Automatic Transfer Switch (ATS) Circuit

In this article we investigate an ATS circuit for initiating an automatic changeover from mains supply to generator supply through many intermediate transfer stages which involves activating the fuel valve, choke valve and the generator starter. The circuit was requested by Mr. Hari, and another dedicated reader from this blog.

Request #1

Dear Swagatam,

I'm Hari, from Indonesia. Thank you for your circuit ideas, i made a battery charger based on your design. Right now, i'm looking for Automatic Transfer Switch (ATS) for my portable generator. It's 5000VA LPG powered generator with electric starter. Buy ready to use ATS is very expensive, i want to make it myself. Can you help me design the ATS?

Right now, i need to shut off the LPG valve manually to turn off my generator. I have plan to add LPG solenoid valve so i can close/open the LPG supply electrically. And add mechanic solenoid (push-pull, normally pull) to automate the choke.

The ATS system feature i need are:

  1. detect main supply, during normal condition (when main supply on), the ATS close the main to load connection and open the generator to load connection
  2. when main supply off, the ATS open the main supply to load connection, but keep the generator to load connection open.
  3. then, the system will activate LPG solenoid valve (normally closed) to open LPG supply to the engine and activate mechanical solenoid (normally pulled) to push the choke grip to START position
  4. after that, the ATS will send signal to the generator starter and start to crank the generator automatically maximum for 5 seconds. If the engine fails to start within 5 seconds, the system will stop for at least 5 second before attempting to start the engine again.
  5. When 3rd trial fails, the system activate an alarm (it could be flashing light or sound).
  6. if the starter succeeds, and the generator runs, the system will wait for 10 seconds then the system will
  7.  deactivate mechanical solenoid so that it pulls  the choke grip back to CLOSE position.
  8. after this, finally the system will close the connection between generator to load.
  9. if the main power back, the ATS will open the generator to load connection, and keep the generator run without load for 2 minutes and turn the generator off by deactivate LPG solenoid valve.
  10. several seconds later, the system will open the generator to load connection, it close the connection between main to load connection


Sir in my area, we have problem of load-shading. i want a circuit (system) to automatically turn ON a self start gas generator (6 KVAR) when Light (WAPDA Supply) goes OFF and load should shift to generator by itself.
And when Light (WAPDA Supply) is back, automatically turns OFF generator and load should be connected to WAPDA Supply..

I know a system using automatic change over and a relay. it is only to automatically turn off generator and shift to wapda..
automatic change over is used to shift from generator to wapda and relay is used only to turn off generator..

Sir, plz tell me a system so that we can make our task easy to turn on and turn off generator..
i think there may be a system such that when light goes off load automatically connects to generator, and we use remote or cell phone to turn on generator..
and to turn off there is already an automatic system...

Circuit Description

The ATS circuit or automatic relay changeover for generator/ mains circuit as shown below can be understood as follows:

For so long as home mains is present T1 base receives the rectified low voltage DC and keeps T2 base grounded.

With T2 base grounded REL1 is held switched OFF along with REL2, REL3 and REL4, the whole circuit thus stays switched OFF.

With REL4 deactivated, the DPDT holds the home mains supply with the load and the load gets powered via its N/C contacts.

Now in a situation when home mains fails, T1 is inhibited from its base drive and it instantly stops conducting.

With T1 OFF, T2 now activates, switching ON REL1, which in turn activates the LPG solenoid valve for allowing the fuel to reach generator combustion chamber.

After a few seconds delay T3/REL2 also activate pushing ON the choke solenoid into start position. The delay may be fixed by the tweaking the values of R7, C3.

REL2 activation switches ON the 555 astable which starts counting upto 5 seconds and triggers T4/REL3 so that the generator starter motor begins cranking the gen.

The astable allows this to happen for 5 seconds, if the generator starts, a 12V supply from a 12V adapter connected at the output of the generator feeds T6 base and disables the 555 astable.

The above 12V from the gen also activates the 4060 timer/latch which counts for about 10 seconds after which its pin#3 goes high.

The pin#3 high pulse latches the IC and also feeds T5 which deactivates REL2 so that the choke solenoid is pulled back to "close" position.

The 4060 output also simultaneously activates T7/REL4 making sure that the load now gets connected to the generator AC via N/O contacts of REL4.

Now suppose due to some fault, the cranking of the generator starter fails to initiate the generator, the astable makes three attempts with 5 seconds interval between each try.

Since the above pulses also reach IC4017 counter, after three pulses the IC4017 output sequence reaches its pin#10 which instantly latches itself due to a high at pin#13, and also disables the 555 astable by grounding its reset pin#4 via T6.

REL3 now stops feeding the crank mechanism.

An additional transistor driver/RELAY may be configured with pin#10 of IC 4017. The N/O contacts of this relay then could be wired with an alarm for the required warning in case the cranking attempts fails to start the generator.

When mains AC returns, T1 receievs the atached 12VDC at its base, however due to the presence of R2, D3, C5, T1 is restricted from the base voltage for a few seconds, until C5 charges.

In the meantime T7 is disabled and REL4 reverted to home mains position by T8, this happens as soon as mains returns, so that the generator gets immediately unloaded from the connected appliances.

Parts list for the above automatic transfer switch or ATS circuit

R1, R4, R5, R6, R7, R8, R9, R10, R11 = 10K

R2, R3 = 100K
C4 = 0.1uF
C1----C5 = timing capacitors, can be between 10uF to 100uF
All transistors are BC547
All rectifier diodes are = 1N4007
All zener diodes (D6, D10, D12) are = 3V, 1/2 watt

REL1---REL3 = 12V/10 amps/400 ohms
REL4 = 12V/40amps or as per load specs

 IC 555 Astable Configuration

IC 4060 Timer Configuration


  1. Hi Swagatam sir,
    wish u &ur family a Happy Diwali...may god bless u...

    1. Hi Sanatan,

      Wish you too a Very Happy Diwali!!

