This system was designed to meet a number of needs:
with all of these in mind I set about designing the system.
To make the unit easy to use, it was intended to be handheld and operate from a PP3 battery. But using a 9V battery meant that a voltage regulator was needed. The standard 3 pin regulator, the 7805, was fine except for one thing, the standby current of 5mA!
Searching through Maplin I found the Holtek HT 1050 Low dropout regulator. This had a standby current of 3.5 uA. The maximum current available was 30 mA at 5V and it will still supply 5 volts with a terminal voltage of 5.1V due to the 100mV dropout voltage.
The 30mA current limit and the 5 uA quiescent supply helped in achieving the Low power consumption figures. The other bonus is that the PIC used in the system will only draw 25uA when in Standby mode. The 30mA max current meant that it was necessary to find an audible device that did not draw too much current. again a suitable buzzer was found from Maplin, drawing 20 mA.
By referring to the datasheets for the PIC 16F84 I noticed that it was possible to make the PIC sleep easily and awake on a keypress. Either by triggering a change on PORT B 4-7 the devices leaves it's low power standby mode. As the system had a low standby power, only 3 buttons were fitted to the unit, one for selecting a 30 second delay, 1 for 60 second delay and one for Reset. see the software listing for some more info.
The Circuit diagram
The software is quite straight forward. I will outline the principle of operation below.
A few features,
Pressing the RESET button silences the buzzer and puts the device into sleep mode.
A picture of the prototype, that has been in use since early 2000.
Updated 27 September 2020