K100 Electronic little heart

The electronic little heart that we present here is a nice construction of visual effects, easy to construct, which can be given as a gift. It consists of 22 bright red LEDs arranged in heart shape, which are flashing in different combinations giving a pleasant visual effect.


The circuit of our construction is based on an AVR microcontroller of ATMEL company, particularly ATmega8 (or the newer ATmega328) in a DIP package. The microcontroller has 28 terminals, 23 from which are input / output controlled by the microcontroller. We use 22 of these terminals in an output mode, which are connected with as many bright LEDs through a resistor of 330 ohms each. The AVR microcontroller has been programmed so that the LED diodes to present a variety of visual effects. LED is a special diode wherein when it biased forward, it emits light. In our construction we use bright LEDs which emit red light. When these are biased forward, they develop a potential difference equal to 2 Volts at their ends. Hence, for the adjustment of LEDs in the outputs of ATmega8 it is necessary for a resistor of value of 330 Ohms to be connected in order to develop the rest voltage.

Circuit Design

The circuit of the electronic little heart appears in the following scheme: Here we can see the circuit symbol of the microcontroller in the centre and the LEDs’ symbols around it. The cathode of every LED is grounded, while the anode is connected to the corresponding terminal I/O of ATmega 8 through a resistor of 330 Ohms. For the power supply needs, the construction was made in two versions. A version with DC power supply 8-15V and another one with power supply 5V from USB port or USB power supply.

circuit for K100 little heart powered by dc pack power supply



The electronic little heart is simple in construction. The PCB we use is double sided with metalized holes with dimensions of 7,5cm X 10cm. At first, we place the resistors, then the LEDs and, finally, the rest of the components. Attention must be given to the placement of LEDs and to integrate circuit which enters the base. The terminals of LED are the cathode and the anode. The cathode is the shortest terminal from the side of the notch in the base of LED and the anode is the other terminal. The cathode of the LED diodes is connected with grounding. The LED diodes can be placed either in the front side of PCB or in the back side of PCB that the components cannot be seen. The function is the same and we choose that we like it. If a LED does not light up, this means that it has been placed upside down. The integrate circuit (microcontroller) is placed so that its notch coincides with it as indicated in the topographic of the PCB.

pcb for K100 little heart powered by dc pack power supply

pcd for K101 little heart powered by usb


The power supply of the construction happens with voltage 5V. Depending on the way of the power supply, there were created the version K100 DC little heart with cylindrical plug for power supply from DC pack from 8V to 15V and the version K101 USB little heart with USB plug with power supply either from the USB port of the computer or from the power supply adapter of 5V with USB plug.

Parts for K100 and K101

R1, R2,…,R22 resistors…. 330R 1/4W
D1, D2,…,D22 bright red diodes 5mm
C1 electr. capacitor………. 2200uF/16V
C2 ceramic capacitor……. 100nF
IC1 ATmega8 AVR programmed microcontroller

Parts for K100

D23, D24 diode…………… 1N4001
CON1 DC power connector
CON2 two screw pcb connector

Part for K101