How to Build a 3-Minute LED Timer

Written by mark stansberry
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Timer circuits can be used to turn on or off lights, like LEDs (light emitting diodes) for a preset amount of time. A three-minute LED timer circuit can often be built with components you already have in your electronic tool box or hobby kit. All you need is a 555 timer component, a few resistors and capacitors, button switches and a LED. Building the timer is simple. Just connect the timer in a standard monostable configuration. This configuration is used to generate a voltage for a time set with the values of the timer's timing resistor and timing capacitor.

Skill level:
Moderate

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Things you need

  • Power supply
  • Electronic breadboard
  • Two 10,000-ohm resistors
  • One 555 timer circuit
  • One 9-million ohm resistor
  • One 10-microfarad electrolytic capacitor
  • One 0.01-microfarad capacitor
  • Two electronic push-button switches
  • One 1000-ohm resistor
  • One LED

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Instructions

  1. 1

    Connect the positive power terminal and the negative terminal of your power supply to your electronic breadboard. Call the positive power terminal connection the "supply connection." Call the negative lead connection "ground."

  2. 2

    Connect the first 10,000-ohm resistor between the supply connection and Pin 2 (the trigger pin) of the 555 timer. Connect the second 10,000-ohm resistor, called the timing resistor, between the supply connection and Pin 4 (the reset pin) of the 555 timer. Connect one end of the 9-million-ohm resistor to the supply connection. Connect the other end of the 9-million-ohm resistor to Pin 7 (the discharge pin) of the 555 timer. Now connect Pin 7 and Pin 6 (the threshold pin) of the timer together.

  3. 3

    Connect the positive lead of the 10-microfarad capacitor, called the timing capacitor, to Pin 6 (threshold pin) of the timer. Connect the negative terminal of the 10-microfarad capacitor to the ground. Connect one lead of the 0.01-microfarad capacitor to Pin 5 (the control pin) of the timer. Connect the other lead of the 0.01-microfarad capacitor to ground.

  4. 4

    Connect Pin 1 (the ground pin) of the timer to the ground connection. Connect Pin 8 (the power pin) of the timer to the supply connection.

  5. 5

    Connect one lead of the first push-button switch, called the trigger switch, to Pin 2 (the trigger pin) of the timer. Connect the other lead of the trigger switch to ground. Connect one lead of the second push-button switch, called the reset switch, to Pin 4 (the reset pin) of the 555 timer. Connect the other lead of the reset switch to ground.

  6. 6

    Connect one end of the 1,000-ohm resistor, called the current limiting resistor, to Pin 3 (the output pin) of the timer. Connect the other end of the current limiting resistor to the anode (positive) lead of the LED. Connect the cathode (negative) lead of the LED to ground.

Tips and warnings

  • After your circuit is built, set the power supply to 5 volts and turn on the power. Next press and release the trigger switch. Your LED should light for about 180 seconds or three minutes. Time how long the LED stays lit. If it is not exactly three minutes, adjust the timing resistor. Increase the timing resistor value if the timer times out to soon and decrease it if the timer takes more than three minutes to time out. Press and release the reset switch to turn off the LED if you want the light to turn off before the three-minute time elapses. Use the formula, T=1.1*R*C for determining resistor and capacitor values for timers that have different time out specifications (see References). You may have to adjust the value of the current limiting resistor to match the current drive specification of the LED you have chosen.
  • Improper use of electronic equipment and components can result in fire, serious injury or death. Always work under the supervision of a safety-certified electronic technician or electronic engineer. Obtain a electronic safety certificate before you work with electronic equipment and components.

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