LCD Instrumentation for Cougar Controller

[ianbartie]

Yep, sender or tank unit - same thing. I have it out & I measured the resistance from full to empty. Empty was 71 ohms & full is 283 ohms so I can replicate this with a trim pot.
The sender has 2 wires - one from the dash panel which has the square wave on it & one which is the return. This return wire is connected to ground somewhere. The trim pot makes gauge go up & down & works equally well connected to the ground frame or the return wire.
There is a bit of damping but there would be less than half a second in sender-gauge response.
Only the peak voltage of the square wave changes going from full to empty.
Average voltage measured with DMM is in a previous post.
I have found something curious though. If I disconnect the trim pot mid stream & reconnect it again, the gauge will stay where ever it was & won't move again until the ignition is turned off & back on again.

These 2 scope shots show the wave at full then at empty. The probe is connected before the trim pot.
The peak voltage at full is 3V & 1V at empty. Going straight across the sender & ground (no trim pot connected) gives the same wave but with a peak of 4.5V.
The spikes are 12.2mS apart. The frequency (80Hz if I have calculated right) does not change, only the peak voltage does.

[harlequin2]

It does look as if the pulse train is produced by some logic device, possibly a micro, with a 5v supply - hence the 4.5 peak unloaded (5v less the driver Vdrop). The gauge reading is then derived from the pulse amplitude, possibly some sort of peak rectifier. Pretty hard to see how a signal fed back into the sender wire can do the job. The only thing I can think of is to put a dc level on the sender wire. ie If you stick 3.5 v on it, the pulse height will be 1v corresponding to the empty reading, stick 1.5V on it for 3v and hence full. You could do this by making something up that rectifies the variable duty cycle o/p of the LCD and turns it into these voltage levels. A sort of DAC with level shifting - probably an op-amp or two!
If you have a variable power supply it would be easy to try and see if what I say does actually work.

[Freyguy]

Well, the low duty cycle sure explains why those average voltages you measured were so low
Looks to me like the computer periodically applies current to the sender/trim pot during ADC sampling. Could be through a transistor or other type of analog switch. May be part of a multiplexing arrangement due to single ADC.

One approach would be RC filter of PWM and buffer with an op amp. I’d avoid having the op-amp connected to sender wire directly. Using a series 71 ohm resistor, the op amp would need to output 0V for empty and approximately 2V for full.
Op-amp needs to be able to sink 14mA and could be powered from the 5V and GND available at TB1 positions 3 and 4 of the LCD brd. VR2 and PWM max still used to tune the empty/full range.
The TLV2472 used on the LCD brd would work. A single version is the TLV2471

-Chris

[ianbartie]

The op-amp approach does not sound too hard. I'll give that one a bash once I get the bits.
What sort of values would you suggest for the capacitor & the resistor in the RC filter?

[Freyguy]

Well, before you invest in the parts, do you have a variable output power supply handy?
Harlequin2 made a good point about testing the voltage theory out. Only difference is insert a series 68~71 ohm resistor from supply output to sender wire. See if the 0 to 2V range actually works.

[ianbartie]

My variable power supply only goes down to 1.34V but I can make up a simple 0-2V tester with a couple of batteries & a variable voltage divider.
Need to go & finish putting in the new, super quiet, vacuum pump first.

[harlequin2]

You'll need to make a low impedance divider if you do it that way. PRobably be OK with a big electrolytic across the bottom leg so the pulses go straight thru.

[Freyguy]

Originally Posted by ianbartie

My variable power supply only goes down to 1.34V but I can make up a simple 0-2V tester with a couple of batteries & a variable voltage divider.
Need to go & finish putting in the new, super quiet, vacuum pump first.

Try putting the power supply in series with a 1.5V cell battery. Power supply negative to battery negative. Then use the battery positive for your ground reference. Relative to this battery positive, the power supply positive can now go a bit below ground.

BTW- What brand of pump is it? My Thomas is a noisy bugger, even after putting in a padded enclosure.

[Freyguy]

Ian,
Attached is a circuit that you could build on a little breadboard and interface to TB1 of the LCD brd. Filtering should be adequate with these values although you could also change line 592 in the code-
(Under the PWM fuel gauge setup section).
Change MOVLW 0x05 to 0x04 ; prescaler changed from 4 to 1 (PWM freq should now be 7.8kHz rather than 1.95kHz)

The code mod listed in post 285 should still be done to invert PWM.
Short across R20 also.

-Chris

[mora]

Oh noes. New PICs give the same black boxes on the upper row of the screen. Current draw is within the limits this time though. Maybe something else fried during first testing. I don't have switches, leds or U8 installed though. I suppose switches should be momentary on, so would it matter if they are not installed anyway? How about leds?