These days, 433-MHz RF transmitter/receiver modules are very popular with electronics hobbyists and tinkerers. These cheap short-range radio modules can be used with almost all microcontrollers for a ...

The receiver is equally simple and also based on a CMOS 555. As long as the sensor picks up infrared light from the transmitter, the reset input of the 555 IC is held low and the buzzer is silent.

Electronic Circuits and Tubes By the Electronics Training Staff of the Cruft Laboratory, Harvard University. Pp. xxiv + 948. (New York and London : McGraw-Hill Book Co., Inc., 1947.) 45s.