Simple circuit times bathroom fan

Maxwell Strange, Goddard Space Flight Center, Fulton, MD -- EDN, 4/27/2000

Forget to turn off the ceiling fan in your bathroom? Install the simple timer in Figure 1. It's located out-of-sight in the fan unit, and you turn it on via the wall switch. The circuit costs virtually nothing, using "junk" parts. When ac power appears, a simple rectifier develops approximately 7V across filter capacitor C. This voltage powers the LM2905 analog timer and simultaneously triggers it via trigger-input Pin 1; output Pin 7 then goes low, turning on the solid-state relay and fan. Low-leakage capacitor C_T and resistor R_T set the time delay; for the values shown, the delay is 1000 sec, or approximately 17 minutes. At time-out, Pin 7 goes high, turning off the relay. For most fans, this relay can be a small, pc-mountable unit rated at as little as 0.5A ac load current. These relays typically turn on reliably at 3V. You can use resistor R to drop excess voltage, thereby reducing loading on the power supply.

The p-channel JFET turns on when the power switches off, rapidly discharging C_T to allow immediate recycling. The 10-kW series resistor makes the circuit inherently safe; in a worst-case failure, line voltage appears across this resistor and develops a harmless 1.4W. You can easily adapt this circuit to other applications. You can change the timing or make it linearly adjustable, and you can program the solid-state relay to turn on instead of off at time-out, by connecting Pin 8 of the timer to Pin 4 instead of Pin 2. You can also reverse the action of the solid-state relay by reversing the control inputs. (DI #2520)