PP3-size 9 V batteries (IEC: 6LR22) have a considerably poorer price/energy ratio than 1.5 V AA (IEC: LR6) cells. This makes it all the more unfortunate when you accidentally leave a device switched on!
The author uses a number of such devices and so started looking for a solution to this problem. His first thought was to use a DC/DC converter to allow the use of 1.5 V cells in 9 V equipment. A perfect device for this application is the Prema PR4401 LED driver.
The IC is small, with just three connections, and the required external circuitry consists of just a coil, a diode, and a smoothing capacitor. The device can convert input voltages of between 0.9 V and 1.9 V to 9 V with acceptable efficiency. The maximum load is around 3 mA.
However, this is only half the story. Even when there is no load at the output the IC still draws current, and so the battery will eventually run flat even if the equipment is switched off. This means that some kind of automatic shutdown circuit is needed. How can we make a timer that runs on less than 1 volt and which consumes only a negligible current? The answer was found in the form of a MOSFET with a very low on resistance and a threshold voltage of just over 3 volts. This voltage is still more than twice the terminal voltage of an AA cell, however; we need to generate a sufficiently high voltage (greater than 3 V) briefly and store it on a capacitor. The capacitor will discharge very slowly into the gate of the MOSFET to which it is connected, which will then cause the connected device to be powered for a few minutes before being turned off.
The self-inductance of coil L is used to generate the higher voltage. When switch S is briefly closed a current flows in the coil. We need to check that the maximum gate voltage of the MOSFET (20 V) will not be exceeded: from the maximum input voltage (approximately 1.6 V), the current that briefly flows through R (approximately 1.5 mA) and the inductance of L we can calculate how much energy can be stored in the coil. When S is opened C is charged via D, and we can then work out the resulting voltage across C. With the component values given, this comes to about 5 V. The author’s prototype gave a power-on period of 15 to 20 minutes.
