Enhanced TCA440 detector

Description: This circuit appears simple due to the significant effort made to streamline its design. It requires only a small number of components and has low current consumption (15 mA). From left to right, the circuit consists of a transducer, a simple transistor preamplifier, a TCA440 integrated circuit (IC) that includes an oscillator and mixer, a second-order low-pass filter, and an audio amplifier using an LM386 IC. The used transducer is a piezoelectric type, which is quite sensitive but has a relatively limited frequency range. By connecting a coil in parallel with the transducer, the response can be somewhat broadened, although this reduces overall sensitivity. The signal from the transducer is first amplified by a simple transistor stage, which consists of a single BC550C transistor biased for a current of approximately 0.7 mA. A capacitor connecting the emitter to ground creates a +6 dB/octave gain slope for frequencies above 16 kHz. The signal then enters the TCA440 at pin 1, where it is multiplied with a signal from a built-in tunable oscillator. The oscillator can be tuned using a potentiometer connected to pin 6, allowing frequencies to range from about 18 kHz to 100 kHz. The potentiometer should be connected so that it has the lowest resistance when the wiper is turned counterclockwise, resulting in a reasonable linear relationship between wiper position and tuning frequency, with counterclockwise yielding the highest frequency and clockwise yielding the lowest. The oscillation frequency is determined by the product of the capacitor between pins 5 and 6 and the resistor connected from pin 6 to the positive supply. The highest frequency is set by the 1.8 nF capacitor and the 2 kΩ resistor, while the lowest frequency occurs when the circuit sees a 2 kΩ + 10 kΩ = 12 kΩ resistor from pin 6 to the positive supply. Therefore, the ratio between the highest and lowest frequency is equal to 12 kΩ/2 kΩ = 6. From pin 16, the signal reappears and enters a low-pass filter, which is a second-order filter with a cutoff frequency of 3.4 kHz, providing a range of about 7 kHz around the oscillator center frequency. Finally, the down-converted and low-pass filtered signal is fed into an LM386 audio amplifier, capable of driving a set of standard low-impedance headphones. A stereo 3.5 mm jack chassis is used, with the left and right leads connected together to provide mono output. Component values are not highly critical; however, metal film 1% resistors are recommended for the input stage. The capacitor connecting pins 5 and 6 of the TCA440 should have a reasonable tolerance. For the 100 µF and 47 µF capacitors, electrolytic types are used (with attention to polarity). It is noted that the capacitor value between pins 5 and 6 has been found to be too low, and a value of 2.7 nF is now recommended to improve frequency range. Additionally, a parallel coil value of 8.2 mH or 5.6 mH is suggested for better broadening of the transducer's frequency range.

The circuit design employs a piezoelectric transducer, which converts mechanical vibrations into electrical signals, making it suitable for various audio applications. The initial amplification stage utilizes a BC550C transistor, providing a necessary gain to ensure the signal strength is adequate for further processing. The TCA440 IC serves a dual purpose: it functions as a mixer and oscillator, allowing for frequency modulation of the input signal. The tunable oscillator feature is particularly advantageous for applications requiring variable frequency response, and the linear tuning characteristic aids in precise frequency adjustments.

The second-order low-pass filter effectively attenuates high-frequency noise, ensuring that only the desired audio signal is passed to the LM386 audio amplifier. The LM386 is well-suited for driving headphones, providing sufficient power while maintaining low current consumption, which is essential for battery-operated devices. The design's consideration for component tolerances and values enhances overall performance and reliability, making it a robust solution for audio signal processing. The adjustments suggested for the capacitors and coils indicate a commitment to optimizing the circuit for improved functionality, ensuring that it meets the performance requirements of its intended application.This circuit may look simple, but that`s because I made considerable effort to simplify the circuit. It requires only a small amount of components and it has low current consumption (15 mA). From the left to the right, this circuit consists of a tranducer, a simple transistor pre-amp, a TCA440 IC that contains an oscillator and mixer, a second-ord er low-pass filter and a audio-amp consisting of a LM386 IC. The transducer that is used is a piezo-electric transducer, which is quite sensitive but unfortunately has a relatively limited frequency range. By connecting a coil parallel to the transducer, the response can be broadened somewhat, at the expense of decreasing overall sensitivity.

The signal from the transducer is first amplified by a simple transistor stage, consisting of a single BC550C transistor. It is biased for a current of approximately 0. 7 mA. The capacitor from the emitter to ground causes a +6 dB/oct gain slope for frequencies higher than 16 kHz.

After this it enters the TCA440 at pin 1. In the TCA440 this signal is multiplied with the signal of a built-in tuneable oscillator. The oscillator can be tuned with the use of the potmeter connected to pin 6, from about 18 kHz to 100 kHz. The potmeter must be connected such that it has the lowest resistance when the wiper is turned counterclockwise.

This gives a reasonable linear relation between wiper position and tuning frequency, but means that CCW is highest frequency and CW is lowest. The oscillation frequency is determined by the product of the capacitor between pins 5 and 6 and the resistor going from pin 6 to the positive supply connection.

The highest frequency is therefore set by the 1n8 capacitor and the 2k resistor, while the lowest oscillator frequency occurs when the circuit sees a 2k+10k=12k resistor going from pin 6 to the positive supply connection. This means that the ratio between the highest and lowest frequency is equal to 12k/2k=6. From pin 16 the signal reappears and enters a low-pass filter. This is a second-order filter with a cut-off frequency of 3. 4 kHz, giving a range of about 7 kHz around the oscillator center frequency. Finally the down-converted and low-pass filtered signal is fed into a LM386 audio amplifier which can drive a set of normal low-impedance headphones.

I used a stereo 3. 5mm jack chassis with the left and right leads connected together to give mono output. Components values aren`t very critical, however I used metal film 1% resistors for the input stage. Also for the capacitor connecting pins 5 and 6 of the TCA440, you should use one with reasonable tolerance. For the 100uF and 47uF capacitors, I used electrolytics (watch the polarity !). Update! The value of the capacitor between pins 5 and 6 has turned out to be too low. A value of 2. 7 nF (instead of 1. 8 nF) is recommended. This improves the frequency range. The value of the coil parallel to the transducer is not optimal. A value of 8. 2 mH or 5. 6 mH is probably better suited to broaden the frequency range of the transducer. See also the page about detuning