How to use 555 timer IC in the circuit to keep the bulb (lamp) on for several minutes and then return to its off state.
555 timer is widely used in electronic field. Today, we will use 555 timer in monostable multivibrator configuration to energize the relay with touch driver (transistor) and operate 230V AC lights when the relay normally open contact is energized.
555 timer for monostable mode connection
The configuration of the 555 timer connected in monostable mode is shown above. This is achieved by connecting pin 6 and pin 7 together (using jumper bridge). Then, connect pin 8 to the power rail (power cord). Then, connect a 10k Ω resistor between pin 4 and power rail, and then connect a 100 μ f 16V capacitor (electrolytic capacitor) to ground (0V or neutral wire of power supply) at the same pin 4.
When the power is initially turned on, the essence of the resistor capacitor network at pin 4 is to slow down or stop the voltage at pin 4 to start immediately. This will eliminate the false triggering of the 555 timer when we don’t even intend to operate nit, starting with the bulb.
Now, starting at pin 7, connect a 100k Ω variable resistor in series with a 10k Ω fixed resistor to the power rail (power cord). That is, the two resistors will be connected in series and one terminal will be connected to pin 7 and the other to the power rail. The fixed resistance is to ensure that the circuit will not be affected in any way after the variable resistance is adjusted to 0 Ω. Now, ground a 1000 μ f 16V capacitor from pin 7 or pin 6 (the negative or neutral line of the power supply).
Finally, for the first stage, a 10k Ω resistor is connected in series with pin 2 (trigger pin or input) and the power rail. Then, connect a button soft switch between pin 2 and ground (neutral). This is to keep the input of 555 timer IC high so as not to “hang” it to avoid triggering it by mistake.
Transistor driver stage
This is just a transistor driver phase. It’s just a transistor connected as a switch. When the output of the monostable multivibrator (555 timer) has voltage, its base emitter node is positively biased, which will cause electrons to migrate from the emitter to the collector, so that a large amount of current flows through the relay, so that the relay is energized, so the contacts of the relay are closed, so our lamp and the relay contacts are opened in series.
The resistance in series with the transistor base is only used to limit the current flowing into the transistor base to a safe level, so as not to damage the transistor or exceed its rated value. The relay coil terminal is connected between the collector of the transistor and the power rail.
Note that a diode must be connected between the collector of the transistor and the power rail. This is to eliminate the back emf that may be generated by the relay coil when the relay coil is de energized, thus damaging the transistor. Finally, the bulb (alternating current bulb) is connected in series with the normally open contact of the relay, and the other end is connected to the live connection of the alternating current source. Then the other terminal goes to the zero line of the AC power supply.
In short, a relay works only in place of a common local switch (bulb switch).