Is this a Zener Diode, & what is it Doing?

Thread Starter

thunderhead289

Joined Nov 28, 2021
7
Howdy,
What I have here is an automotive tachometer circuit, specifically the (-) PWM input to the microcontroller.
From looking at the pinout on the microcontroller used, this circuit seems to convert the (-) PWM from the ignition coil into an analog input signal.
I'm just getting into this stuff, have made a few of my own PCB's with success, but this circuit has me puzzled.
How exactly is it converting the negative/ground PWM to an analog 0-5v signal?

also, my BIGGEST point of confusion is what appears to be a Zener diode on incoming PWM at the very beginning after the current limiting resistor.
what is this Zener doing? I assume its of a 5V persuasion since the linear regulator (78L05) on the same circuit is of a 5v output - but I'm just not sure.

any help in explaining exactly what is going on here would be greatly appreciated!
just getting into this stuff, and really enjoying myself.

(ultimately trying to accomplish the same, receive an analog signal to my own microcontroller. the arduino stuff just isnt fast enough to track an interrupt accurately past a certain RPM point, not to mention handling an interrupt to generate an RPM readout doesn't allow you to do much else, where an analog input signal is much more easy to work with.)
 

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AlbertHall

Joined Jun 4, 2014
11,718
I would expect the zener to be connected between the junction of the two resistors and ground to limit the peak voltage.
If this is a tachometer then then the quantity of interest is the frequency of the input.
This appears to use the transistor to short the capacitor on each ignition pulse so if there is a current source trying to charge the capacitor its average voltage will be a measure of frequency.
 

dl324

Joined Mar 30, 2015
13,857
Welcome to AAC!

What is the part number on the zener diode? Why do you assume that it's 5V?

If you plan to go further in electronics, learn how to draw a proper schematic. We don't use component packages because they don't convey functionality.

This is what I transcribed from your drawing:
1638131040877.png
Note that the flow is primarily from left to right and top to bottom.

D1 doesn't make sense unless the input is AC (unless it's for reverse polarity protection). R1 doesn't make sense.

The zener connection doesn't make sense.
 

LowQCab

Joined Nov 6, 2012
1,635
Getting an Ignition-Signal directly from the Coil can be somewhat hazardous
to Low-Voltage components since it may have all sorts of High-Voltage-Spikes, and other weirdness.

Unless You are working on a very old system, or possibly a Motorcycle,
You can easily find a very clean Square-Wave-Pulse that is driving your Ignition-Module,
quite often it will be 5-Volts, Bonus !!!

If You will provide all the information You can get on your Ignition-System,
I'm sure a very workable solution can be found.
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Thread Starter

thunderhead289

Joined Nov 28, 2021
7
Welcome to AAC!

What is the part number on the zener diode? Why do you assume that it's 5V?

If you plan to go further in electronics, learn how to draw a proper schematic. We don't use component packages because they don't convey functionality.

This is what I transcribed from your drawing:
View attachment 253787
Note that the flow is primarily from left to right and top to bottom.

D1 doesn't make sense unless the input is AC (unless it's for reverse polarity protection). R1 doesn't make sense.

The zener connection doesn't make sense.
thanks for the warm welcome!

I would also agree that I need to learn to draw schematics properly - what platform do you use to generate yours?
D1 doesn't make a lick of sense to me either, but I do know this circuit works as its a commercially manufactured unit (I had to laugh when I opened the casing and there was an attiny24 staring at me).

all I can figure is that the zener has something to do with the voltage for the on/off state of the 2N5551.
I only assumed it was 5v since the circuit its on is 5v, but perhaps its 12v - I plan to recreate the circuit on a PCB and test the analog input signal accordingly and potentially try different zener units on that D1 location.

I also cant wrap my head around how the 2N5551 is actually being triggered - like how does the base ever go high if the input is a ground PWM signal.
 

Thread Starter

thunderhead289

Joined Nov 28, 2021
7
Getting an Ignition-Signal directly from the Coil can be somewhat hazardous
to Low-Voltage components since it may have all sorts of High-Voltage-Spikes, and other weirdness.

Unless You are working on a very old system, or possibly a Motorcycle,
You can easily find a very clean Square-Wave-Pulse that is driving your Ignition-Module,
quite often it will be 5-Volts, Bonus !!!

If You will provide all the information You can get on your Ignition-System,
I'm sure a very workable solution can be found.
.
.
.
dealing with older tech, carburetor era stuff here - the circuit board I show in the picture is a consumer available unit that i can confirm works very well (its the one all the parts stores sell - used them for years).
some of the signal input circuit functionality is a mystery to me other than the fact that a PWM is being converted to analog for the microcontroller input.

my biggest issue with recreating this is figuring out what in the world the zener diode (i think its a zener) is doing on the Signal input, the signal being a ground interrupt/pwm
 

Thread Starter

thunderhead289

Joined Nov 28, 2021
7
also, D1/DW2 in the schematic above MAY NOT be a zener, but the polarity of the diode itself (in my attachment of the PCB) just doesn't seem to make a lick of since if it were a standard diode.
 

dl324

Joined Mar 30, 2015
13,857
I need to learn to draw schematics properly - what platform do you use to generate yours?
I use an old version of Eagle. I think crippled versions are still free, but you have to create an account to download the program.

Using a schematic editor won't help you with component placement or flow. That you need to learn how to do by learning from people who can create well drawn schematics.
I also cant wrap my head around how the 2N5551 is actually being triggered - like how does the base ever go high if the input is a ground PWM signal.
It would make more sense if the PWM signal was positive and the transistor was wired like this:
1638138102208.png
That still doesn't explain what C3 is for. Check the board to make sure you traced it correctly.
 

LowQCab

Joined Nov 6, 2012
1,635
Lets get some nomenclature right.
This is not a PWM Signal, ( Pulse-Width-Modulation ).
It is a Variable-Frequency-Pulse, which is normally at ~12-Volts, and pulses close to Ground.
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Thread Starter

thunderhead289

Joined Nov 28, 2021
7
10-4 on not calling it PWM. variable frequency pulse it is.
I think I'm going to draft up the circuit and order in a few units from JLCPCB and see what I can find also.
perhaps I screwed something up in the transfer - files attached. (track the S/Signal circuit)
(I hope I did transpose this in error, because maybe this will make far more sense)
 

Attachments

Thread Starter

thunderhead289

Joined Nov 28, 2021
7
Sooooooo
I cut the connections on the PCB around the unknown zener, soldered on some leads with a resistor and throttled the voltage on that bad boy.
she ended up being a 5v zener indeed - increased with voltage change until 5v, then stayed at 5v as I throttled her to 12v.
mystery solved - still dont understand exactly to every detail how this circuit works, but I can now recreate the input circuit accurately.
(I dont feel as bad about understanding it since it seems to be functionally an enigma to you guys as well. I agree it makes no sense, but its a commercial/consumer item that I know works correctly as I have used it even myself.)

thanks for all the help my friends.
catch thunderhead289 on youtube.
https://www.youtube.com/channel/UC7ssYyYxo88laKkU0tW9j9A
 
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