Wireless 305 MHz doorbell button. Schematic diagram, how it works.

Today I'll be taking a look at a wireless button that comes with a radio receiver and can function as a doorbell. The device is called the LK-2AS.

This is a circuit diagram of a radio transmitter-button. An encoder is built on two S9014 transistors. It is a rectangular pulse generator with a frequency of 32.768 Hz or 32.7 kHz. The frequency is set by a quartz crystal resonator, which is connected as positive feedback. The signal from the encoder is fed to the radio transmitter, which operates at frequencies of approximately 303-310 MHz. This frequency can be changed by rotating the metal core located in the inductor coil. The frame on the board is the antenna. The signal from the transmitter is sent to the radio receiver, where it is decoded and the bell is triggered.

This is what the encoder signal looks like when it goes to the base of the 9018 transistor.

This is the transmitter signal on the computer screen.

Here are some tests I ran with this wireless button. I replaced the 9018 transistor with a BFR93A. The radio signal weakened and the current consumption increased, so it's best not to do this. I also connected an antenna to the 9018's collector, and the signal became stronger. The antenna is a wire approximately 15 cm long, but connecting it changes the transmitter frequency, which must be adjusted by adjusting the core. This antenna is also noticeably affected by hand movement, although this may not affect operation. I also built an encoder using two 9014 transistors according to the schematic, and it works.