A tutorial on how to make a fastest finger first circuit which can track the first person to press the trigger or Buzzer. This circuit consists of multiple replicas of a module, each of which can be assigned to a single quiz taker or a team. Each module contains a trigger (We used momentary push button in the video) and an LED indicating whether the team corresponding to this module is the first one to press the trigger.
Monostable arrangement can be coupled to the output of this circuit so that a light or buzzer beeps for a second after the first trigger from any quiz taker.
Components Required
Components Required For Each Module:
- 555 Timer IC
- Momentary Push Button Switch
- PN Diode (I used 1N4148)
- LED + Series Resistor (270R)
- 10K Resistor
Components Common For All Modules:
- 2 x 1K Resistors
- LED + Series Resistor (270R)
- Momentary Push Button (For resetting)
- Breadboard
- Few Breadboard Connectors
- (5-12)V Power Supply
Circuit Diagram
[Click on the circuit for full sized image]
How This Circuit Works
Few Basics:
- If Pin-2 of 555 IC sees any voltage less than 1/3rds of the supply voltage, it turns the output ON
- If the reset Pin of 555 IC sees 0V, it resets the output
Apart from positive and negative rails, we used two other rails:
- Reset rail which is pulled up to positive voltage by default using a 1K resistor
- Status / Feedback rail which is pulled down to 0V by default using a 1K resistor. This rail is connected to output of all the modules via diodes. So this rail will be at 0V by default. But as soon as any of the modules output is ON, the voltage at this rail reaches positive voltage via the diode
So when the output of all the modules are in OFF state, the voltage at status rail will be at 0V (default). When any team presses the button, this 0V from status rail is applied at Pin-2. Because 0V is less than 1/3rds of the supply voltage, the output of 555 IC corresponding to the team which pressed the trigger first turns ON.
Immediately after this happens, the voltage at status rail changes to positive voltage because of the feedback via PN diode. So even if other teams press the trigger now, the voltage at Pin-2 of the respective modules will be at positive voltage and the output doesn’t turn ON.
Resetting of states of all the modules is done by applying 0V at the reset pin of all the 555 ICs using reset rail and a dedicated push button.
If you have any queries/suggestions, feel free to post them in the comments section of this video: Fastest Finger First Circuit using 555 IC For Organizing Quizzes