What to use as the solid state "switch"?

[BlackSportD]

I want to replace a sensor that gives the circuit readings via varying resistance to ground, a high/low resistance that fits within certain values.

Trying not to make a whole new circuit to do this, I already have an output at the right timing on/off that I want, and I will soon get the two resistance levels that I need.

I'm wondering what to use as the "switch" that I have displayed as a transistor in this diagram, the switching signal is 0.619vdc to 0.040vdc. As you can see my attempt at switching between two resistance values to ground is a setup were R1 will be in series to ground (high resistance) and when the "high" from the other circuit kicks in, it will put R2 in parallel and making the resistance level go to a lower resistance to ground.
**broken link removed**

 

[kchriste]

Put R2 on the collector of the transistor instead of the emitter. Then add a resistor to the base of the transistor to limit the base current to 1/10th of the max collector current.

 

[Roff]

I think we need to know more about the resistance of the input signal. 0.04V to 0.619V will not reliably switch a BJT, unless some sort of bias is used.
My choice would be an LM393 comparator, with the input threshold set at ≈0.3V.

What is the value of the resistor that is being switched in?
What are you trying to accomplish?

 

[BlackSportD]

I do not have the resistance value of the bias resistor in the 1vdc sensor signal. I will have to look up some notes on how to find a bias resistor's value via readings.

Nor do I have the other values just yet.

I'm trying to replace a Titania sensor element that switches between two resistance levels the 1vdc sensor circuit is looking for, with the current circuit I already have that has the right oscillation-high/low timing the circuit is looking for... but unfortunately at such a low voltage output high/low.

I can get the two "switching" resistance ratings before I can get the 1vdc bias resistor's rating. Is it crucial to have the bias resistor's rating if I find the two resistance levels I need for R1 and R2?

Do I need to provide the current capacity of the high/low 0.619-0.040 signal?

 

[Roff]

I do not have the resistance value of the bias resistor in the 1vdc sensor signal. I will have to look up some notes on how to find a bias resistor's value via readings.

Nor do I have the other values just yet.

I'm trying to replace a Titania sensor element that switches between two resistance levels the 1vdc sensor circuit is looking for, with the current circuit I already have that has the right oscillation-high/low timing the circuit is looking for... but unfortunately at such a low voltage output high/low.

I can get the two "switching" resistance ratings before I can get the 1vdc bias resistor's rating. Is it crucial to have the bias resistor's rating if I find the two resistance levels I need for R1 and R2?


Do I need to provide the current capacity of the high/low 0.619-0.040 signal?

You lost me. I was only asking for two things:
1. The resistance inherent in V1 in your schematic. In other words, if it were at 0.619V, and you shorted it to ground with an ammeter, how much current would flow (don't actually do this unless you know the current is limited to a reasonable level)? We need to know that if we are going to drive the base of an NPN, because current is required in the base.
2. What is the value of R1?

Your schematic shows that v2 is a constant 1V. A search for titania sensors tells me that they are a variable resistance, changing with oxygen concentration. That won't produce a constant 1V.

 

[BlackSportD]

Currently the fuse in blown in my Fluke, so I won't be able to take that amperage reading in the nearest future (have to order over the internet). Is a MOSFET not pratical for this application?

Still working on R1, I do not have the car on hand right now.

V2 is constant. Just like with a MAF sensor having a constant 5vdc and the return to the ECM is the variable, this particular circuit does not have a return to the ECM, its a 1vdc signal from the ECM to ground, in that path is the sensor that has variable resistance dependent of air fuel ratio. So within the ECM there must be a bias resistor for it to read across to figure out the resistance of the sensor, and thus the a/f. I'm not sure if that bias resistor's value is pertinent or not, I'd really like not having to calculate it via readings.

 

[BlackSportD]

I began to take the needed readings and found that the 1vdc supply was not that at all. Its a very low switching voltage 0 to .045vdc. This sensor is not turning out to be any type that I know off and I'm giving up on the project.

Sorry and thanks very much for the help.