  2. if i use 3v input on ic LM338 or LM138, can it produce 5.88v output, assuming R1 and R2 are 270R and 1000R?

  3. Hi Swagatam,
    Thank for your effort to help us. I'll try to make it.
    Warm regards,


  4. Hello sir,
    Wish u and ur family a very very happy dipawali .

    1. Hello Avijeet

      Wish you too and your family a Very Happy Diwali.

  5. Hello sir,
    Actually I want to make remote switch using ht12e, ht12d,rf 434mhz tx and rx but this all are not available in my local market.if u suggest me some sites who sells good quality products and reliable in products delevery .this will help me a lot.

    1. you can try this site:

  6. Please sir, if i power the LM338 with a 6v, 4AH rechargeable battery and regulate its output voltage to 4.7v, and use it to charge 5 mobile phones at once, how many times can i use it to charge the 5 phones before recharging the 6v battery again?

    1. 6V4AH will support at the most two mobile phones, and will need recharging after a single charge of the two mobile phones.

  7. please tell me about ic 4060 there is only two pin use pin 3,16circuit ic 555 circuit there is no nagative line is connected and and ic 4017 i do not understand plz email me

    1. set the rest of the pins as shown here:

      use R1 = 1meg
      R2/R3 = set for 10 seconds
      C1 = 0,22uF

      555 and 4017 remaining pins are configured as per their standard connections, check their datasheets.

  8. Sir,
    Can you please provide me the schematic for 555 and 4017 connection or datasheet.
    I am much interested in that circuit.I will make that one and provide you the feed back.

  9. Pls can you update the circuit about 4017 & 555 ,Thanks.

  10. pls sir can the circuit be use with a remote

  11. pls help me sir am in need of it

  12. pls sir can the circuit about automatic electric switch using arduino

  13. Need to know how to mod ur ATS to fit 3 phase 220/440 home supply to work with single phase genset supply? Cheers!

  14. did anybody build this circuit to to test if all the components and circuit is working properly

  15. Sir plz can you upload a circuit diagram of generator start circuit which start the generator on petrol and then shift it to Gas/LPG.

    1. Junaid, I will try to do it soon, can you provide some more details regarding the application, would help me to design it in a better way.

    2. Sir sorry for late reply....! sir i want to make my generator automatic start on petrol then shift the generator on gas and finally turn on the load and when main supply comes it will auto shift the load to main and turn off the short i want to use this circuit with automatic ATS for my generator which is placed on 2nd floor. i will very thankful to you if you upload the circuit diagram as soon as possible

    3. thanks Junaid, but an automatic start will require a specific mechanism, I won't be able to design the mechanism, I can only suggest the relay switching circuit,

    4. sir its fine for me my generator is already button/self start.i think that relay will do the job (automatic start) but my main purpose is that the generator should start on petrol then after 10 to 15 second automatic shift it to the GAS/LPG..hope you will help me for this.

    5. OK...that will require some kind of solenoid valve changeover mechanism, if possible I'll try to design.

    6. Ok sir I am waiting for your design...please if possible make it soon. thanks in advance.

    7. I'll try to design it soon...please stay tuned.

    8. Junaid, I have published the idea here:

    9. Thanx for uploading the circuit diagram i really appreciate your effort....

  16. What should the value of resistor in 4060 ,R1 ,R2 ,P1?

    1. R1 can be 1.5 meg....R2 = 10K...P1 = 1M pot

    2. Good day Swagatam. I have build this circuit quite some time ago and gone as far as to get a PCB made by a local manufacturer. The circuit works brilliantly but I need to note the following. When the mains AC returns, relay 4's coil remains energized. Relay 4 never switch back to mains power. I have terminated T8 emitter to ground with a diode that solved the problem but the switch back to mains power is instantaneous with no delay, the delay is crucial not to damage any electronics with the switch back. Can you comment and advise what can be done to solve this as the original configuration of T8 emitter to pin 3 of 4060 does not work. Thank you for the great website. Not many people do things like this for free. THUMBS UP.

    3. Thanks very much Chris, I am glad it's working as per the expectations!

      yes it seems there's a "typo"...T8 emitter must be connected with the ground and not with pin#3 of 4060....however a diode at the emitter of T8 may not be necessary, you can connect it directly to the ground line for a better response and an full deactivation of T7....

      when mains returns, the rel4 response should be instant according to me, but if you think that rel4 should respond after a couple of seconds, you can implement it by attaching a resistor between the points where the collector of T8 joins with the base of T7 and the where C1 positive is connected.

      This resistor value should be tweaked along with R1 value such that together they do not hamper the Rel4 activation current specs, and allows REL4 to operate normally.

      another point that requires to be corrected is the IC 4060 resetting back action when mains returns.

      This may be done by connecting pin12 of the IC to grounds via a 1M resistor, and then connecting a 0.22uF cap across pin12 and the 12V from the mains adapter supply :)

      I hope I could explain the mods clearly:) If you have further doubts please do let me know!


  17. can we delay the circuit for 1-3 min to make sure that the power is off before we start the generator

    1. you can do it by removing D3 and connecting C5 directly to the base of T1....but this will also mean an equal amount of delay for the change over when mains returns

  18. hi swagatam
    do you have an ats circuit like this that works from a pic16f876a or similar


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