Transistor to control resistance

[realspido]

Hi everyone,

I need some help with my project - did electronics years ago and getting a bit rusty.
So I have this systems monitor from Blue Sea that monitors your batteries and level of liquids in the tanks. Unfortunately, I can't use the sensors that are listed as compatible and the only way I can measure the liquid level is by measuring the hydrostatic pressure at the bottom of the tank.
I bought a sensor that outputs voltage depending on the pressure from 0V to about 1.6V for maximum water level in my application. So far so good.
Now, the monitor accepts resistive sensors with the range from 10ohm at minimum level to 180ohm at the maximum level.
I'm pulling my hair trying to come up with a circuit that could output a resistance as a function of input voltage. I'm sure the answer is simple, but I just can't figure it out!

Please please help!

 

[Harald Kapp]

Welcome to electronicspoint.

This is not an easy feat. A quick search on the internet doesn't reveal much enlightening stuff.
Provided that the monitor uses a constant voltage source (not a consant current source) to monitor the resistance, you can, if you like, start working from this basic circuit that I "designed" as an exercise some time ago:

This is an LTSPICE schematic.
V1 and V1 are the power supplies for opamp U1.
V3 is the source of the circuit that senses the resistance.
E1 is a voltage controlled voltage source (for values of E<1) a resistive divider can be used. The output of E1 is proportional to the voltage V3.
U1 controls MOSFET M1 such that the voltage across R1 is equal to teh voltage from E1. Thereby you can set the "resistance" as seen by V3 to values proportional to E1.

This circuit neds detailed elaboration, however, it is only a design study.
[Edit] sorry, this is complete nonsense with regard to your application as the "resistor" simulated by this circuit is by no means voltage controlled (took me some thoughts to realize that). The "voltage control" that is used in thhis circuit monitors the voltage across the terminals (V3) whereas you will need something that controls E1.
Again, sorry, forget this circuit, it won't be as easy as this.

If your monitor senses resistance by applying a constant current, however, this circuit will not be suitable. In that case you may try to apply the voltage directly to the monitor's input. You'll have to ensure that the voltage output from the sensor can source current (that is not per se the case). A buffer amplifier may be required to add sourcing capability to the sensor and the buffer may be required anyway to adapt the voltage level to the monitor's requirements.

 

[realspido]

Thanks Harald. I thought I may have to use the FET transistor. So when you say E1 is a voltage controlled voltage source, you mean the output from the sensor with the 0-1.6V range?
How do you adjust the upper resistance limit, i.e. 180ohm on the output (V3)?

I wish I could get my hands on the schematics for either the ultrasonic sensor that they use as standard for this application, or the monitor itself...
What I noticed is that the voltage on the monitor's pin reading the tank level is about 1.6V and every second it briefly jumps to 2.6V as if the device was sending pings (?). When the sensor is not connected, the monitor shows 100% (full) level.
I remember when I was playing with the ultrasonic sensor it was outputting voltage from few hundred milivolts to about 1.6V at maximum level, however applying the same voltage to the monitor's terminal doesn't do the trick.
But when you connect 100ohm resistor, the level indicator drops to about half.

If the ultrasonic sensor was cheaper (they cost $170 each!!!) I would tear it apart and rip the diagram, but they're completely sealed which means I'd have to destroy it with no guarantee I'd figure out how it works...

 

[GPG]

So I have this systems monitor from Blue Sea that monitors your batteries and level of liquids in the tanks.

Link ?

 

[realspido]

Here's the link to the device:
https://www.bluesea.com/products/1801/Vessel_Systems_Monitor_VSM_422

and the sender:
https://www.bluesea.com/products/1810/Vessel_Systems_Monitor_Tank_Sender_-_Diesel_Water_Waste

 

[realspido]

Hi guys,
So I got a response from the manufacturer (below):

"The VSM outputs a constant current source to drive resistive loads. The ultrasonic sensors are able to sink this current and generate the correct voltage. So your output stage needs to look more like a constant voltage source than a current source (which is what your output configuration is). In addition you need to create an offset so that when your input is 0 the output is 0.5 V.
One way to do that is with a summing op amp. Here is an example that is close to what you want (image attached)"

One thing that confuses me is the Vbat source - is this just an equivalent diagram to run SPICE? How do I build the actual diagram? I can't connect the Vbat and Vmeter (output from the sensor) in series as they use the same power source.
Please help.

 

[Herschel Peeler]

Hi everyone,

I need some help with my project - did electronics years ago and getting a bit rusty.
So I have this systems monitor from Blue Sea that monitors your batteries and level of liquids in the tanks. Unfortunately, I can't use the sensors that are listed as compatible and the only way I can measure the liquid level is by measuring the hydrostatic pressure at the bottom of the tank.
I bought a sensor that outputs voltage depending on the pressure from 0V to about 1.6V for maximum water level in my application. So far so good.
Now, the monitor accepts resistive sensors with the range from 10ohm at minimum level to 180ohm at the maximum level.
I'm pulling my hair trying to come up with a circuit that could output a resistance as a function of input voltage. I'm sure the answer is simple, but I just can't figure it out!

Please please help!

Hi, new to the forum here. Are you still looking for this quest? How complex a circuit is acceptable. Is an op amp, +V, -V, a J-FET and a few parts acceptable? Yes a J-FET was my first thought, too, but a 2N5462 needs about -0.65 V in for 0 ohms and about -0.41 V in for 180 ohms. The attached is "close, kind of". I didn't want to put more time into it if this solution wasn't acceptable.

 

[AnalogKid]

I would push back harder on Blue Sea. They sell the sensor and the monitor, so the first question is how do they recommend the two be connected? We're glad to help, but it is their problem to solve since they got your money.

ak

 

[Alec_t]

As the monitor is providing a constant 4.63mA to the sensor, the circuit below should give you the required monitor input from a 0-1.6V sensor. R1 and R2 set the scale and offset, so may need tweaking. I've assumed your supply voltage is a steady 12V, but smoothing/decoupling would be advisable.

 

[cjdelphi]

You can also try this circuit and adjust the values...