An Electronic Stopwatch. Suitable for use in a variety of exciting hobbyist projects, including digital timers, electronic stopwatches, and 24-hour clocks, the ICM7045, Fig. 2, is a complementary MOS LSI precision timer. Manufactured by Intersil, Inc. (10900 North Tantau Ave., Cupertino, CA 95014), the device comprises a crystal-controlled oscillator, a high-frequency binary divider, an intermediate-frequency counter, latch circuits, a multiplexr, a decoder, a control circuit, and segment and digit output buffers. It is designed to operate on a nominal source of 3.6 volts (three rechargeable NiCd cells in series) and will interface directly with a fully multiplexed seven-segment/eight-digit, common-cathode LED display. Supplied in a 28-pin DIP, the ICM7405 sells for $63.80 each in unit quantities.

In operation, the oscillator signal is divided in sixteen binary stages to 100 Hz. Some of the divider outputs are used to generate multiplex waveforms at a 12.5% duty cycle/800-Hz rate. The 100-Hz signal is processed through the counters into the latch circuits which, in turn, are multiplexed into the decoder. The counter section spans the range from 1/100 second to 24 hours, which can be simultaneously displayed on an eight-digit readout. The digit drivers (cathodes) are connected to the multiplex lines through zero suppression logic, while the segment drivers (anodes) are connected directly to the decoder outputs.

A versatile four-mode electronic stopwatch circuit featuring the ICM7045 is illustrated in Fig. 3. Abstracted from the 8-page technical bulletin for the device published by Intersil, the design requires a minimum of components in addition to the IC itself - a quartz crystal, a trimmer capacitor, a suitable LED readout, a 3.6-volt battery power pack, and four switches. The quartz crystal used should have a nominal frequency of 6.5536 MHz when tuned by a total parallel capacitance of 12 pF or less, and an Rs of about 40 ohms. The instrument may be assembled on either an etched circuit board or breadboard, as preferred, but good layout and wiring practice should be observed during construction. In addition, for optimum results, prospective builders should obtain and study a copy of the Intersil bulletin before starting circuit assembly.

With the 8-digit display specified, the instrument's readout is expressed in intervals of 1/100 second, 1/10 second, seconds, 10 seconds, minutes, 10 minutes, hours, and 10 hours, from a minimum of 1/100 of a second to a maximum of 24 hours.