I bought these wireless door bell transmitters from the local supermarket for less than 4 Euro. The printed circuit board contains a sound module but this is not of interest. I removed the silent alert LED and disconnected the npn transistor from the speaker source. The play input of the sound module is provided by the decoder IC ( you can only see the solder switches ) which is redirected to control the base of the transistor. The little beast drains the batteries within a few days so I cut the switch that disables the speaker to have a power on/off switch.I only added the 2.5mm connector, removed the LED and rearranged some connections ( I used the green wires from the speaker ).Unfortunately I only had a mono connector so the middle ring of the 2.5mm plug is permanently connected to the outer ring. Thus, the focus signal is permanently on. However, it works fine so far.

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