Fluid level sensor employing hot wire probe and digital sample and hold circuitry

Claim 1

In a fluid level sensing system which employs an electrical resistance, current excited probe, an electronic control circuit to measure the output of said probe, to memorize its initial value and to provide a warning signal when a preselected threshold is exceeded comprising: a clock generator fed into a two input NOR gate; a resettable binary counter which counts clock pulses generated by said clock generator when presented with said pulses by said NOR gate; an R-2R resistor ladder network which receives the ouput from said binary counter and in response to the increased count from said counter incrementally and proportionally increases its voltage output; an inverting operational amplifier which when presented at its inverting input with said increasing voltage output from said R-2R resistor ladder network will generate an output which is a function of an input resistor and feedback resistor connected to said inverting input and an offset voltage which is presented to the non-inverting input to said inverting operational amplifier; an operational amplifier to compare said output of said inverting operational amplifier with the voltage across said electrical resistance probe and present an output based on said comparison to said two input NOR gate to control the transfer of said clock pulses to said resettable binary counter; a voltage divider means across said probe; an operational amplifier to compare said output of said inverting operational amplifier with a preselected threshold voltage which is generated at the division point of said voltage divider means and which generates a trigger pulse when said threshold voltage is exceeded.