Battery Saver Circuit Diagram

ExtremeCircuits presents a small electronic
switch that connects a battery to the equipment for a certain amount of
time when a push-button is momentarily pressed. And we have also taken
the ambient light level into account; when it is dark you won’t be able
to read the display so it is only logical to turn the switch off, even
if the time delay hasn’t passed yet. The circuit is quite
straightforward. For the actual switch we’re using a well-known MOSFET, the BS170.

A MOSFET (T2 in the circuit) used in this
configuration doesn’t need a current to make it conduct (just a
voltage), which makes the circuit very efficient. When the battery is
connected to the battery saver circuit for the first time, capacitor C2
provides the gate of the MOSFET with a
positive voltage, which causes T2 to conduct and hence connect the load
(on the 9 V output) to the battery (BT1). C2 is slowly charged up via R3
(i.e. the voltage across C2 increases).

Circuit diagram:

Battery Saver Circuit

Battery Saver Circuit Diagram

This causes the voltage at the gate to drop and eventually it
becomes so low that T2 can no longer conduct, removing the supply
voltage to the load. In this state the battery saver circuit draws a
very small current of about 1 µA. If you now press S1, C2 will discharge
and the circuit returns to its initial state, with a new turn-off
delay. Resistor R5 is used to limit the discharge current through the
switch to an acceptable level. You only need to hold down the switch for
a few hundredths of a second to fully discharge C2.

In our prototype, connected between a 9 V battery and a load that
drew about 5 mA, the output voltage started to drop after about 26
minutes. After 30 minutes the voltage had dropped to 2.4 V. You should
use a good quality capacitor for C2 (one that has a very low leakage
current), otherwise you could have to wait a very long time before the
switch turns off! The ambient light level is detected using an LDR (R1). An LDR
is a type of light sensor that reduces in resistance when the light
level increases. We recommend that you use an FW150, obtainable from
e.g. Conrad as part number 183547-89.

When there is too little light its resistance increases and
potential divider R1/R2 causes transistor T1 to conduct. T1 then charges
up C2 very quickly through R4, which limits the current to a safe
level. This stops T2 from conducting and the load is turned off. The
choice of value for R2 determines how dark it has to be before T1 starts
to conduct. The battery saver circuit can be added to devices that use 6
or 9 volt batteries and which don’t draw more than 100 mA. The circuit
can be built on a piece of experimenter’s board and should be made as
compact as possible so that it can be built into the battery powered
device.

Comments are closed.