Design of low power consumption and high sensitivity sound and light controlled LED lighting

Intelligent lighting can change the lighting status in time according to the needs, and save energy as much as possible while ensuring comfort. The specific measures are to automatically turn off the lights in time when no one is there, and to reduce or stop the electrical lighting in time when there is available natural light. For example, voice-controlled lights or induction lights are used in corridors, elevator entrances, public toilets, etc. Such intelligent lights have a great energy-saving effect.

Traditional lighting is gradually being replaced by new high-efficiency, energy-saving, environmentally friendly, and long-life semiconductor lighting fixtures. For example, LED lights used in corridors are in standby mode most of the day and night, so it is critical to achieve low-power standby in the design.

In order to reduce standby power consumption, existing products use piezoelectric ceramics as sound sensors, which have poor frequency response, especially low trigger sensitivity to low-frequency sounds. The lamp can only be lit when a loud sound is emitted, making it very inconvenient to use.

In this paper, a micro-power consumption, sound and light control type high sensitivity LED lighting lamp is designed by using the micro-power consumption characteristics of MK6A11P. This lighting lamp uses an electret condenser microphone as a sound sensor, which has a good frequency response and greatly improves the response to low frequencies.

When the sensitivity is increased, false alarms will occur due to the high sensitivity to environmental noise, especially during the day when there are many noises, and the voice-controlled light may become a "constantly bright light". For example, in the morning or when there is insufficient light during the day, after the LED light is turned on, it works in the "retriggerable" mode, so as long as there is sound, the light will remain on, and at this time the light is sufficient.

If the "non-retriggerable" mode is used, the lamp will always be turned off for 2 to 3 seconds after being lit for a period of time, regardless of whether there is sound or not, and then the ambient light conditions will be detected. If there is a sound signal, the lamp will be lit. This overcomes the above-mentioned shortcomings, but frequently turning off the lights when people are not around will cause discomfort.

This paper uses the intelligent function of the MK6A11P microcontroller to overcome the above shortcomings, and adopts the method of connecting a microphone, a sound amplifier and the MK6A11P microcontroller in series to improve the detection sensitivity and reduce the standby power consumption. The capacitor step-down method is used in the LED drive power supply part, eliminating the complex drive circuit. This circuit has low loss and certain constant current drive characteristics, which can extend the service life of the LED. After the lamp is triggered, its lighting time can be adjusted between 5 and 40 seconds to meet the needs of different lighting occasions.

1 Overall block diagram of LED lamp

Figure 1 is the overall block diagram. After the AC 220 V power supply is stepped down by a capacitor, it is rectified and filtered to become DC power to drive the LED lamp. At the same time, it passes through a high-resistance current-limiting resistor to generate power for the microphone, amplifier, and microcontroller. After the microcontroller receives the sound trigger signal, it detects the ambient brightness through the light detector and controls the LED component on and off through the thyristor. The lighting time of the lamp can be changed by using an external oscillation frequency control resistor.

1.1 Sound and light controlled LED light sound detection circuit

In Figure 2, the AC 220 V power supply is converted into DC after passing through the step-down capacitor C3 and the rectifier bridge D1, and is provided to the microphone and amplifier after passing through the current limiting resistor R1. Usually, the rated operating current of the electret microphone is in the range of 300~500μA, so the selection range of R1 should be able to provide this current value. The average value of the output voltage after full-wave rectification is determined by formula (1).

Where RL is the load resistance Uin=220V at the output end of the rectifier bridge. Since the voltage across RL is much greater than the voltage at points AB, RL≈R1=430kΩ. Substituting into formula (1) yields I0=460μA.

The power loss of R1 is approximately the standby power consumption of the whole machine, P=I0U=0.9I0Uin=0.09 W.

In order to increase the output signal of the microphone and improve the signal-to-noise ratio, R4 is set to 51 kΩ. At this time, the voltage between points AB is about 23 V, and the voltage across the microphone is 2~5 V, which can be amplified normally. Since the output signal of the microphone is relatively high, only one level of amplification is required to meet the high sensitivity requirement.

In Figure 2, resistors R3 and R6 divide the 2.6 V voltage to provide a bias voltage of 0.5 to 1 V to improve the detection sensitivity. The current I0 passes through the white LED to generate a voltage of about 2.6 V and is provided to the microcontroller.

