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07-04-2018

Simple circuit times bathroom fan

National Semiconductor » LM2905

Maxwell Strange

EDN

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 7 V 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 CT and resistor RT 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 0.5 A ac load current. These relays typically turn on reliably at 3 V. You can use resistor R to drop excess voltage, thereby reducing loading on the power supply.

Simple circuit times bathroom fan
Figure 1. Gain control of your bathroom fan by using this simple time-out circuit.

The p-channel JFET turns on when the power switches off, rapidly discharging CT 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 1.4 W. 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.

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