This circuit is very handy as a timer circuit for a lamp, for lighting a staircase, for example, but can also be used as indicator for the front doorbell. A significant advantage of this circuit is that the circuit draws almost no current when in the inactive state. The circuit is activated with push button (S1), after which IC5 (a 555 timer IC) starts to count down the set time. During this time the triac continues to conduct and the lamp is turned on. The ‘on’ time of the lamp is on is determined by the combination of R1 and C2 and can be changed as required by your application or personal preference. R2 and C3 have been added because the 555 expects a ‘negative’ pulse at its trigger input. When the power supply is turned on, C3 holds the TR input of the 555 Low for a short time, which triggers the timer IC.
Depending on the exact type (brand) of 555, the value of C4 (330 nF) may have to be changed to ensure a high enough power supply voltage when in the active state. Note also that you shouldn’t use a ‘too heavy’ version of the triac. The circuit will drive at the most just a little more than 5 mA into the gate of the triac. The circuit worked properly when tested with a TIC206 and the slightly bigger TIC216. When selecting push button S1, take into account the switching current of the lamp. The switch must be able to handle that safely.
In the event of a defective part, a 15- V zener diode is connected across the power supply for protection (D3). R6 and R7 have been added so that C4 will be discharged. In this way no dangerous voltage can remain when the circuit is unplugged. When large values for C2 are used, such as the 470 μF shown here, a good quality capacitor is required for C4. Any potential leakage resistance will then have no influence on the set time. Because of an inferior capacitor in our prototype the time was considerably longer than expected…
