Guess the missing PCB components?

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I was trying to fix my old car's key circuit. It can unlock and lock the car and is the only way to enable the alarm. This key has not worked for years.

I initially changed the batteries. The indicator led lit when the button was pushed but soon realized that the infrared led was not lighting up at all(I saw this through a camera - e.g. TV remote control infra red led is visible through a camera).

After that I examined the circuit carefully and sure enough it seems that two PCB components are missing, I found one of them in the enclosure but not the other.

Could anyone know about these circuits and guide me as to what is missing so I could replace it? Where should the 2.2KOhm resistor go? I don'h have the knowledge to figure out, please help?

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TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

Last Edited: Tue. Oct 8, 2019 - 10:45 AM
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Citroen?

 

Sorry, ours was written-off a few years ago - so no longer have the keys.

 

IR must mean it's really old!

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My guess the 2k2 goes to the top of the board. The other missing one would be a few hundred ohms at most methinks - 3V battery and IR led, so you'd want 20mA to flow so that's 150R.

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Yup, that makes sense to me, one of the components is visibly in series with the LED, so it must be a current limiting resistor; 2.2k seems too much for that. So the 2.2k resistor must belong in the top.

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El Tangas wrote:
n series with the LED, so it must be a current limiting resistor; 2.2k seems too much for that. So the 2.2k resistor must belong in the top.

 

+1

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I thought so too about the 2.2Kohm resistor until I realized that the top resistors connects positive directly to negative whenever the button is pushed. It seems pretty nonsensical to me. Is this common practice?

The few hundreds ohm resistors makes perfect sense for the other position to the right.

The car is a Renault 19. It's an old car but I like it and I like trying to fix stuff on it.

TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

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tellSlater wrote:
I thought so too about the 2.2Kohm resistor until I realized that the top resistors connects positive directly to negative whenever the button is pushed.

Sounds like a pull down resistor when the switch is open, a common practice to prevent floating inputs!

Following the trace, it goes to the base or gate of a transistor.

Jim

 

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Last Edited: Thu. Sep 26, 2019 - 02:31 PM
This reply has been marked as the solution. 
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That's strange, since I just found one of these boards in my back yard, near the tomato plants.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Thu. Sep 26, 2019 - 09:00 PM
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Lol, what are the odds? Anyway, the mystery resistor is just 1 ohm? Seems too low.

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Yes that is insane!! You people never cease to surprise me in a good way=]

Now does that code on that resistor really translate to 1 Ohm? Could this low value be ok since the led is gonna be supplied with a pulse/signal rather than a continuous DC voltage?

TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

Last Edited: Thu. Sep 26, 2019 - 11:24 PM
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The main chip seems to only connect to the battery through the push-button, when the push button is held down. As such, the circuit is working only when the push-button is held down.

Does that not make the use of a pull-down resistor obsolete?

TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

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tellSlater wrote:
Now does that code on that resistor really translate to 1 Ohm?

 

I think so, the marking seems to be 1R0, that's 1 ohm.

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The driver chip is probably doing the current limiting legwork, so 1 ohm probably fine.

You can actually pulse the leds with several amps for a few microseconds without damage (though should verify per application). 

 

funny-I noticed one of those 1R0 resistors in our driveway when I was going to the mailbox.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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My word, either you've got bloody good eyes, or they're remarkably close to ground level! :p

 

Neil

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The units determine the decimal point.
Thus 1R0 is 1.0ohms. If it were R020 that would be 0.02Ohms.

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avrcandies wrote:

 

funny-I noticed one of those 1R0 resistors in our driveway when I was going to the mailbox.

 

And a fully populated pcb in your back yard! Boy do you live in an interesting neighbourhood.

 

Ross McKenzie ValuSoft Melbourne Australia

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The path to the letterbox is paved with good smd?

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It's good to have elves as neighbors surprise

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:
That's strange, since I just found one of these boards in my back yard, near the tomato plants.
avrcandies wrote:
It's good to have elves as neighbors 

I know a guy who sees ghosts, and I used to have a magic can I threw old nuts and bolts into when I was rebuilding motorcycles and if I ever needed some strange bolt or nut I could count on finding it in my magic can, but what are the chances of you finding the exact same board in your garden.  It just sounds so magical.  What is that board doing in your garden?  Elves indeed.

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There's an abandoned AVR mine nearby that I tried to explore once, but was interrupted by stacks of volatile Dragons. 

I managed to push and pull myself away and return without any bites being taken.  

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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The 2.2K resistor would go across the LED points as shown below.  The missing #2 component is most likely a resistor.  Start with a 1K and if theat doesn't work, try a 10K.  I suspect that the missing#2 is a pull-up resistor for the collector or emitter of the transistor directly to the left of the missing component.  Is point A a Vcc point?  Does it have a voltage when the push button is pressed?  (make sure that battery is connected. The battery is most likely +3 volts).

 

 

Opps!  My bad.  I didn't notice the tomato patch dongle photo.  Missing #2 is the 2.2K and the LED is a 10 ohm resistor; it would be too small for a 1 ohm.  Marked with a 10 followed by 0 zeros.

Last Edited: Fri. Sep 27, 2019 - 10:13 PM
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In the end I used a 2.2 KOhm(222) 1206 SMD resistor(the one that was still in the enclosure) for the position up top and

a 1 Ohm(1R0) 1206 SMD resistor for the other position. Works like a charm! Many thanks to all of you :)

TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

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Here is all I figured out

TO THE FINDER... THE ISLE OF KOHOLINT, IS BUT AN ILLUSION... HUMAN, MONSTER, SEA, SKY... A SCENE ON THE LID OF A SLEEPER'S EYE... AWAKE THE DREAMER, AND KOHOLINT WILL VANISH MUCH LIKE A BUBBLE ON A NEEDLE... CAST-AWAY, YOU SHOULD KNOW THE TRUTH!

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When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!