Simple Theremin with Inverter Gates

This simple but complete Theremin circuit is constructed using only two inverter chip plus one regulator IC. This Theremin circuit consist of five functional blocks: power supply regulator, hand controlled oscillator, null oscillator, mixer, and filter

The power supply regulator consist of LP2950 regulator IC, which stabilize the voltage from battery to 5V. You can use more popular 7805 IC for this, but since the power consumption of this Theremin circuit is very small, then you can use 78L05 which is smaller.  CR1 diode is used to protect from inappropriate battery polarity,  shorting the battery voltage together with R8 100 Ohm resistor which prevent the large current when the battery is installed in wrong direction. Although the inverter chip will work well for 9V battery, there is a benefit of using voltage regulation to regulate the battery voltage at lower voltage level, that the voltage will remain constant for until the end of battery life. This will avoid frequency drift of the Theremin’s null oscillator  which should be carefully adjusted to zero the output frequency, which can be affected by the supply  voltage.

Hand Controlled Oscillator

The hand proximity sensor is an oscillator which has antenna extension which shift the capacity coupling in the loop. This capacitance shift occur when we move our hand approaching the antenna. Since this change is very small in percentage, we need this oscillator to be high enough to produce notable frequency difference.  This oscillator block is built around U1A, U1B, and U1C. This oscillator give oscillation at around 73kHz. This frequency is not directly audible, we have to process this signal further to produce audible signal.

Null Oscillator

Null oscillator is employed to produce a constant frequency oscillation that will be used to produce differential frequency which is audible.  This oscillator block is built around U2A, U2B, and U2C. This null oscillator should be adjustable to set the null point where the Theremin should produce no oscillation at certain hand position.  At this point, the null oscillator should be adjusted to have  same frequency with the hand controlled oscillator since the audible Theremin output is the product of the difference between hand controlled and null oscillator frequencies.

Frequency Mixer (Differentiator)

The mixer is used to mix the signal from two oscillators, the hand controlled and the null oscillators. This mixer produce an output which contain many frequency components, not only the difference but also the original and the sum, since the amplifier U1F is basically a digital inverter which has non-linear transfer function. Fortunately, all the frequency components, except the difference, will be much higher than the needed signal and inaudible. This make it easy to remove by simple low pass filter to obtain only the differential frequency component.

Low Pass Filter

As stated before, we need to obtain only the audible frequency component by low-pass filtering. Although the high frequencies is inaudible, we still have to remove it since it can cause damage in audio power amplifier is the level is too high. It can distort the audible signal, and it can eat the electric power like ghost, frying your amplifier or at least make it overheated. This  Theremin circuit use simple low pass filter consist of C4, R5, and R7 for the passive stage, and C2 inside the inverter amp loop.

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True Stereo Indicator Detects L-R Signal Difference

This true stereo indicator is different from what we usually find on FM radio receiver, which is usually a pilot tone detector. A stereo broadcast from FM radio station contain pilot tone, but a presence of pilot tone doesn’t necessarily a stereo broadcast signal since a mono FM transmitter ca broadcast pilot tone as well.  Since this circuit detect the difference between left and right channel, this circuit can detect a real stereophonic programs. When there is no difference between R and L input signals, the output A1 and output A2 is at the same potential. That will make a a virtual ground rail at half the supply voltage.

The A1 will supply a negative or positive voltage when A1 detects a difference between R and L input signals with respect to the virtual ground rail. The C4 will be charged via D2 an C3 via D1. The LED is turned on by the comparator A3/A4 via OR circuit D3/D4. The input signal level should be greater than 100mV to compensate for the drop across D2 or D1. P1 is used to adjust the sensitivity of stereo indicator.

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53 dB Stereo Preamp for Tape or Phonographs

With the circuit shown in the following schematic diagram, both channels of this stereo preamp is constructed using  RCA CA3052 quad AC amplifier.  this pre-amplifier circuit is featured with tone control (bass – treble). Make a similar circuit to complete the other channel, since the circuit for left and right channel are similar. To be high-fidelity, total harmonic distortion should be kept minimum, this circuit gives less than 0.3% distortion level at  at 1-kHz testing signal with 1-V amplitude.  Gain or amplification at 1 kHz is 47 dB,  and the tone control will curve the response with 11.5-dB boost at 100 Hz and 10 kHz at maximum bass and treble boost.  For minimum bass and treble knob position,  the attenuation will be 10dB  at 100 Hz  and 9dB at 10kHz. This circuit is operated by a single ended supply for wider range environment. Inputs can be from tape recorders pickup or magnetic-cartridge phonographs.

