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This circuit creates a set of LED-based electronic birthday candles that are just as much fun as blowing out wax candles, but are also reusable, scalable, and even eco-friendly. It uses a thermal sensor that’s maintained at a temperature above the ambient temperature. When you blow air over the sensor, the resistance changes. The circuit detects this change and turns off the eight LEDs. When you stop blowing, all but one of the LEDs turn on. This cycle continues until you’ve blown across the sensor eight times and all LEDs remain off.

The “blow sensor” consists of a 47-O, 1/4-W heating resistor attached to a 180-O NTC thermistor. To attach the resistor, scratch the paint off one of the leads of the thermistor bead to expose the metallic contact. Then cut one of the resistor’s leads near the cap. Scrape the paint from the resistor cap to expose its metallic surface. Then solder the resistor to the exposed thermistor contact (Fig. 1).

Soldering ensures firm thermal contact between the devices. When voltage is applied to the heating resistor, the thermistor’s temperature rises and its resistance decreases. If you blow air on this sensor, it cools down and its resistance increases. Thus, the sensor detects air blown over it.

The circuit runs on 6 V, either four AA cells or a rechargeable lead-acid battery. A mains-connected supply was avoided as a safety precaution. The blow sensor consists of heating resistor R1 and the thermistor R4 (Fig. 2). R1 is fed through a small resistor, R2, in series. The thermistor forms a voltage divider with R3. Node X is connected to the inverting terminal of op-amp U1A (LM324). Node Y of the divider formed by R5 and R6 is connected to U1A’s non-inverting terminal. U1A amplifies the difference voltage between nodes X and Y. Q1 (TIP 122) boosts U1A’s output current.

A T-filter (R9, R10, and C2) smoothes the voltage fluctuations in U1A’s output. These fluctuations are caused by the varying current drawn by the blinking multicolor LEDs. The filtered voltage is then fed to the inverting terminal of comparator U1B, whose non-inverting terminal is held at 0.5 VBAT by voltage divider R11 and R12. The comparator’s output passes through Schmitt-trigger inverters U3A and U3B (74HC14) and is connected to the clock input of shift register U2 (74HC164).

The shift register comprises an 8-bit serial-in/parallel-out device that’s used to turn off the LEDs one after the other. Inputs A and B of the shift register are tied to logic high. The clear pin is connected to the power-on reset circuit, which consists of C1, R14, and Schmitt-trigger inverter U3C. The outputs of the shift register are connected to eight PNP transistors, Q2 through Q9. In the collector circuit of each of these transistors, LED1 to LED8 are connected. Power for ICs U2 and U3 is derived from VBAT using a Zener regulator (D1) of 5.1 V.

At power ON, Q1’s emitter voltage is zero because voltage at node X is higher than at node Y. As heater resistor R1 heats thermistor R4, the voltage at node X starts decreasing. Gradually, voltage at Q1’s emitter starts to build up. The power-on reset circuit clears the shift register contents and all eight outputs of U2 become low. Therefore, transistors Q2 to Q9 turn on. As Q1’s emitter voltage builds up, the LEDs gradually turn on. The emitter voltage stabilizes at about 4.7 V. This process takes about 40 to 50 seconds and all LEDs glow with full brightness. The circuit is ready for use.

When the birthday honoree blows air on the sensor, the voltage at node X increases, reducing the voltage at Q1’s emitter. This reduces the brightness of the LEDs. When the LEDs fully turn off, the voltage at the non-inverting terminal of comparator U1B goes below 0.5 VBAT. U1B’s output then goes high and generates a clock pulse to the shift register. Since shift register inputs A and B are high, output QA goes high and transistor Q2 turns off. When all of the LEDs are off and the blowing stops, the LEDs again turn on gradually. But LED1 will not turn on since transistor Q2 is off. To turn off all of the LEDs, repeat the process eight times.

The circuit was assembled on a circular printed-circuit board PCB) with the eight LEDs equally spaced on the periphery (Fig. 3). The sensor was located on the front side and the supply wires on the back side. The finished unit includes a CD used as a reflector to enhance the light output from the LEDs—and to hide the components on the PCB (Fig. 4).

Implementers can customize this circuit in a number of ways:

• The circuit was tested at an ambient temperature between 20°C and 30°C (68°F to 86°F). For lower temperatures, you may have to reduce the value of R2 to compensate for higher heat losses.
• To speed up the turn on and turn off of the LEDs, U1A’s gain can be increased by boosting the value of R8.
• You can increase the number of LEDs by having more than one LED in parallel for each transistor.
• You can reduce the number of cycles for turning off LEDs by controlling more than one transistor with one shift register output.