We conclude this month with the final part of our four-part series of "perpetual" projects. All are based on one small p.c.b. called a Uniboard. Each is powered around the clock – perpetually – by a single, high quality, memory back-up capacitor and a small solar cell (no battery). Each is designed to run unattended for months at a time – in fact for years!

Gate Sentinel: Be on your guard against unwanted visitors.
This Sentinel is intended to sound when a person enters your front gate. It is not intended so much as an alarm, but rather to give the impression: "You have been noticed." It therefore emits a pip-pip-pip tone. In order not to give the impression that this is merely a "dumb" device, the Gate Sentinel does more than merely switch on when your gate is opened, and off when it is closed – in that case, an intruder would know that it was directly linked to the gate. It emits some ten pips after the gate has been closed, so as to give the impression: "I’m more clever than a simple on-off device."

Solar-powered Bird Scarer: Ruffle a few feathers with this harmless "electronic scarecrow"!
It has been shown that birds particularly dislike a rasping sound around 200Hz, and will, for a while, avoid it. The present project (which could also be called an Electronic Scarecrow) was given to a farmer to test on his grapes, and proved very effective over a radius of three to four metres. In fact the farmer was quite excited about the results. Since this project will only be needed during the daylight hours (most birds go to roost in the darkness), it allows a higher current consumption during the day, and puts the circuit to sleep at night.

Solar-powered Register: Is anyone at home?
Our final Perpetual Project is a Register. This device is useful especially to indicate whether a person is in or out. It will flash Green if a person is In, and Red if a person is Out. A series of these devices together in a hallway could inform residents of a small "digs" as to who is in and who is out – residents would simply touch a touch-switch as they passed to indicate that they were coming or going. Alternatively, it could send secret messages to the street from your bedroom window. A green flashing l.e.d. might indicate: "See you at the bowling alley tonight," while a red flashing l.e.d. might indicate: "I’m grounded."