This circuit is designed to take a square wave signal and double it to drive a piezo buzzer more effectively.

A circuit diagram showing an inverter and charge pump connected to a buzzer.

Let’s break down how it works:

The Basic Concept of a Charge Pump

A charge pump is a type of DC-to-DC converter that uses capacitors to store and transfer electrical energy to create a higher voltage. Think of it like filling and emptying buckets of water to move water uphill.

In this circuit, the charge pump section is the heart of the voltage doubling process. Here’s how it operates:

  1. Charging Phase: When the input signal is low, one of the capacitors (C1 or C2) charges through its respective diode (D1 or D2) to the supply voltage (+5V).
  2. Pumping Phase: When the input signal switches high, the input side of the capacitor is pushed up, which forces the output side of the capacitor to go even higher - now carrying a voltage equal to the supply voltage PLUS the input voltage. This works because when you charge the input side of the capacitor the output side is unable to discharge instantly.
  3. Output Delivery: This higher voltage is then available to drive the piezo buzzer.

The Circuit Components and Their Roles

Looking at the specific components:

  • Capacitors C1 and C2 (100nF): These are the “buckets” that store and transfer charge. They temporarily hold energy that gets pumped to a higher voltage.

  • Schottky Diodes D1 and D2: These act as one-way valves that allow the capacitors to charge but prevent them from discharging back into the supply. Schottky diodes are used because they have a lower voltage drop than regular diodes, making the circuit more efficient.

  • Resistors R3 and R4 (100Ω): These limit the current flow and protect the components during charging and discharging.

  • Transistors Q5-Q8: These act as switches that control when the capacitors charge and discharge. They effectively convert the input signal into the alternating phases needed for the charge pump operation.

Dual Charge Pump

The first pair of transistor splits the input into a inverted pair of signals, with either signal going into one half of the dual charge pump. This way we get two inverted charge pumps working out of phase with each other.

If your input voltage is lower than the supply voltage (example: 5v supply with 3v signals) you could add another identical pair of transistors to act as a buffer.

Why This Works Well for Piezo Elements

A piezo buzzer
A piezo buzzer

Piezo elements have some unique electrical properties that make this circuit particularly effective:

  1. High Voltage Requirements: Piezo elements create sound by physically deforming when voltage is applied. The higher the voltage, the more they deform, and the louder the sound. While they might make some noise at 3V or 5V, they become much more efficient and louder at higher voltages.
  2. Capacitive Nature: Piezo elements themselves act like capacitors. They don’t draw much current, but they do need voltage to create mechanical movement. This makes them perfect matches for charge pump circuits, which excel at providing higher voltages but can’t deliver high currents.
  3. Differential Drive: We can drive the piezo louder using two outputs that are out of phase with each other. By connecting these to opposite sides of the piezo element, the effective voltage seen by the piezo is doubled! When one side goes positive, the other goes negative, creating a larger voltage difference across the piezo than would be possible with a single-ended drive.
  4. Voltage doubler: We further double the voltage using the dual charge pump, providing a second doubling of the loudness.

In simple terms, this circuit is like pushing and pulling on opposite sides of the piezo element at the same time, making it bend more dramatically than if you just pushed from one side.

Summary in Everyday Terms

Imagine trying to make a drum vibrate by hitting it. You could hit it from one side with a certain force. But what if you could simultaneously hit it from one side while pulling from the other? You’d get more movement with the same effort.

This circuit does something similar with - it uses a clever arrangement of electronic components to “hit” the piezo element from both sides at once, using electrical energy more efficiently to create a louder sound than would be possible with the original input voltage alone.

Alternative components

CD4007

Since we have three identical pairs of transistors, a CD4007 could be used instead of six individual transistors.

You typically cannot use a buffer because a buffer will keep signals at the system voltage.

Schematic diagram of a PAM8904Q integrated circuit connected to a piezo sounder/speaker.

PAM8904

Instead of this circuit you could use a PAM8904 IC, which does basically the same thing. The PAM8904 costs about 30 cents, but the parts of this circuit should cost less than one cent each.