Projects

01/18/08

DIGITAL CLOCK

Josh Heckathorn

Josh is a junior at Rocky Grove High School.

Josh built this digital clock for a class project. The clock is one of many projects that Josh has built in this class.

The Basics
If you have read, How a pendulum clock works you know that all clocks (regardless of technology) have a few required components:

	A source of power to run the clock In a pendulum clock, the weights or the springs handle this role.
	An accurate time base that acts as the clock's heartbeat In a pendulum clock, the pendulum and escapement handle this role.
	A way to gear down the time base to extract different components of time (hours, minutes, seconds) In a pendulum clock, gears serve this role.
	A way to display the time In a pendulum clock, the hands and face serve this role.

A digital clock is no different. It simply handles these functions electronically rather than mechanically. So in a digital clock, there is an electrical power supply (either a battery or 120-volt AC power from the wall). There is an electronic time base that "ticks" at some known and accurate rate. There is an electronic "gearing mechanism" of some sort -- generally a digital clock handles gearing with a component called a "counter." And there is a display, usually either LEDs (light emitting diodes) or an LCD (liquid crystal display).

A few notes on the parts used:

	The difference between the AC transformer we are using here and the DC transformer we used in the article on gates is that the AC transformer preserves the 60-Hz sine wave found in 120-volt household current. If you want to use your volt-ohm meter to measure the voltage of an AC transformer, be sure you use an AC voltage range rather than a DC range.
	We use the bridge rectifier to convert the AC to DC. One of the terminals on the rectifier will be marked with a "+" -- from that you can find the minus and AC inputs. There is no polarity to an AC transformer, so it does not matter which transformer lead you connect to which AC lead of the rectifier.
	The 7805 and capacitors are wired just like they were in the electronic gates article.
	The resistor and the zener diode extract a 60-Hz signal from the transformer's sine wave. A diode is a one-way valve for electrons. A zener diode is also a one-way valve, but it also passes electrons in the other direction if they are above a certain voltage. The zener diode therefore turns a 10-volt sine wave into a clipped wave oscillating between 0 and 5 volts. This is perfect for clocking the TTL counters. The 1-K-ohm resistor makes sure that the current to the zener diode is limited so we do not burn out the diode. The diode will have a band painted on one end -- this band should be the end connected to the resistor.