SN74AHC05: Paper sensor is always detected without paper also

Hemantha Kumara1

Part Number: SN74AHC05
Other Parts Discussed in Thread: SN74LS06, LM393

Dear Team,

we are getting issues with SN74AHC05 IC issues in printer. pls find the below circuit and need your support on this issue.

Issue:

Paper sensor is always detected without paper also. Earlier we used SN74LS06 IC and now we are using SN74AHC05.

1. Paper sensor is connected to J3 Connector, output of paper sensor is compared with LM393 IC and PESENSE signal connected to SN74AHC05 IC.

2. PESENSE Signal connected to SN74AHC05. Output of PESENSE1 signal is connecte to MCU pin.

3. PESENSE1 signale is connected to MCU 91st pin

Seeking your help here!!

Thanks,

Hemantha Kumara N K

over 6 years ago

0 Clemens Ladisch over 6 years ago

Guru 317370 points

The schematic did not get attached.

Did you measure the voltages at the input/output pins of the logic gate?

0 Dylan Hubbard over 6 years ago in reply to Clemens Ladisch

TI__Mastermind 46045 points

Hi Hemantha,

Could you share the schematic you are referencing?

0 Hemantha Kumara1 over 6 years ago in reply to Dylan Hubbard

Prodigy 110 points

Dear Sir,

Please find the attached file for the schematic and please do let me know if any details needed.

Needed your valuable help.

Thanks and Regards,

Hemantha Kumara NK1207.Schematic.docx

0 Hemantha Kumara1 over 6 years ago in reply to Clemens Ladisch

Prodigy 110 points

Dear Sir,

Yes , Please find the attached file for voltage details.0602.Schematic.docx

0 Hemantha Kumara1 over 6 years ago in reply to Clemens Ladisch

Prodigy 110 points

Dear Sir,

Yes , Please find the attached file for voltage details.3872.Schematic.docx

0 Clemens Ladisch over 6 years ago in reply to Hemantha Kumara1

Guru 317370 points

The LM393 has open-collector outputs. Connecting it directly to a TTL input happens to work (but is very sensitive to noise), but a CMOS input will just stay at the old value, or float. You need a pull-up resistor on PESENSE.

1.72 V is not a valid logic level. Where does it come from? How can the comparator output a value that is not at the rails? Is there anything else connected to PESENSE?

0 Dylan Hubbard over 6 years ago in reply to Clemens Ladisch

TI__Mastermind 46045 points

Hi Hemantha,

Is there a pull-up connected to the output of the LM393?

0 Dylan Hubbard over 6 years ago in reply to Dylan Hubbard

TI__Mastermind 46045 points

Hi Hemantha,

I haven't heard back from you in a few days so I assume this has been resolved. If not feel free to post a response and I will be happy to continue support.

0 Hemantha Kumara1 over 6 years ago in reply to Dylan Hubbard

Prodigy 110 points

Dear Sir,

we tried 2 options and it is working fine.

1) we have changed firmware and started to using this device.

2) we checked with resistor and it is working fine.

Thanks for your valuable feedback & help.

Warm regarrds,

Hemantha Kumara NK

0 Dylan Hubbard over 6 years ago in reply to Hemantha Kumara1

TI__Mastermind 46045 points

Hi Hemantha,

I'm glad you were able to resolve you issue! I also appreciate you marking a verified answer for others who may also be experiencing a similar issue.

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SN74AHC05: Paper sensor is always detected without paper also