HeartLEDPCB

Heart LED PCB

A collection of Valentine's Day-themed PCB designs featuring LED animations in the shape of a heart. Perfect for pendants or keyrings!

Main Branch

The main branch features a design built around the ICM7555 timer IC, configured in astable operation mode. The timer output drives a CD4017B decade counter, which sequences through LEDs to create an animated pattern. Power is supplied by a CR2032 3 V battery through a TPS6122 boost converter.

Components Overview

ICM7555IBAZ-T

The ICM7555 is a CMOS RC timer that provides significantly improved performance over the standard SE/NE 555/556 and 355 timers. It is a direct replacement for these devices in most applications. Enhanced features include low supply current (60 μA), a wide operating supply voltage range (2V to 18V), higher frequency performance, and no requirement to decouple the Control Voltage pin for stable operation.

Although the ICM7555 itself consumes very little current, the total system supply current can increase unless high-impedance timing components are used. To address this, a high value for R and a low value for C were selected.

The frequency of the astable operation is determined by the formula:

To achieve a period of approximately one second, R was set to 100 kΩ and C to 4.7 μF.

CD4017BM96

The CD4017B is a decade counter with 10 decoded outputs. Each output remains high for the duration of one full clock cycle, sequentially cycling through the outputs with each clock pulse. It operates over a wide supply voltage range of 3V to 18V, making it compatible with various applications.

TPS61222DCKR

The TPS6122x family devices provide a power-supply solution for products powered by single-cell, two-cell, or three-cell alkaline, NiCd, or NiMH batteries, or a single-cell Li-Ion or Li-polymer battery. They offer up to 95% efficiency under typical operating conditions.

In this design, the specific application of the device is a fixed output voltage supply of 5 V at up to 60 mA. The TPS61222 DC/DC converter is suitable for systems powered by a total typical input voltage between 0.7 V and 5.5 V.

In the fixed-voltage version, the output is set by an internal resistor divider. To ensure proper operation, a suitable inductor must be connected between pin VIN and pin L. Inductor values of 4.7 μH provide good performance across the full input and output voltage range. Using inductor values below 2.2 μH is not recommended.

An input capacitor with a value of at least 10 μF is recommended to improve the transient behavior of the regulator and reduce EMI in the total power supply circuit. A ceramic capacitor placed as close as possible to the VIN and GND pins of the IC is advised.

For the output capacitor (C2), small ceramic capacitors are recommended, placed as close as possible to the VOUT and GND pins of the IC. A minimum capacitance value of 4.7 μF should be used; 10 μF is preferred.

BOM & Schematic Diagram

HT7750A Branch

In this branch, the TPS61222DCKR converter is replaced by the HT7750A. The HT77XXA series is a family of step-up DC/DC converters known for their high efficiency and low ripple. These converters feature an extremely low start-up voltage and high output voltage accuracy. They require only three external components to provide a fixed output voltage of 2.7 V, 3.0 V, 3.3 V, or 5.0 V.

BOM & Schematic Diagram

MCU Branch

This branch replaces the timer and decade counter with an ATtiny816 microcontroller. The ATtiny816 is a member of the tinyAVR® 1-series family featuring advanced analog features and core independent peripherals. This design allows for more sophisticated LED control and animations through programmable patterns.

Programming is accomplished via the Unified Program and Debug Interface (UPDI), a one-wire interface for external programming and debugging. The circuit incorporates a tactile button connected to a GPIO pin for user interaction.

BOM & Schematic Diagram

Simple NE555 Branch

Finally, this branch replaces the ICM7555 timer IC with a conventional NE555, offering a cost-effective alternative that requires only one SMD component - the IC itself. This design imitates a popular DIY kit, where the NE555 generates a 1 Hz signal at pin 3, alternating between high and low levels. When pin 3 is low, the upper row of LEDs illuminates, and when high, the bottom row lights up instead, creating an appealing visual effect.

While the circuit can operate with a CR2032 3V battery, one LED row will appear dimmer due to the NE555's minimum voltage requirements. For optimal brightness across both rows, a 5V power supply connector can be soldered to the board as an alternative power source.

BOM & Schematic Diagram