Inexpensive DIY EV Monitoring System (Arduino based)

[Astro]

Originally Posted by mechman600

I tried this with a 2N7000, but I could not make it work for the life of me. It either stayed switched on (no matter what PMW value) or if I switched resistors to different spots (like a 5K from the 2N7000's gate to ground), my test LED switched on when PWM was 1 but the current did not increase as I turned it up.

Something like this maybe?

[P-hack]

also, the pwm frequency doesn't need to be anymore than visual limits, like 60hz. But 7/8ths is plenty good too

[mechman600]

Astro, the schematic you posted above is the exact circuit I attempted and could not make work. I suspect the opto is not very good at outputting a nice square wave, and since the transistor will stay on with the most minuscule voltage on the gate, it simply never turns off. I suppose I could try to slow down the PWM frequency (easy enough to do) but this throws all the delay timers off. And besides, I don't feel like doing it.

The solution I did come up with is simple enough - a 100 ohm bypass resistor between gauge signal and ground. At 0 PWM it sits on the empty line and at 255 PWM it is just short of the full line. Good enough!

Here is the schematic of the whole entire thing:


INTERESTING BITS:

Warning Lamp: the ex-low oil pressure warning lamp

Vac Pump Switch/"Run" +12V: monitors the vacuum pump switch when the car is on; if the switch remains closed for more than 10 seconds, the warning lamp is flashed and "VACUUM PUMP FAILURE" is displayed

"ACC" +12V: turns the display on when the keyswitch is turned the first click; I was going to trigger a pin on the Arduino (using lcd.display() and lcd.noDisplay() functions) but I ran out of pins ; opening the display's contrast pin also turns the screen off.

All my bits and pieces have arrived from Digikey. All I am waiting for is my Hong Kong sourced 20X4 LCD display.

[mechman600]

Originally Posted by Daox

I used one of the Haas current sensors that Paul's open revolt uses in my BMS, but it wasn't as accurate as I'd like it to be.

Did you include the capacitors that are in the datasheet? Omitting them would certainly cause erratic readings.

The 47nF cap between 0V and 5V isn't necessary because the arduino already has a much larger cap between its 5V and ground. But the 4.7nF cap between output and 0V will no doubt smooth out signal irregularities.

[P-hack]

mucking about with resistor values:

tap   divider   sense  watts   resolution per bit
12v   20k/10k   4v     0.005   0.01171875v
24v   50k/10k   4v     0.010   0.0234375v
36v   80k/10k   4v     0.015   0.03515625v
48v  110k/10k   4v     0.020   0.046875v
60v  140k/10k   4v     0.025   0.05859375v
72v  170k/10k   4v     0.030   0.0703125v

max bit level rounding error @ 72v = +-0.123046875v

[Daox]

Originally Posted by mechman600

Did you include the capacitors that are in the datasheet? Omitting them would certainly cause erratic readings.

The 47nF cap between 0V and 5V isn't necessary because the arduino already has a much larger cap between its 5V and ground. But the 4.7nF cap between output and 0V will no doubt smooth out signal irregularities.

Its been a while so I'm not absolutely sure. I believe I did. In any case, I suggest testing things to make sure they're accurate before you go driving off into the sunset.

You're doing a great job! I look forward to more updates.