fastestingerfirstcircuit - Make this Fastest Finger First Indicator Game Circuit

Make this Fastest Finger First Indicator Game Circuit

We all have probably seen the famous game show "who wants to be a millionaire" and many similar quiz shows, where in the initial stages a few of the participants are asked to go through a small test, the participants are asked a question, and the member who answers it first by pressing the buzzer is given the opportunity to occupy the "hot seat".

Well everybody might not be as lucky to get into such game shows, but you can certainly enjoy making and playing with this little fastest finger first decider circuit right in your home.The circuit is very simple, utilizes just a couple of ICs and some LEDs.

Circuit Description

Four press to ON type of switches are placed at the input of the IC 7475 which is a 4-Bit Bistable Latch with Complementary Outputs.

The button which is pressed first, triggers the relevant output of the IC 7475, which in turn prompts the IC 7420 to instantly deactivate the other inputs from the other push buttons, rendering the whole system inactive, except the one which had the first trigger through the push button.

This output concerning the triggered latch illuminates the relevant LED, indicating the button number which was pressed first.

The is a simple game circuit which can be built by all electronic hobbyists and also school kids.

fastestingerfirstcircuit 1 - Make this Fastest Finger First Indicator Game Circuit


39 thoughts on “Make this Fastest Finger First Indicator Game Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. I'm still an electronic nube, so may be a dumb question, but I don't see where the voltage input is in the circuit. I see the grounds after the player and reset buttons, which leads me to believe that the input voltage is the trace that goes across the top right corner of the circuit. Which means the reset basically drains all power from the circuit to start it back up clean. Like turning it off and back on. Is this accurate? Also since this is a TTL circuit is it a 5 volt input?


    • Thanks for the pointing it out. Yes the rail which joins the upper 1K resistors, the LED cathodes and pin5 of the IC is the positive line.

      Surely it must be operated with 5V since it's a TTL device.

      I haven't tested it practically so not very sure how it would work.

  3. Hi sir swagatam..
    I'm mark.. and im a beginner in electronics..

    .sir i made the prototype of that diagram and it didn't work at all..
    .where part do you think i've made a mistake?

    .is it correct that i connect the negative input in the push buttons line? Because it didn't work..

    .is my diagram correct wherein i connect the terminals of ic 7475 to ic 7420 in this manner..

    9 to 13
    10 to 12
    15 to 10
    16 to 9

    and then i connect the 8,2,4,5 terminals of ic 7420

    • Let's go step wise.

      Initialy remove the 7420 IC, and short the reset switch pins, and check the outputs by pressing the buttons, the LEDs should light up and stay latched.

      Please confirm this first then we'll proceed with the remaining functions.

  4. .sir if im not mistaken the cathodes are the negative terminals… Sir im just confused why the cathodes in that diagram are connected in the positive line..

  5. sir swagatam i revised my prototype and still nothing happens.
    sir i try to remove the ic 7420 and still the led's didn't illuminate..
    sir what do you think the problem of my prototype?
    i need to pass this project immediately.. and i cant make it work..

  6. .sir how to make 6 volts to 5 volts?
    .maybe thats my error because i first supply it by 6 volts.. I will change the ic tomorrow and try it again.. And i will update you sir of the result.. Thanks for your help sir..

  7. .thanks sir. I got it…
    .. :D..
    .sir please i need your help tomorrow. Can you give me some of your time if i have any questions.. Because i need to finish that fastest finger first until nxt nxt day..

  8. .sir i think theres something wrong in the circuit..
    .because the led's didnt illuminate.. Even one …
    .sir still i cant make it work.. 🙁

    • The circuit is correct now according to the datasheet,

      remove the lower IC and confirm the upper IC functioning first.

      Initially all the 4 outputs will show high voltage (=5V), as soon as one of the switches is pressed the relevant output will become low (0V) and the particular LED will light up.

  9. .sir still its not working. I check my pcb design how many times and it match your schematic diagram..

    .yet its not working.. I try to remove the ic 7420 and still nothing happen..
    .maybe sir theres a problem in the connection of terminals in ic 7475… Because theres no current flow everytime i place it..

  10. The led which is connected to the pin 15 and 16 of ic 7475 glows immediately when the power source is given….
    if the 4 th switch is pressed , the led connected to pin 9 glows….
    if the 3rd switch is pressed , the led connected to pin 10 glows…..
    but if either 1st or 2nd switch is pressed the led connected to pin 15 and 16 which is aldready glowing , glows brigther than the normal……..
    And the reset button is of no use or not working……
    pins 1,4,8,11 of ic 7475 is not used in the circuit , but in the data sheet these pins are mentioned as complementary input and output and enable . what about this ….whether it is related to this project…please give the solution for these problems as soon as possible………..

    its urgent , because the project must be submitted on thursday (28/08/2014)

    • I have modified the circuit a bit by adding a capacitor with pin 13/14 of the IC 7475..this is done to stabilize the circuit during power ON. Please do it accordingly.

      If still the problem persists could possibly mean a fault in your circuit connections or in the IC….remember the ICs work only with 5V.

    • you'll have to study the functioning the IC pinouts and then troubleshoot yourself…It would be difficult for me to suggest without practically checking your assembly

  11. Sir,
    Here all leds are glowing but as per your principle the push button which will be press first, only that led which is corresponding to its particular push button will glow is unfortunately not occuring in my case.
    And reset button is not working.
    I need your help, please give me some solution.

    • Kaustubh, this circuit was referred from some other site…to troubleshoot the fault you will have to understand the logical behavior of the ICs. you can take the help of the datasheet of the specific ICs to view their internal gate configuration and then compare it with my article explanation, this might help you to figure out why your LEDs are always ON.

    • sorry but just reading will not help you must try to understand how it is supposed to work

      7475 is a bistable latch…so may be due to switch ON surge all the bistables are latching together. the 0.22uF is actually placed to eliminate this switch ON surge…you can try shorting this capacitor manually to shut off the LEDs, if that doesn't help then your IC may be faulty or some other hidden fault might be there…actually pressing the reset MUST also reset the 7475, as this is supposd to cause a low logic at the output of 7420 bottom gate…you can attach an LED across the 7420 bottom gate output and ground to check how the reset button is responding.

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