Voice-controlled small light working at 4.5V DC voltage

This voice-controlled light is used to control a small light bulb powered by 4.5V DC. It can be used as a student experiment or as a voice-controlled night light. The circuit is mainly composed of a 5G555 time-base integrated circuit and some discrete components, as shown in the figure below:

Working Principle:
The piezoelectric ceramic piece B, the transistor VT1, the resistor R1, and the resistor R2 form a sound-controlled pulse trigger circuit, the time-base integrated circuit IC, the resistor R3, the capacitor C, etc. form a typical monostable delay circuit, and the transistor VT2, VT3, the resistor R4, R5, etc. form the power drive amplifier circuit of the small electric bead H.
Usually, because the bias resistor R1 of the transistor VT1 is large, VT1 tends to be cut off, and its collector output voltage is higher than 1/3VDD=1.5V (VDD is equal to the power supply voltage, i.e. 4.5V), and the low-potential trigger terminal 2 of the time-base integrated circuit IC connected to it is at a high level, and the monostable circuit is in a steady state. The two ends of the capacitor C are short-circuited by the transistor turned on inside the IC through the 7 and 1 pins of the IC, and the 3 pin of the IC outputs a low level, VT2 and VT3 are cut off without bias current, and the small electric bead H does not emit light.

When you clap your hands within the effective distance, the sudden sound waves are received by the piezoelectric ceramic piece B and converted into a weak electrical signal. The positive half cycle of the signal is amplified by VT1, and a negative pulse is output from its collector. The 2nd foot of the time base integrated circuit IC obtains a voltage that is instantly lower than 1/3 VDD = 1.5V. The low-level trigger signal triggers the monostable circuit composed of IC to enter a temporary stable state (i.e., a delayed state). Pin 3 of IC outputs a high level, VT2 obtains a suitable bias current and turns on, VT3 enters a fully saturated conduction state, and the small electric bead is energized to emit a bright light. As pin 3 of IC becomes a high level, the transistor turned on inside IC is cut off, and the short circuit to capacitor C is released. Battery GB starts to charge capacitor C through resistor R3. When the charging voltage across C (i.e., the potential of pin 6, the high-potential trigger terminal of IC) reaches 2/3VDD=3V, the monostable circuit flips and recovers to a steady state, the transistor inside IC turns on again, C discharges through pins 7 and 1 of IC and is short-circuited again, and pin 3 of IC outputs a low level again and turns on VT2. VT3 loses the bias current and turns off, and H automatically extinguishes when the power is off.

In the circuit, the length of time that the small electric bead H is delayed each time depends on the time constant of the resistor R3 and the capacitor C in the monostable circuit, which can be estimated by the formula: T=1.1R3C. According to the figure, the value of R3 and C is selected, and the delay time of H is about 1 minute. When the current amplification factor β of the transistor VT1 and the resistance value of R1 are determined, the potential of the 2nd pin of the IC in static state can be adjusted by changing the resistance value of R2. In other words, by properly adjusting the resistance value of R2, the sensitivity of the voice control can be controlled.

Component selection:
IC uses a 555 time-base integrated circuit with a CMOS process that has very low static power consumption. The static current of this CMOS time-base integrated circuit is very small, only about 75uA (measured at a working voltage of 4.5V), and the working voltage is low (actually measured to work at no less than 2V). The "555" time-base integrated circuit produced by the commonly used ordinary TTL process is not suitable for use in this production because of its high power consumption and high working voltage requirement (≥4.5V).

VT1 and VT2 are both 9014 (maximum collector current ICM=0.1A, maximum collector power consumption PCM=310mW) or 3DG8 silicon NPN low-power transistors, requiring VT1's current amplification factor β>200, VT2's current amplification factor β>100, VT3 is 9012 (ICM=-0.5A, PCM=625mW) or 3CG23 silicon PNP medium-power transistors, requiring current amplification factor β>50.
R1-R5 are both RTX-1/8W carbon film resistors. C uses high-quality CD11-10V electrolytic capacitors with very low leakage. B uses φ27mm piezoelectric ceramics, such as FT-27, HTD27A-1, etc., and requires a simple plastic or metal resonant cavity box. H uses a 3.8V, 0.3A small electric bead commonly used in flashlights (special for flashlights powered by three dry batteries). GB is made of three No. 5 dry batteries connected in series (must be equipped with a plastic battery rack), with a voltage of 4.5V.

If the voice control sensitivity is not high enough, it can be adjusted by increasing the resistance of resistor R2 appropriately; on the contrary, if the voice control sensitivity is too high, it can be adjusted by reducing the resistance of R2 appropriately. The general range of R2 resistance is 10-150K. If the resistance of R2 is too large (pin 2 of IC is already at a low level (< 1.5V) when static), the monostable circuit cannot work properly. The small electric bead H will always be on. If the delay time of 1min is too short, it can be adjusted by increasing the resistance of resistor R3 appropriately; on the contrary, it can be adjusted by reducing the resistance of R3 appropriately. For example, when the resistance of R3 is 560K, 1.8M, 2.4M, 3M and 3.6M respectively, the corresponding delay time is 30S, 1.5min, 2min, 2.5min and 3.1 minutes respectively.

Since the delayed night light uses a piezoelectric ceramic piece B with a high resonant frequency (about 4K) as an acoustic sensor, it is sensitive to sudden clapping and collision of hard objects, but not sensitive to people's speech and other low-frequency noise in the environment. In other words, the circuit has a relatively good anti-false triggering performance. Of course, when the circuit voice control sensitivity is adjusted to a higher level, the anti-false triggering ability will be reduced accordingly. This point should be mastered and taken into account when debugging the voice control sensitivity.　

Since the static power consumption of the entire circuit is very small, the measured static total current is less than 130μA, so the circuit does not set a power switch.