Problem
Our customer asked us to produce a small hand-held data logger that would work with a variety of "smart" sensors. The sensors would communicate to the data logger through a serial cable. The data logger also could communicate with a computer serially to transfer data and programs.
Solution
The device had a thin flex circuit dome keypad for local programming and a two-line LCD. A high speed microprocessor handled the communications and data storage.
The high speed processor drew far too much current to allow continuous operation. The processor did not have a low power sleep mode so we needed a way to start and stop it when necessary. A second small low power processor was used as a clock-calendar to schedule data logging events. It controlled power to the high power circuits to enable them for scheduled events. It also monitored the serial communications lines for activity and wakened the communications processor when necessary.
One of the challenges we faced was building a power supply for the unit that consumed little power. The low power processor circuit drew only 22 microamps. Power regulator ICs typically drew several milliamps from the power source - about 100 times as much as the processor! We designed a very low power voltage regulator circuit to step down the 9V battery output to the 5V power for the data logger circuit. This supply drew only 11 microamps.
We used an off-the-shelf plastic case that had a 9V battery enclosure. A few simple machining operations created the openings for the LCD and serial connectors. We worked with a manufacturer of graphic overlays and keyboards to design the custom keyboard for the case.
Results
The unit worked as the customer had expected. Battery life was excellent. A test unit operated for several years on a single 9V alkaline battery. We produced a variety of "smart" sensors for use with the product.