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Simple 9V Wireless Microphone FM Transmitter

This FM transmitter circuit can be used as a wireless microphone,  can be received by an ordinary 88- to 108-MHz FM broadcast receiver. For the power supply, this transmitter circuit is powered by a 9 V battery. To comply with the radiation limit of FCC rules, keep the antenna length under 12 inches. L1 is 6 turns of #24 wire wound around a pencil or a 1/4″ form, with turns spacing of 1 wire diameter. C6 is a gimmick capacitor which has value about 1 pF.

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LM380, Simple 2 Watts Audio Amplifier

Small audio amplifier is useful for audio troubleshooting or simple audio projects. It should be a low cost and easy to build. Using integrated circuit for this purpose is the right choice. With LM380  audio amplifier integrated circuit, 2 Watts power can be delivered. For better thermal endurance, a large copper track can be printed in the board to provide heat sinking, this copper track should be soldered to pins 4, 3, 10, 5, 12 and 11.

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Discrete Sliding Tone (Frequency Ramp) Doorbell

This doorbell circuit produces a low tone that will slide up to higher frequency. The equivalent total resistance connected between the base of Q1 and ground (Rbg) , and coupling capacitor  C1  determines the AF oscillator’s frequency. The resistance (Rbg) is equal to (R2+R1)R3. 

The R2 is used to set the initial bias condition, adjusted to produce a pleasant low starting frequency doorbell tone. D1 will start to conduct when Capacitor C3 charge through R6 until it reaches D1 bias  voltage level. Then the value of Rbg is paralleled by R4 and D1, and R5-D2-D3, and the values of diode’s equivalent resistance is gradually decreased as the C3 voltage ramp up.  This decreasing resistance value make the output tone slides up in frequency.  Two different diode path is provided to extend the linear area of diode conduction transition slope. With two path with different biases, after the single diode path has saturated, the second path provide further linear increase at higher voltage level.

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LM3900 Audio Mixer

This audio mixer circuit doesn’t use a low impedance input to mix no ideal  sources, but use many amplifiers to provide  ideal sources before mixed through simple resistors. Ideal source means a sources with low impedances, make the interaction between signals in the output doesn’t affect the input.   Please note that this mixer circuit has high impedance output, so you’ll need an next processing equipment with appropriate  input specs. This audio mixer circuit is designed around an LM3900 quad op amp and combines 2 line and 2 mike inputs and sums them at the output terminal. To vary the gain (around +23 dB), we can change the R7 through R10.

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TDA7056 3W BTL Mono Audio Power Amplifier

For mono output amplifier application,  TDA7056  IC can be your option. Compact but powerful, this integrated circuit is contained in a 9 pin medium power package. This device is designed for battery fed portable equipments such as mono recorders, radios and television. To attract the market, TDA7056 has many features such as low power consumption. For more reliable operation, TDA7056 also has short circuit proof and ESD (Electro Static Discharge) protected on all pins.  Designing application with this IC should be easy since no external components is needed. To make sure you’ll love this chip, this device also has no switch on/off clicks. Overall, TDA7056 has good stability.

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Compressor Circuit with 570/571 Compandor IC

Compressor circuit provide high gain for low amplitude input and provide low gain for high amplitude input. This action, in effect, produce a nearly constant amplitude even though the input has very high dynamic range (very high amplitude variation from time to time). The action of compression like this is needed in some situation, such as in maximizing modulation depth in broadcasting, or sustaining electric guitar signal which has very high variation between the plucking time and fading out.  The following circuit has complementary input/output characteristic and unity gain at 0.775 VRMS input. Voltage gain through compressor is square root of 0.7/Vin. Vin is average input voltage. This circuit  uses Signetics dual channel compandor IC.  570 has lower inherent distortion and higher supply voltage range (6-24 V) than 571 (6-18 V).

