It might seem like magic but a boost converter or step-up converter is a handy little device that can output a voltage greater than its input voltage. This makes it very useful for getting a consistent voltage in battery powered devices or running a circuit from fewer cells than would otherwise be required. If you don’t need much capacity it can also be a good way of using the last remaining power from batteries that other devices have deemed too flat, connect a few up to a boost converter and you can still get some useful power out of them for a while longer. For more capacity you can add more cells (as long as you keep the input voltage under the output voltage) or use higher capacity batteries such as C or D cells.
The Maxim chip I am using here has an adjustable output (2.7-5.5V) and will work with an input voltage as low as 0.7V.
Obviously it isn’t really magic and this extra power can’t be generated from nowhere, P=IV and all that. What this means in practice is that by increasing the output voltage the available output current must be lower than the source current and will decrease as the input voltage decreases (as the batteries deplete), as seen in the graph on the left below. Efficiency is also dependent on the input voltage and the output current, peaking at around 87% with a 3.3V input and a 5V 200mA load as seen on the right hand graph.
Wikipedia has a better explanation of how a boost converter works than I could give so have a read of that if you’re curious about the details. The Maxim MAX757 I’m using here is an 8 pin DIP chip and requires minimal external components which makes it ideal for a stripboard build, it has an adjustable output voltage in the range 2.7V to 5.5V and can operate down to a 0.7V input voltage, the data sheet seems to imply that it needs at least 1.1V initially to start up but this doesn’t seem to be the case, at least it wasn’t in my testing. This output range combined with the low voltage capability means it is easy to get the two common voltages used with microcontrollers and other digital electronics of 3.3V and 5V from even a single AA or AAA cell although the extra capacity probably makes two (or more) more practical for most uses.
The output voltage on the MAX757 is set by a voltage divider between ground and the output which is connected to the feedback input (pin 2), the formula to calculate the required resistors is:
VOUT = (1.25) [(R2 + R1) / R2]
To get 5V I have used 30K for R1 and 10K for R2, for 3.3V you could use 18K for R1 and 11K for R2.
If you can find it an alternative part is the MAX756 which is a bit easier if you only need an output of 5V or 3.3V as the voltage can be set by connecting pin 2 low for 5V or high for 3.3V which negates the need for the voltage divider arrangement.
These chips also have a low battery output that can be used to light an LED to notify of a low battery, it brings pin 4 (LBO) low when the voltage at pin 5 (LBI) drops below the converters internal reference voltage of 1.25V. This means the lowest voltage you can trigger the warning is 1.25V so you would probably want to leave this off if using a single cell. For a warning at 1.25V you just need to connect pin 5 (LBI) to VIN and an LED with a suitable resistor to pin 4 (LBO). For a warning at a higher voltage you would need to use a voltage divider to output the desired voltage to the LBI pin.
Here is my stripboard layout for the MAX757, the MAX756 would be the same except R1 and R2 would be omitted and pin 2 would instead be connected to ground to select 5V output or to VIN to select 3.3V output.
1 x MAX757 Adjustable-Output Step-Up DC-DC Converter
1 x 8 pin DIL socket
1 x Piece of stripboard, minimum 8 rows by 21 holes
1 x 22 uH Inductor (0.03 Ohm or less, 1.2A or more)
1 x 150 uF electrolytic capacitor
1 x 100 uF electrolytic capacitor
1 x 100 nF ceramic capacitor
1 x 1N5817 Schottky Diode
1 x AA Battery holder of desired capacity
1 x LED (optional)
1 x 100 Ohm resistor (optional)
For 5V output:
1 x 30K resistor (R1)
1 x 10K resistor (R2)
For 3.3V output:
1 x 18K resistor (R1)
1 x 11K resistor (R2)
For other output voltages calculate suitable resistors using the formula VOUT = (1.25) [(R2 + R1) / R2]