Electronics Projects: How to Make an LED Flasher Using a Timer Chip

In this project, you build an electronic circuit that uses a 555 timer chip to alternately flash two LEDs on and off. Then, you modify the circuit so that the circuit is controlled by two pushbuttons that function as a set/reset switch.

Build an electronic LED flasher circuit

For this circuit, the 555 is configured in 555 in astable mode, and the resistor values are chosen so that they cause the high and low timings to be very close to one another, about one-tenth of a second each.

LED1 lights when the output is high, and LED2 lights when the output is low. Note that this circuit uses both sinking and sourcing of the output current.

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If the circuit doesn’t work, here are a few things to check:

  • Check the battery voltage.

  • Double-check all the jumper wires and resistors to make sure they're inserted in the proper holes.

  • Make sure the diodes and the electrolytic capacitor are inserted correctly. For the LEDs, the anodes must be on the positive side of the circuit and the cathodes on the negative side. For the capacitor, the negative side must be in the ground bus.

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Modify to add a set/reset switch

In this section, you modify the circuit that you built in Project 2-2 so that the circuit is controlled by two pushbuttons that function as a set/reset switch. When you connect the power to this circuit, LED1 turns on and stays on. When you press the set pushbutton, the two LEDs start flashing alternately and continue to flash until you press the reset pushbutton.

The circuit for this project uses two 555 timer chips. The first is configured in bistable mode with two pushbuttons acting as set and reset switches. The second is configured in astable mode, almost identical to the 555 timer that was used in Project 2-2.

The difference is that instead of connecting the astable 555 Timer chip’s supply voltage pin (pin 8) directly to the battery, it is connected to the output of the first 555. Thus, the first 555 controls the power to the second 555, so the second 555 flashes the LEDs only when the output of the first 555 is high.

This project requires that you use two pushbutton switches that are not designed to be inserted into a solderless breadboard. To make the switches easier to use with the breadboard, solder short lengths of 20-gauge solid wire to the switch terminals.

Project 2-3 shows you how to build this circuit, and the completed project is pictured here.

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