Many young children will insist on keeping the bedside light on for a couple of minutes after the storybook has been closed and father or mother has gone downstairs. They are also prone to fall asleep with the light on, which is a waste of energy, and a problem for the parent because the light has to be switched off without waking the child.
The timer shown here is an elegant solution to this little domestic problem. Simple to build from a handful of inexpensive components, it lets you determine how long the bedside light remains on after you have said goodnight and actuated the timer.
Pressing switch S1 causes bistable ICib to toggle, and produces a debounced clock pulse at the input of the second bistable, ICia, whose Q output goes high, triggering a low-power thyristor, Th]. The complementary bistable output, Q, goes low and enables timer IC2. The load, a small bulb (max. 60 W) is switched on and remains on until counter IC2 resets ICia. The counter, a Type CD454I, has an onboard oscillator that operates at a frequency, f, given in Hz by
f = 1/2.3 Rtc, Ctc,
where RTC and CTS are the resistor and capacitor connected to pin 1 and 2 respectively. The resistor connected to the Rs input, pin 3, has a value of about 2R-rc. The scale factor of the 4541 is set to 65 536 (216) hereby tying its A and B control pins to the positive supply rail. This means that the OUT pin changes state after 32 768 clock pulses. The logic levels defined at pins 5, 10 and 9 select a logic low level at the OUT pin when the RESET pin is logic high. Hence, the delay, t, in seconds, introduced by the circuit can be calculated from
t = 2.3 x 32 768 X R5 X C5.
The circuit is powered directly by the mains. Transistor T1 forms a 10-V zener diode. A LED, D2, is used as an orientation aid for the child. The LED forms part of the ITW (Digital) push-button, S1.
It should be noted that the actual supply voltage of the circuit may lie between 6 V and 12 V, depending on the characteristics of T1. The actual value is of little importance, however, as long as ICia is capable of supplying a trigger current of about 200 μA to the thyristor.
The circuit is constructed on the printed-circuit board shown in Fig. 2, and fitted in a suitable ABS enclosure. In the interest of safety, make sure the input and output cable are properly insulated and secured with strain reliefs. The clearance for the key top in the top panel of the enclosure must be made as small as possible to prevent any risk of the circuit being touched.
WARNING. Since the circuit carries dangerous voltages at a number of points, it is essential that proper electrical insulation is applied. Never work on the circuit when the mains is connected to it. Make sure that no part of the circuit can be touched when it is being set, adjusted or used.
R1 = 180 kΩ, 0.5 W
R2 = 220 kΩ
R3, R4 = 470 kΩ
R5 = 180 kΩ
R6 = 390 kΩ
C 1 = 22 μF, 16 V
C2 = 100 nF
C3, C4 = 1 nF
C5 = 47 nF
D1 = 1N4007
D2 = LED, high-efficiency (see S1)
D3 = 1N4148
B1 = B380C1500
T1 = BC547A
Th1 = BRX49
IC1 = 4027
IC2 = 4541
S1 = digitast push-button switch with integral LED
K1, K2 = PCB-mount terminal block, pin spacing 10 mm ABS enclosure about 100 γ150 γ125 mm