Level Sensor Circuit

gjscott said:

I do have the old IC and the dimple for pin 1 was in the top right corner as my photo is oriented. My electronics trouble shooting skills are limited but I've been building boards since I was a little kid for my father who was as electronics engineer so am well aware of how to connect components and IC's, that's why it's bizarre to me that the polarity and IC may be reversed.
However, this sensor has never worked since I've had the bike since 2011 and I can't confirm if it ever worked. And having owned several Lancias in my life I know how bad Italian automotive electrical systems can be, maybe the circuit was built wrong in the factory and it never worked.

So, shall I reverse the LM393 and the supply polarity and give that a go ?

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alec_t said:

That's how they justify their fees and how auto manufacturers justify the rip-off prices for vehicle spares/parts
Well, it's one possible way of using the component values from your circuit. I wouldn't say it's definitely what the original designer would have done.
The changes from your drawing essentially are (I think):
1) swap the + and - power lines,
2) rotate the IC through 180°,
3) check the transistor pin-out and re-orient as needed,
4) 2k2 goes between transistor base and comparator output.
5) connect lamp between transistor collector and ground,
6) supply - = circuit ground.

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I reversed the supply polarity and the LM393 and the lamp is staying on regardess of switch state.

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alec_t said:

Well that's points 1 and 2 taken care of. I suggest you next try 3-6, which hopefully will cure the lamp-stays-on problem.
I find it's easy to get confused with IC pin numbering and orientation when turning a board over to check top and bottom sides; a mental flip of the IC is needed.

Edit:
Can you check the polarity/resistance of the diode (with no power to the circuit)?
Is it forward or reverse biased when meter + probe is on the blue end and meter - probe is on the brown end?

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alec_t said:

Right. Here's what I think the circuit was intended to be:
View attachment 72121
In my view a much simpler circuit (not much more than 1xR, 1xC, 1x MOSFET) would have done the same job more cheaply

Edit:
Here's the cheapo version
View attachment 72123

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Regarding the transistor, the 2FZ in NPN, your circuit has a PNP so I'll need to change it ?

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alec_t said:

According to your post #1 the 2FZ is a PNP. That's what's needed.
We should be able to get your board working. Here's the revised version:
View attachment 72152
Note that +12V is now at the top, Gnd (0V) is at the bottom, the PNP has been rotated (assuming the pin-out was correct in your photo) so that the emitter goes to the top rail, the circuit track under the board beneath the 'X' needs cutting to isolate the collector from Gnd, the lamp connects between collector and ground, the caps have + at the top and - at the bottom, the IC is oriented with pin 1 bottom left, the 2k2 near top-right is removed but a 2k2 (same one) is connected between IC pin2 and ground, the fuel switch connects between pin 2 and +12V. We'll keep the cheapo version as 'plan B'

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How does that change things ?

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alec_t said:

Well if you want to try the board mod in post #26 the transistor needs to be PNP, e.g. a 2N2907 or a 2N2905A. May help wiring-up if it's not a surface-mount type. BTW, in that mod I forgot the base resistor. Apologies. Here's a corrected version:
View attachment 72169
I think the diode is a zener for limiting the voltage across the IC in the presence of transients, the blue band denoting the cathode. But from your measurements it seems fried (a zener should be relatively low resistance in one direction). I suggest replacing it with a 15V zener or a suitably-rated varistor.
If the capacitors have been subjected to reverse voltage they may have failed. Do you have spares?

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