Commercially available baby monitors are ridiculously overpriced, usually only allow one-way communication and cannot be used for any other application. Once the child has outgrown the device, it is useless. In addition, the radio range of the devices is usually very limited.
An alternative would have been smartphone apps or the baby monitor function of our landline phones. However, neither of these appealed to me as a permanent solution.
I then had the idea of using a walkie-talkie with Auto-VOX function. A hands-free function that is automatically activated when an acoustic threshold is exceeded. I decided on the Motorola T82. The practical problem was that the threshold value had to be exceeded for at least 1 second in order to activate VOX. If the child repeatedly cries only briefly, VOX remains switched off.
I decided to use the Max9814 microphone breakout for the control. It is very well documented and there are various libraries available. The gain can also be easily changed. With an Arduino Nano, the signal of the Max9814 is read out via the analogue input. If a threshold set by me is exceeded, a digital output is switched for 2 seconds. This in turn (very inelegantly, I know) controls a relay. The power supply is provided by a 18650 Li-Ion battery, which can be easily charged via the TP4056 charge controller using a micro-USB cable. The DC-DC converter MT3608 raises the cell voltage of the battery to 9 V for the Ardunio Nano.
If the walkie talkie has an external trigger, the transmission can be controlled externally. The subsequent modification of the device, e.g. by simply soldering two cables to the send button to bridge this, leads to an end of the operating license and is illegal.