Here is a simple and low cost automatic bathroom light circuit. The light automatically turns on when the bathroom door is opened, and remains lit in the occupied state. When the door is opened again the lamp turns off after a very short delay. This automatic circuit saves energy and greatly assists your visitors, especially at night. The “single-chip” circuit, built around the popular decade counter CD4017 (IC1), is very compact, and can be powered from any 5VDC/500mA mobile phone travel charger/similar smps adaptor. After construction, try to enclose the whole circuit in a suitable plastic case for total convenience. Next, arrange the door switch (S1) such that when the door is closed it should be in closed state.
How does the automatic toiled light works?
The heart of this circuit is CD4017 (IC1), CMOS counter with decoded output. On each pulse received on its clock input, this IC drives high the next output. Output 0 (pin 3) of IC1 drives a green “Toilet Free” indicator (LED1), and output 1 (pin 2) of IC1 drives the 1W light source (LED2) through a driver transistor (T1). Note that T1 (2N2222) is here wired as an emitter-follower to avoid the use of a base resistor.
Initially when the circuit is powered, IC1 is resetted by the RC network R1-C1 and as a result LED1 lights up. When the door is opened for the first time, switch S1 is opened and as a result IC1 is clocked by the clock pulse generator R2-C2. Now LED1 goes off and LED2 lights up. IC1 stays in this condition to provide the interior lighting, even if the door is closed. However, when the bathroom door is opened again IC1 is clocked again, and this resets the chip by the signal from Output 2 (pin 4) of IC1 through D1 (1N4148). When IC1 activates its Output 2 and reset itself, LED1 lights up again!
Regarding components selection, nothing is really critical. But try to use a good quality “normally-opened” (N/O) switch as S1. The 3.6V/1W White LED (LED2) current is limited by resistor R3. It may become necessary to adjust this value to get the best from your circuit.
Since the clock generator for IC1 is realized using a tricky RC network, erratic switching may be occurred. Although the bumps are rare, you can defeat this by adding a manual reset switch (again one N/O push switch) between +5V and pin 15 of IC1.