HP Printer (Deskjet 6840) PhotoInterrupter fix - Page 1
Forum section: electronics / beginners

author: Rentium
author_rank: Contributor
timestamp: September 19, 2015, 08:55:33 pm
content: Hi!This is my first topic and i'm not sure where i should post it, it's a fix, but it's also a newbie question so i figured this is the right place.My printer recently suffered a common problem where the lever that detects the main cover position broke off and makes it impossible to print anything since it's stuck in the open position.I have little knowledge about the operation of a photo-interrupter and i'm wondering if i can replace it easily soldering a few wires and a manual external on-off switch that i can use to send the "cover open" or "cover closed" signal to the processor so i can send the closed cover signal to print or the open cover signal to change the cartridges (They get locked if the processor thinks the cover is closed).I read a few common photo-interrupters datasheets and my conclusion was that the output was 0V for low logic signal and 5V for high logic signal however i'm not sure if (and how) i can simulate that with a on-off switch.Here´s a link to a imgurl album with a few pics of the photo-interrupter.http://imgur.com/a/ageZEThanks in advance,Rentium.

author: SL4P
author_rank: Super Contributor
timestamp: September 19, 2015, 09:18:25 pm
content: Test your problem by putting a piece of opaque cardboard or plastic in the slot/gap of the opto (photo interrupter).that will stop light getting from the emitter to the detector - telling the printer the (broken/missing) lever is back in place.The material you use should be opaque to IR, and make sure to completely blocki the optical path. |Ou

author: Rentium
author_rank: Contributor
timestamp: September 19, 2015, 09:53:17 pm
content: Thanks, the opto is operating correctly and i can tell the printer the lid is closed or open by blocking/unblocking the path just like you said, the problem lies on a lever that just broke off and seems like a common fault in this model:http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01065334&cc=us&lc=en&dlc=en&product=385458.The lever is broken in a way that i can't put it back together and the opto is located deep inside the printer, that makes it very hard to try and fix the (odd shaped) lever hence i wondered if i can replace it with an external switch and operate it manually when needed.Thanks again,Rentium.

author: SL4P
author_rank: Super Contributor
timestamp: September 19, 2015, 10:05:23 pm
content: Yes - you could replace the opto with a switch -- at least that lets you 'return the cartridges to the centre when you want to load/change them.In the attached image - you basically want to put a switch in place of the RHS of the opto.If you decide leave the opto in circuit on the board (no issue yes or no) - then you must block the IR light path, or the switch is effectively <always> closed - because the LEDs light can always reach the photo-transistor

author: Rentium
author_rank: Contributor
timestamp: September 20, 2015, 02:24:03 am
content: Like you said, I soldered two wires and a switch ( A desk lamp switch) across RHS , covered the sensor with a piece of cardboard and insulating tape and voiláworks perfectly, Thanks.? Thanks to you.

author: SL4P
author_rank: Super Contributor
timestamp: September 20, 2015, 02:26:54 am
content: Very glamorous!