Hey Folks!

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.

The above figure shows a Paper schematic of 555 adjustable timer. The 555 is a highly stable device for
generating accurate time delays or oscillation. Additional terminals are provided for triggering or resetting if desired. In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor. The circuit may be triggered and reset on falling waveforms, and the output the circuit can source or sink up to 200mA or drive TTL circuit.

In the Monostable mode, the LM555 timer acts as a one-shot pulse generator. The pulses being when the LM555 timer receives a signal at the trigger input that falls below a 1/3 of the voltage supply. The width of the output pulse is determined by the time constant of an RC network. The output pulse ends when the voltage on the capacitor equals 2/3 of the supply voltage. The output pulse width can be extended or shortened depending on the application by adjusting the R and C values.

Step 1: Required Hardware

The following components have been used:

1. X1 IC 555 timer

2. X2 3KΩ Resistor

3. X4 10KΩ Resistor

4. X1 1MΩ Potentiometer

5. X2 IN4004 Diode

6. X2 Tactile Momentary Push Buttons

7. X1 SPDT Slide Switch

8. X2 100uF Capacitor

9. X2 0.1uF (100nF) Capacitor

10. X1 2 Pin Screw Terminal

11. X1 3 Pin Screw Terminal

12. X1 12VDC Relay

13. X1 12VDC Adapter

14. X1 5mm LED

Step 2: Circuit Schematic & Working

The above figure shows the circuit schematic of a 555 adjustable timer. 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.

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.

When pin 4 of the LM555 is triggered (by shorting it to the ground) through the momentary switch S2, the timer is reset.

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.

Step 3: PCB Design

The figure shows the circuit PCB Design of 555 adjustable timer using Eagle software.

Parameter consideration for PCB design:

1. Trace width thickness is minimum 8 mil.

2. The gap between plane copper and copper trace is a minimum of 8 mil.

3. The gap between a trace to trace is a minimum of 8 mil.

4. Minimum drill size is 0.4 mm

5. All the tracks which have current path need thicker traces.

Step 4: PCB Manufacturing

You can draw the PCB Schematic with any software as per your convenience. Here I have my own design and Gerber file. After you generate the Gerber file you can send it to the manufacturer. I personally recommend LionCircuits.

I upload my Gerber files on their online platform and get an instant quote to place an online order. They charge less for the prototypes and I receive my PCBs in just 6 days.

Stay tuned guys! I will post the second part of this instructable when I receive my boards.