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Thursday, March 26, 2009

VO Fuel Controller

A small circuit to avoid cross contamination of diesel and VO in dual tank vehicles. This is a schematic for a vegetable oil fuel controller, the function is to enforce that VO goes back to the VO tank and diesel goes back to the diesel tank.

Notable Parts:

  1. K1 is the return fuel line relay
  2. K2 is the send fuel line relay
  3. S1 is the primary switch
  4. S2 is the bypass switch
  5. S3 is the purge button
  6. S4 is the On/Off switch
  7. LED1 is the return line indicator
  8. LED2 is the send line indicator (download schematic)


When the vehicle is warm enough so that it can run on vegetable oil turn on S1. The send line will immediately switch to VO and the return line will stay on diesel for a user specified amount of time. To determine correct timing switch your engine to VO and time how long it takes for the diesel to be purge from the system. Now you set the time in the circuit by changing R1 to the correct value based on 1.1 * R1 * C2. To make it easier, I set C2 at 1000µF, so if you want about 45 seconds use the closest value below 45Kohms (45,000 ohms). In the circuit as set up above R1 is 39K ohms giving a timing of just under 45 seconds (1.1 * 39 = 42.9).

When you are a few minutes from home turn off S1 and press S3. By turning off S1 you will switch the send line back to diesel and by pressing S3 you will keep the return line on VO for a user specified amount of time. To set timing use the same value resistor for R4 as you did for R1.

If you stop for a short period of time and the engine is still warm enough to run on VO when you restart it then either switch on S2 for a minute or so or press the purge button. In either case you will bypass the on-delay timer and keep the VO going to the VO tank.


Do not expect exact timing from this circuit because capacitors are not perfect and voltage leakage will increase the time to some extent. When I timed the above circuit I found that it varied approximately 2 - 5 seconds (though I used a stopwatch and might have hit the start early or late, so YMMV). The timing can also be affected by length of time of discharge of the capacitors. If you turn off the circuit and turn it on again pretty quickly the timing can be much shorter than expected. I do not consider this an issue because the time it takes for the vehicle to cool down should be well longer than the time it takes for the capacitors to discharge. If this does become a problem use a lower value capacitor and a higher value resistor, for instance you can use a 220uF capacitor and a 180K ohm resistor to get approximately the same amount of time but the timing errors I initially stated may become more noticeable.


If you want the circuit to be more automated so you can just switch it on when you turn on the vehicle and it will wait until temperatures are high enough before switching from diesel to VO, just add a thermostat into the circuit directly before S1. Use a NO (normally open) thermostat set to close it's contacts when the desired temperature is reached.

Parts List
  1. (1) 7805 voltage regulator
  2. (2) 1N4148 diodes
  3. (2) SPST switch
  4. (1) DPST switch
  5. (1) N/O momentary push button switch
  6. (2) LM555 timer
  7. (2) 1000uF polarized capacitors
  8. (1) 0.01uF non-polarized capacitor
  9. (3) 0.1uF non-polarized capacitors
  10. (2) LEDs
  11. (2) 500 ohm resistors
  12. (2) 100K ohm resistors
  13. (2) resistors chosen for timing value (R1 and R4)
  14. (2) solid state relays capable of handling the current your solenoid valves draw

All capacitors should be rated at least 25 volts, anything higher is fine.
Resistors should be rated for 1/4 watt.
7805 is a generic voltage regulator, if it says 78L05AZ or something it's still fine. visit page

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