This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
The operational amplifier used in this project no longer exists, but the circuit can be adapted to...
With S1 closed, when dark, the triac will supply the load with the power set in R4. The LDR is...
Toshiba LDO Regulators Toshiba has a wide variety of low voltage drop (LDO) linear regulators...