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I have a remote like this enter image description here

I removed the plastic sticker and below, we have a series of push buttons. Two pieces of copper in SSS form. When you push and the plastic bends, a little metal piece touches the SSS and you close the circuit.

The device seams to be performing some kind of scan over the rows and the columns since the buttons clearly share "pins" of the "chip".

I tried short-cutting with a piece of wire the two sides of one button and I can easily fake a finger press.

I tried substituting my finger by a transistor powered from the RasPi and .. there is no way to get it to work :(

I removed the battery of the remote and powered straight from the 3V3 of the Pi. The device works (short cutting still works).

I connected emitter and collector to the proper places. (To discover the polarity I short cut it with a diode. It only works in one direction as expected.)

I then connected a 200 omh resistor to the base and connected to the 3V3 and ... nothing happens ...

What can be happening? I tested the transistor in a protoboard with an LED and it works as expected.

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    \$\begingroup\$ Post a schematic. Also, someone was asking a very similar question recently, and you may want to read through it. electronics.stackexchange.com/questions/310259/… \$\endgroup\$
    – user57037
    Jun 17, 2017 at 21:44
  • \$\begingroup\$ Alternatively, use the rpi to control a IR led directly and forget about the remote. Make the RPi the remote instead. \$\endgroup\$
    – Passerby
    Jun 17, 2017 at 22:01

3 Answers 3

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This most likely is a matrix keyboard. This means there is no common potential you could refer to, but you need that to place transistors across more than one of the keys, and use a raspberry to control them.

I recommend YOU to use reed relays. They are small and they don't give you a headache.

There are ways to get around that limitation but they are not easy to implement and require to re-engineer the schematic first.

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  • \$\begingroup\$ any recommended cheap small reed relay ? \$\endgroup\$
    – javirs
    Jun 17, 2017 at 21:58
  • \$\begingroup\$ This depends on the distributor. Where do you buy? \$\endgroup\$
    – Janka
    Jun 17, 2017 at 21:59
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This is similar to How do I read a Board? in that the author was trying to perform a similar modification.

enter image description here

Figure 1. A typical keyboard matrix. Scanning the keyboard is done by pulling each row low in sequence and reading which, if any, column is pulled low by a switch. This arrangement allows a matrix of R x C buttons with only R + C pins on the chip.

Note that with the arrangement shown in Figure 1 that neither side of any switch is connected to ground so your simple transistor idea won't work.

Check the battery supply on the remote. If it is 3 V and the Raspberry Pi is 3.3 V then you can probably remove the remote battery and power it from the Pi. This will make control much simpler.

See the linked question for one means of performing the switching action using a CD4016 CMOS analog switch.

Another idea would be to try an FET opto-isolator.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Pulling the GPIO pin low will turn on D1 in the FET opto-isolator shorting out the switch contacts.

A regular transistor opto-isolator might work too but you'd have to get the transistor connected right way around on the matrix. The FET should work either way.

I've never tried this but it might work!

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I understood that you got a proper fake pressing generated by connecting a diode. Then you shoul succeed also by inserting optocouplers.They do not care the potential difference between your Pi and the matrix keyboard reader.

I have once generated fake pressings by connecting 2 analog multiplexers between the X and Y rows of an existing matrix keyboard. It was year 1982. I wanted to get high quality printed output from a computer that had Centronix type printer interface. I built a box that made an electronic typewriter to Centronix compatible printer. The needed X and Y numbers for each character were stored in EPROM.

It's well possible that you can do the same, except no EPROM is needed, only 2 analog multiplexers and six outputs to it from PI. This unfortunately needs exact knowledge of voltages and polarities. If you haven't it, then try optocouplers or even relays that are even more robust.

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