1.2 LED lamp driver circuit

LED is a semiconductor diode element with nonlinear volt-ampere characteristics and negative temperature characteristics of its voltage drop. Therefore, a small change in voltage will also produce a large change in current. Once it exceeds its rated value, the LED will be damaged. In order to extend its service life, the system should adopt a constant current drive method.

The forward voltage range of low-power white LEDs is generally 2.8~4 V, and the operating current is 15~20 mA.

Figure 3 is an LED driver circuit. After the voltage is stepped down by the high-voltage ceramic capacitor C3, it is bridge rectified and filtered by C6 to drive 60 white LEDs, providing a power of about 3.6 W. R8 is a high-voltage discharge resistor, R15 is a current limiting resistor that can reduce the pulse current when the thyristor Q4 is turned on, and R9 is a current limiting resistor for the LED component to protect the LED component.

In the capacitor step-down circuit, because the load is a component of 60 LEDs connected in series, there is a voltage drop of 3.1×60=186 V, so the current in the actual capacitor is not a continuous sine wave, but a pulse wave as shown in Figure 4.

Table 1 lists the changes in the LED operating current when the input voltage changes in the range of 200~240V.

Assuming that the system operates at 230 V as 100%, when the input voltage changes within the range of 200-240 V, the operating current of the LED changes from 71% to 108% of the rated value, which is a small change and has a certain constant current characteristic.

1.3 Sound and light controlled LED lamp single chip control circuit

The MK6A11P microcontroller has strong anti-interference ability and contains RC oscillator, watchdog, etc. Compared with other series of microcontrollers, it saves a lot of peripheral components and is low in price, suitable for the design of various small products.

Figure 5 is a detection and control circuit composed of MK6A11P. In order to reduce the impact of the light detection circuit on the 2.6 V operating voltage, a high level of 40 ms is first output through the PB1 port during detection. The response time of the photoresistor is usually 20-30 ms. Then, the high and low level states of PB3 are judged. After the judgment, PB1 outputs a low level.

In the light detection unit, the low-priced, small-sized, and highly sensitive photosensor R4 is used as the light detection device. R4, R17, D4, and VR1 together form the ambient brightness detection circuit, and the voltage is detected through PB3. The change in the resistance value is converted into a change in the voltage signal through the voltage sampling circuit, and this voltage value is compared with the reset level of the MK microcontroller to determine whether to trigger the voice control unit, thereby determining the working state of the lighting system.

As shown in Table 2, the relationship between the oscillation frequency and the operating current when MK6A11P operates at 2.6 V voltage.

In the control circuit, VR2 and C10 form an oscillation frequency control circuit. Changing the VR2 value can change the clock frequency, thereby changing the delay time. From Table 2, it can be seen that the maximum value of the working current is 67μA, and the regulated current through D2 is I0=460μA≥67μA, so MK6A11P can work stably.

Figure 6 is a timing diagram of the working process. When the sound is detected, the microcontroller outputs a high level of 40 ms at the PB1 port to detect the ambient brightness. If the PB1 port is at a low level, the LED light is off, as shown in timing 1-2 in Figure 6; if the PB1 port is at a high level, the LED light is on. When the LED is on, the ambient brightness is not detected, but the sound state is detected. If a sound is detected, it is delayed again for 20 s, as shown in timing 3-5 in Figure 6; if the PB1 port is at a high level, the LED light is on. When the sound signal is detected multiple times, it is automatically turned off for 0.5 s after a maximum delay of 40 s, and the ambient brightness is detected. If the conditions are met, the number of sounds detected when the LED was on last time is judged. If it is >2, the LED is automatically lit, thereby preventing discomfort caused by frequent opening and closing, as shown in timing 6-12 in Figure 6.

2 Conclusion

The experimental results show that this LED energy-saving lamp has a good frequency response to low frequencies, greatly improved voice control sensitivity, and a wide voltage range when the standby power consumption is less than 0.1 W. The lighting effect of a 3.6 W LED lamp is subjectively equivalent to that of an existing 11 W energy-saving lamp, which has certain energy-saving significance and broad market prospects.