Simple DC - DC Boost Converter Using 555

It is often useful in a circuit to have higher voltages. Either to provide +ve and -ve rails for an op-amp, to drive buzzers, or even a relay without the need of an additional battery.

This is a simple 5V to 12V DC converter built using a 555 timer and a couple of 2N2222 transistors. Dedicated ICs already exist to perform this function and they do so much more efficiently than this design - this project is fun to experiment with and have an intuition for how these circuits work.

The circuit functions by closing the transistor, effectively grounding the inductor. This causes a large current to flow into the inductor. When the transistor is open the magnetic field collapses in the inductor causing the voltage to rise, often much higher than the battery voltage. If the voltage generated is higher than the voltage stored in the capacitor the diode closes and allows the capacitor to charge.

Using a signal generator to drive the transistor I found that for my component values (parts that I salvaged from discarded electronics) I need a frequency of around 220KHz to generate 15V. A feedback network will then control the frequency to try to maintain a steady 12V at various loads.

There are various 555 oscillator circuits online, but I built mine this way.

The output, pin 3, is used to charge and discharge a capacitor via a resistor. The voltage at across the capacitor is monitored to toggle the output pin.

If using a 6V supply it is easy to see the op-amps have a 2V and a 4V reference voltage. Both op-amps are monitoring the capacitor voltage and thus the pins (2 and 6) are wired together.

If the voltage rises above 4V the top op-amp goes high Resetin the latch, the capacitor begins to discharge until falling below 2V at which point the bottom op-amp will go high and Set the latch. Once again charging the capacitor.

The yellow scope trace shows the capacitor charging and discharging while the blue trace shows the output pin 3 generating a square'ish wave at 190KHz.

The requirement for the feedback loop is to lower the frequency when the output voltage gets too high, and to raise the frequency when the voltage gets too low.

The easiest way I could think of to do this was by using a transistor to bleed away current during the capacitor charge cycle.

During this cycle the DISCHARGE pin 7 is active low, allowing the bleed circuit to steal current from the capacitor.

The base voltage - 0.65V is present at the emitter, this voltage over a fixed R resistor will maintain a steady current, which must come from the capacitor charging current, slowing down the cycle and lowering the frequency. The higher the voltage, the more current is bled away from charging and the lower the frequency. Which fits our requirements exactly.

Experiment with component values, but I selected 3K for the base resistor for this reason:

At it's lowest point the capacitor sits at roughly 2V. From a 5V supply this means 3V across the 3K resistor will begin charging the capacitor with 1mA.

With 1V preset at the emitter across a 3K resistor will draw 1/3 of the current, or 333uA... which I thought would be a good bleed current. The base voltage comes from a potentiometer, forming a voltage divider with the voltage we wish to monitor, ie the 12V output. As the potentiometer is adjustable the emitter resistor value is not critical. I selected a 20K potentiometer for this.

I only had a surface mount diode available which can be seen soldered to the bottom of the board.

The circuit was tested from a 5V supply from an Arduino, and effectively drives a 12V buzzer, DC motor, 12V relay or a series of diodes without the need for an external 12V supply.