OK. I never saw a p-ch used as a sinking device. So it is seeing a gate voltage of something like -2.8, and is not turning on all the way so you're getting the excessive drop across it. Too bad you are out of IO pins. You could combine the two switches onto one analog pin and free up a pin. Then use 2 n-ch. But the blinking light is what most people will want!

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Hey I like that idea. I suppose one switch would short the input to ground and the other would short it to +5VDC. And it's probably unreliable to leave the input floating, so a resistor would have to go in parallel with each switch.

The other day I connected the circuit to my 300SL to read codes from the top controller. It was nice to be able to short an input, walk away, and return several minutes later to find all the codes read. In this case it found 11, 20, and two others above 20. It was nice to be relieved of the tedium of counting all of those pulses. And of course erasing them all was equally simple -- short two inputs and walk away. That's when I concluded this circuit was worth building and offering to the forum.

That technique is actually used in cars quite a bit. It is used for steering wheel switches so only one wire is needed for 3 switches for example. They make a multi tap divider out of 4 resistor so each switch sends a different voltage. The rear window switch in the door of my C230 uses this system to detect rev, off, 1/2 fwd and fwd using only 1 wire to the window controller.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I just realized I can't wire both buttons to a single pin. I would have 10 outputs and 2 analog inputs, but one pin of the 16F684 has to be a digital input (I'm need an analog input to sense the pulses). But I've decided to increase the current limiting resistors as suggested and have set them to 220 ohms.

I was thinking more about your interesting use of the p-ch fet and I think it might actually work better with a regular PNP transistor like a 2N3906 etc. I think a base resistor of 1k would probably work fine.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I agree. A BJT would make the left digit brighter. A program change could also help by displaying the left digit 70% of the time and the right digit 30%, for example.

The difference in brightness is just noticable, and as I mentioned the photo exaggerates the difference somehow. But I suppose one may as well add another FET and resistor to the parts count and build the variation below.

A friend of mine works for Harris Semiconductor in Melbourne. He suggested adding small capacitors across the voltage regulator pins and a zener diode to protect the PIC from voltage spikes on the diagnostic socket. I don't think I'd bother with those. This is the same friend that originated the fix for flashing lights on early '90s instrument clusters (replacing the 470u capacitor on the circuit board).

I think I'm finished with and ready for my next project: one-touch operation of the power top for the R129 from the remote.

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I need to ask a simple question to tack onto the end of this subject.

I have always wanted to build the simple code reader that I have seen on this site over the years, but have never understood why I need one.

I have a 1990 300te California car. It has the push button diagnostic unit near the battery with 16 numbered holes (2 of which are used by the push button and the red LED).

I push the button and count the flashes to find out any stored codes. I then fix the problem and clear the codes by holding down for 8 secs and turning off the key.

What more would the home built code reader do for me?

>>What more would the home built code reader do for me?>>

It would get you to the MAIN Modules CODE storage banks.

The one you are using [ built-in ] accesses ONLY emmissions codes ..
which trigger the CE lamp.
If you really want to know whats up with your car, you don't want only emmissions codes , you want Systems Management codes. Which are accessed at the other sockets.

E320 16 PIN DTC Charts inc. pin 8, pin 14

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A Dalton

1990 300te
California Car
147,000 miles
124.090 body
engine 103.983

Ok, I built Arthur's home brew code reader as a project with my 10-yr old son. It came out great and works like a charm. Tested the car, found tons of codes, cleared them one at a time and then drove some errands.

Upon return I got the following:

Pin 3 - 17 Oxygen sensor is shorted to positive or ground
Pin 14 - 3 TN/TD signal (RPM) interrupted

I have 2 questions:
Is pin 3 picking up the same Oxy sensor fault that I get when I use the built-in diagnostic unit on my car that reports on the emission system?

What does the message on the Pin 14 fault mean?

Thanks in advance.