Hi all
Sorry if this has been asked before, I searched and all I could find were discussions dealing with tying into a typical float-on-an-arm type factory sending unit.
I'm not afraid of electronics, I can read a schematic, I understand the basics, and can design and build simple circuits, but I could use some help here cause this is a bit over my head and I don't know exactly what I'm looking for.
I need to make a low fuel light circuit (automotive type application, in case it's not obvious!) which drives an LED. This application DOES NOT use a typical fuel level float on an arm supplying a resistance which varies with the fuel level, but instead uses a thermistor (?) which is cooled by the fuel surrounding it. When the fuel level drops below the thermistor, it heats up and it's resistance goes down (or up?)
(Before anyone jumps about using something that heats up in a fuel tank, it's perfectly safe - simple chemistry here, it can't explode or incinerate...)
Hondas have historically used this method to illuminate a standard filament bulb in the dash. One problem with the old honda circuits that drove a bulb - it was a direct driven current/resistance circuit, so as the thermistor heats, the bulb comes on slowly, dim at first and then brightening. If you take a fast corner, the bulb goes out as fuel sloshes over and cools off the sensor. This will be used on a motorcycle, where fuel will be sloshing around even more. in 2010, we can do better than that...
Here's what I have in mind - as best as I can explain - a comparator circuit, where when the voltage drop across the thermistor reaches a threshold, it latches a circuit into an ON state to illuminate an LED, and stays latched until voltage is removed (ignition cycled). The reason for this is to avoid the light continuously turning on and off when the fuel level is right at the level of the thermistor.
Ideally, the voltage drop is "buffered" by a capacitance circuit of some sort so it's not latching on falsely when fuel uncovers the sensor for a short period of time (such as when taking a corner or going through a long curve on the road) - so the voltage drop would need to stay low (high?) consistently over the course of maybe 2+ minutes before the circuit would latch on.
I understand principles and generally what the basic electronics components do and how to use them, but I don't have the direct experience or knowledge to know what specifically to use or how to to put it together, so saying "use a voltage divider" doesn't do much for me, (while I can imagine that it's probably two resistors in series, I really wouldn't know it if it bit me!) I need hand-holding, more specific help or pointers to existing schematics...
beenthere's suggestion in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=9053
was the closest I could find, but while I followed what he was saying, I don't know how to use his suggestion to do what I'm looking for.
Any help, direction or resources would be greatly appreciated, and I'd even pay someone to design this *relatively* simple circuit for me.
TIA!
Sorry if this has been asked before, I searched and all I could find were discussions dealing with tying into a typical float-on-an-arm type factory sending unit.
I'm not afraid of electronics, I can read a schematic, I understand the basics, and can design and build simple circuits, but I could use some help here cause this is a bit over my head and I don't know exactly what I'm looking for.
I need to make a low fuel light circuit (automotive type application, in case it's not obvious!) which drives an LED. This application DOES NOT use a typical fuel level float on an arm supplying a resistance which varies with the fuel level, but instead uses a thermistor (?) which is cooled by the fuel surrounding it. When the fuel level drops below the thermistor, it heats up and it's resistance goes down (or up?)
(Before anyone jumps about using something that heats up in a fuel tank, it's perfectly safe - simple chemistry here, it can't explode or incinerate...)
Hondas have historically used this method to illuminate a standard filament bulb in the dash. One problem with the old honda circuits that drove a bulb - it was a direct driven current/resistance circuit, so as the thermistor heats, the bulb comes on slowly, dim at first and then brightening. If you take a fast corner, the bulb goes out as fuel sloshes over and cools off the sensor. This will be used on a motorcycle, where fuel will be sloshing around even more. in 2010, we can do better than that...
Here's what I have in mind - as best as I can explain - a comparator circuit, where when the voltage drop across the thermistor reaches a threshold, it latches a circuit into an ON state to illuminate an LED, and stays latched until voltage is removed (ignition cycled). The reason for this is to avoid the light continuously turning on and off when the fuel level is right at the level of the thermistor.
Ideally, the voltage drop is "buffered" by a capacitance circuit of some sort so it's not latching on falsely when fuel uncovers the sensor for a short period of time (such as when taking a corner or going through a long curve on the road) - so the voltage drop would need to stay low (high?) consistently over the course of maybe 2+ minutes before the circuit would latch on.
I understand principles and generally what the basic electronics components do and how to use them, but I don't have the direct experience or knowledge to know what specifically to use or how to to put it together, so saying "use a voltage divider" doesn't do much for me, (while I can imagine that it's probably two resistors in series, I really wouldn't know it if it bit me!) I need hand-holding, more specific help or pointers to existing schematics...
beenthere's suggestion in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=9053
was the closest I could find, but while I followed what he was saying, I don't know how to use his suggestion to do what I'm looking for.
Any help, direction or resources would be greatly appreciated, and I'd even pay someone to design this *relatively* simple circuit for me.
TIA!