How to Build a Digital Countdown Timer

Digital timers come in handy anywhere from the kitchen to the gym.

Digital timers are used for a variety of activities, such as cooking and running. The typical timer runs on an oscillator. The oscillator vibrates at a particular frequency depending on what type of timer it is used for. The accuracy of the time depends on the accuracy of the oscillator. A circuit counter controls the pace of the digital timer. An example pace would be counting down one second at a time. The resulting binary codes from the circuits are then decoded and displayed on a digital screen. To make your own digital timer, you may need a background in building small electronics.

Things You'll Need

• Breadboard
• Jumper wire
• Power supply
• IC (4069) Hex inverter
• IC (4060B)
• Quartz crystal (4.194304 Mhz)
• 2 integrated circuits (ICs) (74LS90)
• IC (74LS00)

Instructions

1. Insert two 74LS90 counter chips on to the breadboard. Configure the chips to decade mode. This will enable a regular numeric display of 1-10, which is appropriate for a digital clock display.

2. Create an oscillator circuit. For the greatest accuracy, use a quartz oscillator. See the Resources section on how to build an oscillator circuit.

3. Use a 4069 hex inverter IC and a 4.194304 MHz crystal to build a crystal oscillator. See the Resources sections for details on building an oscillator.

4. Feed the signal from the crystal oscillator into a 4020B divider IC. Insert the 4020B divider IC into the breadboard to do this. Make the connection with jumper wires.

5. Create a divider circuit using a 4060B counter. Since you are using a quartz oscillator, you need to divide the frequency up so that it is usable.

6. Insert the 7490s into the breadboard in any pattern that you would like. Configure one 7490 to 0-9 mode and the other to 0-59 mode. The first is to count hours. The second is to count minutes.

7. Wire the first stage of the 7490 input to the divided output of the oscillator. The overflow from the first 7490 will go to the second 7490 creating the appropriate cycle of hours and minutes.

8. Create a buzzer that will go off when the timer hits zero. Do this by creating a NAND circuit. See the Resources section for details on how to do this. In this circuit, if an overflow is detected from the 74LS00 chip to the 74LS90 chip, a signal will be released from the 7400 logic chips. This signal can be used to set off a buzzer or LED light.

9. Insert a reset button into the breadboard. This will interrupt the flow of the oscillators and reset the 7490 chips. The timer can be reset with this system.

References

• REUK: Making Very Accurate Timers
• Make your own Chip: Decade and Binary Counter
• SGS Thompson Microelectronics: Hex Inverter

Resources

• Electroschematics: Crystal Quartz Oscillator
• Make your Own Crystal Oscillator
• Using NAND or NOR Gates to Make up Circuits