Lighting Governor Using 555 Timer IC

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.

Circuit diagram :

Lighting Governor -Circuit Diagram

Lighting Governor Circuit Diagram

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.

Author : Peter Jansen - Copyright : Elektor

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