All,
This is my first post. I'm not a EE, but sometims need to make circuits and such for my work.
I want to build a circuit which will read a 0-5 VDC signal which is proportional to a chemical tank level. If the level is too low (e.g. 10%--> 0.5VDC) I want a normally open relay to be de-engergized (shut off the pumps). But I don't want the relay to re-energized until the tank leve reaches 20% (i.e. 0-5 VDC signal = 1 VDC).
My circuit box already has 24 VDC available I will use that to power my chips. My current plan is as folllows:
Use 4000 series logic chips to engergize DC to AC relay as needed.
Use two differnt Op-amps with different resistors added to accomplish different gain on the incoming 0-5VDC signal.
OP-AMP 1 will be set with gain of 12 (R3=22.0kΩ, R4=2.0kΩ)
OP-AMP 2 will be set with gain of 6 (R5=10.0kΩ, R6=2.0kΩ)
Thus V1 = 6VDC at 10% (de-engergize relay)
and V2 = 6 VDC at 20% (re-energize relay)
So if I power a 4000 seires NAND gate with 12VDC, anything above 6VDC = "ON" and anything below 6VDC = "OFF".
But once the relay has bee de-engergized, I don't want it back on until after the tank has reached 20%. This means I need some kind of latching logic circuit.
I found a NAND Gate latch circuit which will do most of what I want, but doesn't allow an input state of (0,0). My quesitons are:
1. Does my circuit make sense as far as what I'm trying to do?
2. Is there an elegant way to use a NAND gate latch or something like it so as to avoid the (0,0) input problem?
3. How do I upload pics--I wasn't able to do so yet?
Thanks a lot for any help.
rsi77