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IC 555 Timer for Switching Two Alternate Loads with Predetermined Delay

IC 555 Timer for Switching Two Alternate Loads with Predetermined Delay

In this post we learn how to make a simple IC 555 based alternate relay timer circuit for toggling a couple of loads alternately with a specified length of delay, as determined by the calculated values of the relevant components. The idea was requested by Mr. Sanjoy.

Circuit Objectives and Requirements

  1. I am a regular reader of your excellent posts. Here I would like to request a circuit design.
  2. I am trying to build a laboratory paper coating set up for which I am going to employ 2000 watts hair driers for drying of the coated paper.
  3. The problem is that those driers can't be run continuously.
  4. Therefore I decided to use two driers alternately for three minutes each. But alternate switching of these driers manually always is tiresome.
  5. So I request a circuit which will be able to switch on and switch off the the driers alternately and automatically for a preset period of time and continue doing this until switched off.

The Design

Referring to the below shown alternate switching relay circuit, or we can also call it an alternate switching flasher circuit, the idea may be understood with the help of the attached explanation.


The circuit is built around a standard IC 555 astable configuration, which basically produces an alternating high and low or an alternating 12V and zero volt at its pin#3 when switched ON.

This alternately switching output accompanies certain delay between its ON/OFF switching as determined by the component values of R1, R2, and C.

In the proposed design the values of these timing components are suitably calculated to produce an approximately 50% duty cycle, and having a delay length of around 180 seconds or 3 minutes.

With the shown arrangement, only the value of the 470uF capacitor needs to be altered for achieving other preferred time delays across the output pin#3.

The pin#3 can be seen connected with a transistor relay driver stage which responds to the high/low pulses from the pinout and accordingly switches the relay contacts across the N/C and N/O contacts.

Since the two loads are connected across these two contacts of the relay, these are also alternately switched from ON to OFF and vice versa with a delay of 3 minutes between each switching.

The two LEDs connected across the supply pins and the pin#3 of the IC helps to indicate which load may be in the switched ON or OFF position at a given instant.

The above explained alternate switching relay timer circuit can be also implemented for other identical applications and the ON/OFF periods can be independently adjusted for achieving different ON/OFF sequences, by suitably altering the R1/R2 timing components of the astable.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

13 thoughts on “IC 555 Timer for Switching Two Alternate Loads with Predetermined Delay”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. I am looking for an alternating relay that will simply replace a manual 24 volt 3 position toggle switch; load A, off, load B, which operates two irrigation solenoid valves. When my mechanical timer shuts off, there is “no” power to the system, when the mechanical timer comes back on power is restored and i need to switch to the other valve, and so on. All the relays i have found require continous power to work. Can you tell me if i am looking for something no one makes???? I appreciate any input.

    • It may be possible to switch loadA and loadB alternately, but how’s the off position supposed to be switched, i can’t figure it out? Please clarify this!

  3. Hi sir I really appreciate your concern I love so much for that you rely my message God will bless you and your family, pls I will he in touch if I need some support over circuit thanks.

  4. I need a circuit that will time the duration of an induction heater. The heater run time can be from 1 to 30 seconds, depending upon part size and configuration, at which time it turns off, shutting off the power relay to the heater. It would then trip a second relay with operates a solenoid to remove the part from the heater and reset everything to the initial state. A push button would initiate the heating/part ejection cycle.

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