In the circuit below, is there any reason why we can't combine R1 and R2 into one 4 kΩ resistor in either location?
I am reverse engineering a window controller board, which is no longer available.
The board has three input switches powered by an unregulated 24 V supply using this circuit to level-shift the inputs to the MCU's 3.3 V input pins. D2 and D3 represent an optoisolator (1/4 of LTV-846s). D1 is a visible LED that indicates when the input is pressed. R1 and R2 are 1/4 of a 8-resistor SOIC-16 SMD package (Bourns 4816P-1). Since there are three of these circuits, two of the resistors in the package are left unconnected.
Question 1: Is there any reason why this circuit would need two 2 kΩ resistors, instead of one 4 kΩ resistor? And why would they have used a SOIC-16 package for six resistors, instead of just three 0805's? The board has plenty of other discrete 0805 resistors.
I thought maybe it would be for power dissipation but unless I am making a stupid mistake, it's only dissipating 1 milliwatt which even small SMD resistors could handle.
I am going to use the same optoisolators (LTV-846s) in my new replacement board. Am I risking anything by replacing that resistor network chip with three 4 kΩ 0805 discrete resistors?
Question 2: I would be tempted to get rid of the level-shifting circuit and just use the 3.3 V supply for the switches. If the 18 AWG wires to the switch are at most 100 ft, would it be a problem to use only 3.3 V? (I think that's about 1 ohm and 1 nF in 100 ft of wire). Would I be risking activating the input, from noise picked up from the wire?