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Sound-Activated Lamp (Relay/Switch)

This simple circuit shown int the schematic diagram actives the switch using sound. We can use this circuit for various applications, such as automatic (sound-controlled) disco light or car’s LED light show.  The Q1 amplify the audio from mic. The R1 is used to adjust the peak of signal to greater than about 0.7 volts, act as sensitivity adjuster. A certain level, the signal coming from microphone, after amplification by Q1, will trigger the SCR and light lamp I1. If we change the lamp with a relay, then we can get a sound-activated relay/switch, which can be used to control more powerful / high wattage high voltage lamps. If we use a relay, place a 1N4007 diode in parallel with the relay coil to prevent the back-emf from  relay coil destroying the SCR.

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Basic Complementary (Push-Pull) Power Amplifier Circuit

This amplifier circuit is very popular audio power amplifier circuit type. We call it a complementary since the final transistors is an NPN-PNP pair, each with the same characteristics. This circuit produce an AB class amplifier, since each transistor works in slightly more than half cycle of the signal. There is overlap area when both transistor conduct a current, and this area will be around its stationary current (when the input signal is zero). This circuit is also known as push-pull amplifier circuit since each transistor in the pair is working alternatively.

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LM3900 Audio Mixer

This audio mixer circuit doesn’t use a low impedance input to mix no ideal  sources, but use many amplifiers to provide  ideal sources before mixed through simple resistors. Ideal source means a sources with low impedances, make the interaction between signals in the output doesn’t affect the input.   Please note that this mixer circuit has high impedance output, so you’ll need an next processing equipment with appropriate  input specs. This audio mixer circuit is designed around an LM3900 quad op amp and combines 2 line and 2 mike inputs and sums them at the output terminal. To vary the gain (around +23 dB), we can change the R7 through R10.

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Voltage-Controlled Attenuator (Volume Control) Using FET

Using the circuit shown in the schematic diagram below, we can control the low-level audio signals with ±3V variable DC voltage. This attenuator circuit uses a field effect transistor (FET) to shunt the signal to ground. The R2 is used to control the output level (the attenuation level), but you can use other source of voltage signal to control the grid of the FET, such as DAC output, just remember that this voltage is a negative going signal (you can use with DAC which uses symmetric power supply system). The minimum output of this circuit is when gate bias is zero. When the gate bias is set close to pinchoff value, the circuit will produce maximum output with value that equal to input level. Here is the schematic diagram of the circuit:

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60-dB Range Compressor For Audio

A compressor circuit shown in the schematic diagram below can produce consistent output of 1.4V P-P over entire 60dB range that is very useful for audio level/volume stabilization.  As the main components, this circuit uses a 741 opamp and JFET. With input range of 20mV to 2oV, this circuit has delay time of 0.4s (decay time)  and response time of 1.2ms (attack time). The JFET is used as  voltage-controlled resistor in peak-detecting control loop of 741 opamp.

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Phase Delay Network for 3D Audio Enhancement

3D enhancement is needed to create a fully 3-dimensional sound for most stereo multimedia products. Usually, simple phase-delay circuits is used to produce a widening effect on the perceived sound field. However, there is transaural acoustic crosstalk effect. The following figure shows transaural acoustic crosstalk effect and schematic diagram of simple phase-delay circuits :

Transaural acoustic crosstalk effect is a condition where some sound from the right-hand stereo speaker reaching the left ear, and vice-versa.

This is an active circuit that uses first-order section to present a phase-shift filter with ft of 1kHz for the quadrature signal  and ft = 10kHz for the linear signal. It will produce 90° phase shift between the quadrature and linear signals over the audio bandwidth of 1kHz to 10kHz.

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Audio level meter (vumeter)

This circuit uses just one IC and a very few number of external components. It displays the audio level in terms of 10 LEDs. The input voltage can vary from 12V to 20V, but suggested voltage is 12V.

The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs  providing a logarithmic 3 dB/step analog display. LED current drive is regulated and programmable, eliminating the need for current limiting resistors.

The IC contains an adjustable voltage reference and an accurate ten-step voltage divider. The high-impedance input buffer accepts signals down to ground and up to within 1.5V of the positive supply. Further, it needs no protection against inputs of 35V. The input buffer drives 10 individual comparators referenced to the precision divider. Accuracy is typically better than 1 dB.

