Normally, 9V PP3 batteries last less than a couple of hours if used continuously. These cannot be used if the voltage is below 7.8V. Say goodbye to these expensive 9V batteries having a short life. Two AA-size alkaline cells in conjunction with this DC-DC converter give 9V at a low current and last much longer.The power supply is designed using a boost converter with fixed ‘on’ time and variable ‘off’ time. The variable ‘off’ time regulates power to the load. The converter consists of transistor T2, inductor L1 and capacitor C2. The conductance of transistor T1 controls ‘off’ time of the oscillator in conjunction with capacitor C2. IC TL431 (IC1) monitors the voltage across capacitor C4. When the voltage exceeds 2.5V at the reference pin (Ref) of IC1, the opto-coupler conducts more and reduces the conduction of transistor T1.
The frequency of oscillations mainly depends on the time constant (R-C) of feedback capacitor C3 and the input stage impedance (R1 plus VR1). Adjust preset VR1 to tweak the circuit for efficiency. The converter works with a single cell also. In that case, keep the output current-drain minimal.
EFY note. We have measured maximum output of 8.7V at 28mA current. Above this current, the output becomes zero.