Hi, I'm trying to fix a device I have, the part that is broken is a temperature sensor and display on a machine I have. Fortunately I also have a second identical machine where the circuit is working.

With my oscope, it appears (although this seems like a strange design to me) that the display sends out a 12v signal to an analog to digital adapter box and based on the analog voltage value coming in from the temperature sensor, the adapter box will sink (pulse) a ground signal to the display wire creating a digital square wave signal on that wire, that varies based on input from the temperature sensor.

Here are some scopes to explain what I mean, the first is when the display wire is disconnected from the adapter box, you can see how the display side is 12v analog and the adapter box side is 0v.

Here is a scope when the wires are joined together, you can see that a digital signal is formed, the pulse width changes based on the value of the analog input voltage value from the temperature sensor.

Could the adapter box be pulsing a ground/sinking current from the supply (temperature display) to create a square wave signal and then the display be displaying a temperature value from this? This seems very backwards to me, and I have not seen a circuit designed this way before, but wanted to get a few more opinions before I tried to simulate the adapter box.

 

I think you've got it right. The adapter is converting an analog voltage to a digital signal of some sort. The unknown is whether the adapter's digital signal is a frequency (voltage controlled oscillator) or a duty cycle (PWM) that changes in proportion to the analog input voltage. I suppose another possibility is a digital word but that seems less likely to me.

A few model numbers and data sheets would reveal all mysteries!

 

I wish I had a data sheet!!!

I just don't understand how a gauge can reflect what happens to an analog voltage it supplies, with an adapter box that then turns the frequency to a digital frequency. How does it even read that if it is supplying the voltage as analog?

I have samples from different temperature ranges though.


50C

60C

70C

80C

90C

 

Any ideas from the few samples I posted?

 

My interpretation is that the adapter is sending out fixed frequency (2.28Hz) pulses with a duty-cycle which decreases with increasing temperature; i.e it generates a PWM signal. The 12V 'from' the display is just the display supply voltage presented via a pull-up resistor at the display input.

 

That's an interesting idea.

So how can I verify this?

Should I try a higher resistance resistor (~100k) and ground it and see if it pulls down the pull up voltage on the display side?

And then also create my own supply through a ~100k resistor to the adapter box to see if the behavior duplicates?

I could even try different pull down resistors to determine the value of the pull up resistor on the display side by trying to build a resistor ladder and choosing the value that put me at 6v, correct?

 

Certainly try what you suggest. Start with 100k and reduce it if necessary.

 

Will do this weekend and I will report back with the results.

 

Well this weekend turned into 3 weekends later, lol.

I tested it today and I was able to get it to drop below 50% (to 46%) with a 3k resistor, so a 3.3k or 3.4k would be the value that cuts the voltage in half. I'm guessing this rules out a pullup resistor, being that the value would be 6.6k or 6.8k?

Does this also mean that the gauge is the source and the adapter box is the drain?

 

Your test suggests the display input has a 3.3k pull-up resistor. I would guess that the adapter has an open-collector transistor driving the display input.

 

Doesn't that seem like a low value for a pull up resistor?

Either way, I think you are right. I built a 555 circuit last night with a 2.25hz frequency and a fixed duty cycle of about 50% and I was able to drive the display

So now I need to figure out how to keep the frequency constant, but vary the duty cycle based on a 0 to 5v input. Can that be done with a 555 timer ic?

Your test suggests the display input has a 3.3k pull-up resistor. I would guess that the adapter has an open-collector transistor driving the display input.

 

3.3k seems reasonable for a pull-up in this context. A 555 is probably not the best option for voltage-controlled pwm. Personally i'd try 2 opamps or comparators; 1 used as a triangle-wave generator and the other as a comparator where the 0-5v is the reference.