Introduction: 555 Adjustable Timer (Part-2)

Hey guys!

Learn how to make a precisely adjustable timer with a variable delay from 1 - 100 seconds that use a 555 IC. The 555 timer is configured as a Monostable Multivibrator.

Let's pick up from where we left last time. For people who didn't see Part-1 CLICK HERE.

Step 1: Fabricated Board

The figure shows a fabricated PCB board from LionCircuits. Their automated platform makes it easier to just place an order online and receive good quality and quick turn PCBs.

Let’s starts with the assembly of this board.

Step 2: Components Assembled Board

The above figure shows all the components are assembled on the PCB board. Double check components with polarities. Lastly, solder the Power adapter to the PCB. Once every component is soldered onto the PCB, you can connect the load across the relay terminals.

The LM555 has a maximum typical supply voltage rating of 16V while the relay's armature coil is enabled at 12V. Hence a 12V power supply is used to minimize the number of components such as linear voltage regulators. When pin 2 of the LM555 is triggered (by shorting it to the ground) through the momentary switch S1, the timer is started.

Step 3: Working

The timer generates an output pulse with an ON time period determined by the RC network i.e t = 1.1RC. In this case, the fixed value of the capacitor is 100uF. The value of R consists of a 10KΩ resistor in series with a 1MΩ potentiometer. We can vary the potentiometer to change the time period of the output pulse.

For example, if the potentiometer is set to 0Ω, the value of R is equal to 10KΩ. Hence t = 1.1 x 10K x 100u = 1 second.

But if the pot is set to 1MΩ, the value of R is equal to 1MΩ + 10KΩ = 1010KΩ. Hence t = 1.1 x 1010K x 100u = 100 seconds.

Step 4: Output

When the timer starts, the relay turns ON. Hence the Common(COM) terminal of the relay is shorted to the Normally Open (NO) terminal. A high power load can be connected to this terminal such as a light bulb or water pump. A transistor Q1 acts as a switch an ensures sufficient drive current is provided to the relay. Diode D1 acts as a flyback diode which protects the transistor Q1 from voltage spikes caused by the relay coil.

LED2 turns on in order to indicate when the relay is turned ON. LED1 indicates the circuit is powered ON. An SPDT switch S3 is used to switch the circuit ON. Capacitors C2 and C4 are used to filter noise in the supply line.