time delay relay circuit

Posted on Feb 6, 2014

A diesel tractor that has a magnetic reed switch in the fuel tank to indicate low fuel. This switch is open when there is fuel in the tank, and closes when the level drops below it. When this switch closes a 12v relay is energized to disengage the fuel control and shuts off the engine. The problem is, sloshing fuel causes the float switch to intermittently open/close, which shuts off the engine! What I am looking for is a

time delay relay circuit
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time-delay circuit, so that when the float switch closes a delay of 10-20 seconds starts. If the switch STAYS closed longer than 20 seconds, the delay timer energizes the 12v relay and kills the engine. If the float switch OPENS during that 10-20 seconds, the timer needs to reset. My electronics experience is limited, and I have looked at perhaps a 555 chip to do this, but I cannot figure out the reset portion at all. Could someone show me a circuit diagram to do what I need Any and all help is greatly appreciated! ! While I can understand a schematic and get something assembled, I really don`t know that much about individual transistors and their abilities/functions. and so will need a pretty complete schematic to pull this off. Guess I need some skoolin`! does this fit my criteria correctly At first glance it appears to. I added the transistor to carry the load of the relay, which I believe is. 16 amps. I THINK I understand this! One question I have is: does this keep the relay energized after the time delay, or is it just a pulse that will energize then de-energize I would prefer it to keep the relay energized if possible. In this circuit, the input will normally be low, Q1 will be turned on and the capacitor will be charged. Th 555 output will be low and the 2N3055 will be off. If I am understanding you correctly, then this is just what I need. The input (float switch) bounces up and down as the tractor is moving, so...

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