This is all "theory". I would like to do this but it's too expensive right now. But it's a great transistor learning point. Since they are so mystical and black magic.I thought about a EFI system to replace the carb in my 1973 dodge power wagon w200. Problem is, I have two fuel tanks. And the electric pump for it needs a return line. Now how would one run a return line to two tanks? Without just moving fuel from one to the other? Well I figured it out! On the truck is a fuel tank switch. Well there are two but one is only for the fuel gauge. I replaced the nasty "voltage regulator" with a proper 5v regulator. Original is a pulsed duty cycle 5v "regulator". The 5v goes into the meter which is not a nice responsive analog needle. It's a bi metallic strip which a heater wire is wrapped around kinda meter. Super (not) fast. It can take up to 2 minutes to fully respond.
Now this is where my problems first came. The fuel switch is just a switch that changes the needle to read from one tank to the other. The fuel senders in each tanks are 10-70ish ohms based on fuel level. 10 when full equals maximum reading and 70 is minimum. I designed a circuit as best I can for now to use a op amp as a voltage comparator. That way it's either max voltage or no voltage. Just right to use a high power transistor as a switch for a relay coil! I used a 2n3055 as my switch. It's so overpowered it never gets remotely warm. Which is great since it would be behind the dash which has poor to no ventilation.
Next is my fuel tank switch. The real switch that selects which tank delivers fuel to the carb. Well how was I to do that? My current is a manual switch with two off positions, one for internal tank and one for external. The off is no fuel to carb at all! And what about my return line? I need the pump to return fuel to the tank which it takes from. Otherwise I might as well ditch one tank(not good with 10mpg or less)! Well online I found a fuel transfer switch with just what I need! It's motorized so my circuit works. And it has two sections! That perfect for fuel from two tanks and a return line going to two tanks! I'm not sure how this switch is powered as I'm not going to spend $100 to find out unless I go for this project. So I'm guessing there's a ground and two 12v inputs. On input is say open and the other say closed. I'll post pictures of my schematic and the switch I found. As a project to help me learn transistors a little better it sure worked.
Problems:
On pin two of my op amp(741) it needs 1.92v or less to trigger my relay. Not sure how my meter and it's unknown resistance and my fuel senders resistance will affect that without more testing in vehicle. Maybe changing the resistors on pin 2 ground and pin 3 ground from 1kohm to another may change that. For simple tests it works.
Let me know what problems you have with this and opinions. It's just a test for now to help me understand those black magic things called transistors. Dry theory is fine but real tests help more then that. My schooling talked about using transistors as switches but did it stick? No! I had to google it and play with it to really understand how to use it! And plus, I could just use a switch in the dash for this but that doesn't help me understand them. And I don't want to put a hole in my dash for that. I already have a switch in the dash to switch between tanks. Just need to know how to use itto do the job it should!
Ps: It took two times of running out of gas before I figured out my dash switch doesn't really change between tanks! I had to learn my damn floor switch! Man that sucked! It caused me to be towed twice, replace a fuel pump and filter without need and it caused embarrassment! But I do love the truck anyways!
Pps: Don't hate on my schematic man! The upwards black triangles are connected and lower white are connected. I like using lower white triangles instead of the proper ground symbol. Don't like it? Well too bad. It's my style, not yours!
