Telephone Baby Monitor

The usefulness of baby monitor alarms using (FM) radio has always been limited by poor range and its susceptibility to interference. This novel design uses a microcontroller to take a fresh look at the problem.There are several different types of baby monitor on the market. The most common provides a short-range radio link (typically 100 m) between the transmitter unit in the nursery and the receiver unit carried by a parent. Another type uses audio to modulate a carrier frequency sent along mains wiring to the parents receiver plugged into a mains outlet. The RF model has a limited range and the mains modulation method requires that the parent cannot wander too far from a mains socket. Both systems are susceptible to interference.

In the circuit diagram, the value of C8 should be corrected to read 470 pF.