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Output Relay Delay Audio Amplifier Circuit

Output Relay Delay Audio Amplifier Circuit

This is a simple circuit which I built to one of my audio amplifier projects to control the speaker output relay. The purpose of this circuit is to control the relay which turns on the speaker output relay in the audio amplifier. The idea of the circuit is wait around 5 seconds ofter the power up until the spakers are switched to the amplfier output to avoid annoying “thump” sound from the speakers. Another feeature of this circuit is that is disconnects the speaker immdiatly when the power in the amplifier is cut off, so avoinding sometimes nasty sounds when you turn the equipments off.

Output Relay Delay Audio Amplifier Circuit Component list
C1 100 uF 40V electrolytic
C2 100 uF 40V electrolytic
D1 1N4007
D2 1N4148
Q1 BC547
R1 33 kohm 0.25W
R2 2.2 kohm 0.25W
RELAY 24V DC relay, coil resistance >300 ohm
Circuit operation
Then power is applied to the power input of the circuit, the positive phase of AC voltage charges C1. Then C2 starts to charge slowly through R1. When the voltage in C2 rises, the emitter output voltage of Q1 rises tigether with voltage on C2. When the output voltage of Q2 is high enough (typically around 16..20V) the relay goes to on state and the relay witches connect the speakers to the amplifier output. It takes typically around 5 seconds after power up until the relay starts to condict (at absolute time depends on the size of C2, relay voltage and circuit input voltage).
When the power is switched off, C1 will loose it’s energu quite quicly. Also C2 will be charged quite quicly through R2. In less than 0.5 seconds the speakers are disconnected from the amplifier output.
Notes on the circuit
This circuit is not the most accurate and elegant design, but it has worked nicely in my small homebuilt PA amplifier. This circuit can be also used in many other applications where a turn on delay of few seconds is needed. The delay time can be increased by using bigger C2 and decreased by using a smaller C2 value. Note that the delay is not very accurate because of simplicity of this circuit and large tolerance of typical electrolytic capacitors (can be -20%..+50% in some capcitors).

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Phase Delay Network for 3D Audio Enhancement

3D enhancement is needed to create a fully 3-dimensional sound for most stereo multimedia products. Usually, simple phase-delay circuits is used to produce a widening effect on the perceived sound field. However, there is transaural acoustic crosstalk effect. The following figure shows transaural acoustic crosstalk effect and schematic diagram of simple phase-delay circuits :

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Audio Visual Ringer

 Many a times one needs an ex- tra telephone ringer in an ad- joining room to know if there is an incoming call. For example, if the telephone is installed in the drawing room you may need an extra ringer in the bedroom. All that needs to be done is to connect the given circuit in parallel with the existing telephone lines using twin flexible wires. This circuit does not require any external power source for its operation. The section comprising resistor R1 and diodes D5 and LED1 provides a visual indication of the ring. Remaining part of the circuit is the audio ringer based on IC1 (BA8204 or ML8204). This integrated circuit, specially designed for telec- om application as bell sound generator, requires very few external parts. It is readily available in 8-pin mini DIP pack.

Resistor R3 is used for bell sensitivity adjustment. The bell frequency is controlled by resistor R5 and capacitor C4, and the repeat frequency is controlled by resistor R4 and capacitor C3. A little experimentation with the various values of the resistors and capacitors may be carried out to obtain desired pleasing tone. Working of the circuit is quite simple. The bell signal, approximately 75V AC, passes through capacitor C1 and resistor R2 and appears across the diode bridge comprising diodes D1 to D4. The rectified DC output is smoothed by capacitor C2. The dual-tone ring signal is output from pin 8 of IC1 and its volume is adjusted by volume control VR1. Thereafter, it is impressed on the piezo-ceramic sound generator

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Audio Light Modulator

Audio light modulations add to the enjoyment of music during functions organised at home or outdoors. Presented here is one such simple circuit in which light is modulated using a small fraction of the audio output from the speaker terminals of the audio amplifier. The output from the speaker terminals of audio amplifier is connected to a transformer (output transformer used in transistor radios) through a non-polarised capacitor. The use of transformer is essential for isolating the audio source from the circuit in The sensitivity control potentiometer VR1 provided in the input to transistor T1 may be adjusted to ensure that conduction takes place only after the AF exceeds certain amplitude. This control has to be adjusted as per audio source level. The audio signal Proper earthing of the circuit is quite essential. The diode bridge provides pulsating DC output and acts as a guard circuit between the mains input and pulsating DC output. Extreme care is necessary to avoid any electric shock

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