-ohms dark, and peak spectral response at 700 nm. (c) Cell 3: 0.8 K-ohms at 2 fc, 2M-ohms dark, and peak response at 7150 Angstroms. Which of the three cells would be the best replacement?Explanation / Answer
We are given that photocell operates on infrared beam.
Hence we must choose photocell which has peak response in infrared range.
This is the most important criterion is chossing the photocell.
Now let's investigate each cell
1) cell 1 has peak response at 5150 Angstroms = 515 nm which is in visible range and not in infrared region of electromagentic spectrum.
2)cell 2 has peak response at 700 nm which is at end of visible range and start of infrared region of electromagentic spectrum.
3)cell 3 has peak response at 7150 Angstroms = 715 nm which is in infrared region of electromagentic spectrum.
Comparing peak response wavelenth it is clear that for best response we must choose cell 3 as it's peak spectral response is in infrared region of spectrum.
Hence we will choose Cell 3.