Electronics Projects: How to Make a One-Shot Timer

In this project, you build an electronic circuit that uses a 555 timer chip in monostable mode. When a trigger switch is pressed, an LED lights and stays lit for approximately five seconds. Then, the LED goes dark until the button is pressed again.

The circuit is based on this schematic for the monostable circuit.

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An LED is the output (pin 3) and the resistor and capacitor values are included in the capacitor charging circuit.

This project uses a pushbutton switch that isn’t designed to be inserted in a solderless breadboard. To make the pushbutton usable with the solderless breadboard, solder a small length of 18- or 20-guage solid wire to the end of each lead of the test strip. You can then insert these leads directly into holes on the breadboard.

When you are finished, the circuit will look like this.

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To test the circuit, push the pushbutton. The LED should light, stay lit for just over five seconds, and then go back off. It should light again only when you press the pushbutton again.

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

  • Make sure the battery is good. (Use a voltmeter to test it.)

  • Carefully double-check all the jumper wires and other components to make sure they're connected properly.

  • Make sure the LED isn’t inserted backwards. As a test, pull it out and insert it with the leads reversed.

  • Make sure the electrolytic capacitor is inserted with the negative end on ground side of the circuit.

  • Make sure the solder connections to the pushbutton are solid.

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