Any other issues you can see with the schematic. Looks like I can drop the ZD1, R6, C2, D1 and D2.
Hello there bryan,
ZD1 is most likely a zener diode, voltage unspecified, but probably 6 to 10 volts. R6 limits the current through that diode.
C2 is a filter capacitor to help keep transients from the input from affecting the circuit too much.
The action of ZD1, R6, and C2, is to keep transients that are often found in an automobile off of the supply for the circuit. Without them it may act a little erratic depending on how bad the 12v supply (from the car) really is.
D1 and D2 act to create a minus supply voltage at the cathode of D2. So the cathode of D2 is negative 1.4v with respect to ground. This does a couple of things...
First, it helps A1 to cope with voltages near ground even though it may do well without it.
Second, it helps the output of A1 to go all the way to zero or below.
Third and more importantly, in this circuit the LM35 can work from about +2 to +150 degrees C, but with the addition of the two diodes D1 and D2 it is possible to extend that range from -55 to +150 degrees C. What is missing however is an 18k resistor from the output of the LM35 to ground (GND) which is also needed to get that extended range. So to get that range you need to add an 18k resistor. You'd also have to connect the voltage reference to the minus supply voltage though, and then the two diodes might have to be connected through one or two wires to bring them out to the same location as the LM35 so their temperature changes along with the LM35 instead of with the other circuit elements.
Now the other components would also have be checked to see if they can stand +150 degrees C too, so maybe the top end will be more limited than that, but to get the negative temperature range down to -55 deg C you need the two diodes and add an 18k resistor.