Electronic projects

Six timing positions suited to different skin types
Timing affected by sunlight intensity

Circuit diagram

Parts:
R1 47K 1/4W Resistor
R2 1M 1/4W Resistor
R3,R5 120K 1/4W Resistors
R4 Photo resistor (any type)
C1,C3 10µF 25V Electrolytic Capacitors
C2 220nF 63V Polyester Capacitor
D1,D2 1N4148 75V 150mA Diodes
IC1 4060 14 stage ripple counter and oscillator IC
IC2 4017 Decade counter with 10 decoded outputs IC
Q1 BC337 45V 800mA NPN Transistor
SW1 2 poles 6 ways Rotary Switch (see notes)
SW2 SPST Slider Switch
BZ1 Piezo sounder (incorporating 3KHz oscillator)
B1 3V Battery (two 1.5V AA or AAA cells in series etc.)

Device purpose:
This timer was deliberately designed for people wanting to get tanned but at the same time wishing to avoid an excessive exposure to sunlight.
A Rotary Switch sets the timer according to six classified Photo-types (see table).
A Photo resistor extends the preset time value according to sunlight brightness (see table).
When preset time ends, the beeper emits an intermittent signal and, to stop it, a complete switch-off of the circuit via SW2 is necessary.

Photo-type, Features and Exposure time
I & children Light-eyed, red-haired, light complexion, freckly 20 to 33 minutes
II Light-eyed, fair-haired, light complexion 28 to 47 minutes
III Light or brown-eyed, fair or brown-haired, light or slightly dark complexion 40 to 67 minutes
IV Dark-eyed, brown-haired, dark complexion 52 to 87 minutes
V Dark-eyed, dark-haired, olive complexion 88 to 147 minutes
VI The darkest of all 136 to 227 minutes
Note that pregnant women belong to Photo-type I

Notes:
Needing only one time set suitable for your own skin type, the rotary switch can be replaced by hard-wired links.
A DIP-Switch can be used in place of the rotary type. Pay attention to use only a switch at a time when the device is off, or the ICs could be damaged.