What is it?

PCB size: 7.5 * 10 CM double-sided conventional process (minimum via 0.305mm, network spacing 0.152mm)

Power supply: TypeC 5V 100mA (or DC-05 interface 7-12 V internal positive and external negative)

Classic average working current: 35-38mA (when not connected to speakers or headphone loads)

LFO oscillation frequency: 3-4Hz

Playback track: The first half of "The East is Red", F key 3/4 beat

Playback speed: 20 seconds/time

The whole machine has only one power switch, and the machine automatically resets after turning on the power. There is also a volume adjustment knob.

After the initial assembly is completed, just unplug IC18 and IC17, and then adjust R13-R18 so that the output frequency of the corresponding calibration point is equal to the frequency marked on the back of the circuit board, then plug in IC18 and IC17, adjust RP2, and calibrate the 3.2Hz square wave calibration point. After the calibration is completed, no other operations are required.

The most common LM324 (quad op amp version of LM358) is used as the filter, synthesis and vibrato control voltage driver. The circuit is specially optimized for the effective dynamic range of LM324 at 5V (0.8-3.6V). This op amp basically meets the requirements of this design, with low power consumption and low cost.

The basic principle of generating vibrato (FM modulation) and attenuated sound (AM) modulation is the same as the third version.

The basic principle of using NE555 to realize FM modulation: NE555/CMOS 555's 5th pin is generally the ground terminal of the bypass capacitor, but if an external voltage is input to this port (a small AC signal with a DC bias of 2/3 VCC), it will affect the upper and lower trigger thresholds of the internal comparator of 555, thereby slightly changing the oscillation frequency without changing the timing resistor and capacitor. The smaller the control signal amplitude, the better the linearity within the adjustment range. When the control signal is generated by LFO, a synthesizer-like FM vibrato effect can be achieved at the output.

Basic principle of using NE555 to realize AM modulation: NE555 / CMOS 555's 7th pin is generally used to discharge the oscillating part, but it is also a natural open-drain output. Square wave multiplication can be realized by simply changing the power supply voltage of the external pull-up resistor. If the power supply voltage decays exponentially, a reduced amplitude output can be obtained at the discharge end. At this time, the discharge end no longer participates in the oscillation, and the original output end (3rd pin) participates in the astable oscillation.

Device selection suggestions:

74 chip: Due to the simplification of the circuit, such as the simplified design of power-on reset, diode encoder, and gate driver, the logic chip has low input current and high output pull current requirements, so CMOS devices should be selected, such as 74HC, 74AC, 74ACT, 74AHC, etc.

555 timer chip: Four dual 555 chips, namely 556, are used in the circuit. CMOS versions should also be selected: ICM7556, TLC556, 5G7556, HA17556, etc. The CMOS version of 555/556 has a small input current, symmetrical output, and low power pollution when the output level is flipped (see ICM7556 datasheet for details).

Op amp and audio amplifier: Whether you use LM324, RRO version LMV324, RRIO version SGM324, etc., it is all OK. Since the actual median voltage of FM modulation is 3.33V, it is only necessary to ensure that the maximum output swing of the op amp when it works in the effective dynamic area under the current power supply is greater than 3.33V+0.2V. LM386 can be selected, with suffixes of -1, -3, and -4.

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What makes it special?