Digital timers use an electronic oscillator, counter circuits and a binary-to-decimal decoder to produce timings of different lengths. The oscillator is tuned to vibrate at a specific frequency, usually a tiny fraction of a second. For example, digital wrist watches are tuned to 32.768 kHz, or 32,768 vibrations per second. The more accurate the oscillator is, the more accurate the timer will be. The rest of the circuit consists of counters to provide the primary circuit, built from flip-flop circuits, to divide the oscillation down to the desired rate, such as one second or one minute, depending on the application. Finally, the binary coded values of the count are displayed on a digital readout by a decoding circuit.
Place 74LS90 counter chips on a bread board. The 74LS90 is a 4-bit counter with preset modes, and it can be configured into decade mode (0-9) or binary mode (0-15). For the purpose of a digital timer, to be read by humans, use the decade mode. Two of these chips are needed for a one-minute timer.
Build an oscillator circuit. A 555 timer is inexpensive and can be built to produce oscillator periods of virtually any length. However, its precision is determined by external resistors and capacitors and is therefore limited to about 90 per cent accuracy, which means in the period of an hour, the timer may be off by 6 minutes or more. A quartz oscillator is far more accurate. Both types are easy to build.
Build a crystal oscillator circuit. Using a 4.194304MHz crystal and a 4069 hex inverter IC, a reliable signal can be generated, which can be fed directly into a 4020B divider IC.
Build the divider circuit using the 4060B counter, if a quartz crystal oscillator is to be used. Since quartz crystals operate at a high frequency, they will need to be divided to a usable frequency. For example, to obtain a 1 second pulse output from a 4.194304MHz crystal, the frequency will have to be divided by 2, 22 times. 2^22 = 4,194,304.
Configure the 7490s in the desired pattern. Setting one 7490 in 0-5 mode and another in 0-9 mode and then cascading them together will produce a total count of 0-59, which is perfect for counting seconds.
Wire in the oscillator's divided output to the input of the first stage of the 7490 circuit. As the oscillator ticks, the first 7490 will begin counting. When that 7490 overflows, it will send a pulse to the second 7490. Eventually, the second 7490 will also overflow, and the counter will reset to zero.
Install the logic circuits. Configure a NAND based circuit, using 74LS00 chips to detect the presence of an overflow on the second 74LS90 chip. If an overflow is detected, the 7400 logic chips will output a signal which can be used to trigger an alarm, buzzer, or LED indicator.
Add a reset button and a start/stop button. The reset button simply sends a pulse into each 7490 chip to reset them. A start/stop button operates by interrupting the flow of pulses from the oscillator.