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Capacitive fingerprint sensor not working
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Please be positive and constructive with your questions and comments.

Capacitive fingerprint sensor not working

by Rocketmagnet on Thu Jan 27, 2022 6:42 am

I am trying to use one of the ultra slim Adafruit fingerprint sensors: https://www.adafruit.com/product/4750.

Background
To enrol a new fingerprint into its memory, you place your finger on the sensor twice, and each time it create a model of the print. It's meant to then compare the models to check they are from the same finger before storing the model in its database. This sequence is driven by commands sent over UART using the Arduino library.

However, this process doesn't seem to be working for me.

Problem
During enrolment, if I place two different fingers, it always thinks they match, though they obviously shouldn't.

Debugging process
I can upload the fingerprint images taken during this process, and they look correct. (two different fingerprints)
Next I would like to view the model it creates of the fingerprints, but I cannot find any documentation of the format of the model data.

Questions
Is this the expected behaviour of the sensor?
Is the format of the model data documented anywhere?

Rocketmagnet
 
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Re: Capacitive fingerprint sensor not working

by rpiloverbd on Thu Jan 27, 2022 8:07 am

Never used this. Let's see what Adafruit has to say. Personally, I found this one reliable: https://www.adafruit.com/product/751

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Re: Capacitive fingerprint sensor not working

by adafruit_support_carter on Fri Jan 28, 2022 2:22 pm

You're running this example:
https://learn.adafruit.com/adafruit-opt ... th-arduino
and when it asks to "Place same finger again", you intentionally place a different finger but it sees it as same and says "Prints matched!"?

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Re: Capacitive fingerprint sensor not working

by Rocketmagnet on Thu Feb 03, 2022 7:26 pm

Thanks for the reply. Yes, I am running the "enroll" example code from the Adafruit Fingerprint sensor library. This is the output:

Code: Select all | TOGGLE FULL SIZE
Adafruit Fingerprint sensor enrollment
Found fingerprint sensor!
Reading sensor parameters
Status: 0x4
Sys ID: 0x800
Capacity: 120
Security level: 3
Device address: FFFFFFFF
Packet len: 128
Baud rate: 57600
Ready to enroll a fingerprint!
Please type in the ID # (from 1 to 127) you want to save this finger as...
Enrolling ID #5
Waiting for valid finger to enroll as #5
...............................................................................................Image taken
Image converted
Remove finger
ID 5
Place same finger again
.......................................................................................................................Image taken
Image converted
Creating model for #5
Prints matched!
ID 5
Stored!
Ready to enroll a fingerprint!
Please type in the ID # (from 1 to 127) you want to save this finger as...

In this case, I intentionally placed two different fingers, and it still claims that they matched. This always happens. I purchased two sensors, and they both exhibit this behaviour.
Am I doing something wrong?
Can you recreate this behaviour with one of your sensors?
Would it help for me to post the contents of the packets going to/from the sensor?

Rocketmagnet
 
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Joined: Thu Jan 27, 2022 6:35 am

Re: Capacitive fingerprint sensor not working

by Rocketmagnet on Thu Feb 03, 2022 7:48 pm

Interestingly, the sensor still seems to work.

It stores the both fingerprints in the ID slot, and will match a fingerprint to the lowest ID slot it finds itself in. For example, if I make the following enrolments:

ID 0: index finger twice.
ID 1: middle and ring fingers. It should have said "No match".
ID 2: ring and pinky fingers. It should have said "No match".

Now, if I touch my fingers on the sensor, these are the IDs it returns:

Index finger: ID 0
Middle finger: ID 1
Ring finger: ID 1
Pinky finger: ID 2

Both the middle and ring fingers return ID 1, because that slot has both templates stored in it.

I guess I can still use this sensor; I'll just have to be careful when enrolling. Still, it would be nice to know what's going wrong with the matching during enrollment.

Rocketmagnet
 
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Re: Capacitive fingerprint sensor not working

by adafruit_support_carter on Mon Feb 07, 2022 3:29 pm

Thanks for the additional info. Seems like you're going through the enroll process as intended. There is nominally a check for mismatch:
https://github.com/adafruit/Adafruit-Fi ... l.ino#L203
but that does not seem to be working in your case.

Opened a new issue for this here:
https://github.com/adafruit/Adafruit-Fi ... issues/104

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Re: Capacitive fingerprint sensor not working

by adafruit2 on Mon Feb 07, 2022 3:31 pm

it could be its much less strict on the matching stage - the library just sends the commands over to the sensor using a standard protocol!

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Please be positive and constructive with your questions and comments.