The switch circuit, intended for low-voltage halogen lights, extended the life of the lamps, because it ensures that the filament current is increased gradually, thus obviating the high peak currents that flow through the lamp with normal switches, The addition of a timer would enable the circuit to switch the lamp off again after a preset period of time.
The lamp is switched via T1m a MOSFET that has a channel resistance of only 0.08 Ω, which ensures that losses are low in the prototype <250 mW. Control is by means of pulse-width modulation, which also tends to minimize losses.
The circuit is switched on and off with S1. Bistable IC2b is a debounce circuit that clocks binary scaler IC2a. When the Q output of the scaler is high, the lamp is on or is coming on; when the Q output is low, the lamp is out or going out. The lamp may be switched off automatically by IC3 after a preset time The time, t, in seconds, is calculated from
where R4 is in ohms and C7 in farads. With values as shown, the time is 700 s (11 min).
When the reset input of IC2a is earthed, the lamp can be controlled only via S1: timer IC3 and the associated RC network can then be omitted.
When the lamp is switched on with S1, the voltage across C6 rises slowly. Because of D3, even at standby, there is a potential across C6 at a level just below that necessary to toggle comparator IC1b. As soon as C6 is being charged, the comparator will, therefore, toggle almost immediately. This starts rectangular-wave generator IC lc. However, it is not the rectangular signal that is used here, but the triangular signal across C8. That signal is compared With the voltage across C6. This results in a 25 kHz rectangular sign. al at the output of IC1d, whose pulse width increases slowly. That signal is used to drive T1, and thus the lamp, which will gradually begin to, light.
The voltage across C6 continues to rise until the toggle level of comparator ICia is reached. That circuit then toggles, which stops generator IC1c, but T1 is kept conducting by ICid. The potential across C6 is kept just above the toggle level of ICia by D2. This arrangement makes it possible, if required, for the lamp to be turned down almost immediately after S1 is pressed or the time set for IC3 has elapsed.
When IC1a toggles again, the triangular voltage is compared with the falling potential across C6. so that the pulse width of the output signal from 1C1d decreases. When the voltage across C6 has reached the level at which IC lb toggles, the generator is switched off again, but this time T1, and thus lamp, remains off.
Finding switch S1 in the dark is facilitated by using a switch with integral LED (D4). The power supply consists of a suitable mains transformer (which is probably already present for the lamp) and a bridge rectifier rated at 3 A. The current is drawn primarily by the lamp: with a 20 W lamp, it amounts to 1.6 A.