Simple Two Op Amps Instrumentation Amplifier Circuit

In standard instrumentation amplifier, there are a three op-amps instrumentation amplifier circuit is common, but if you wanted a smaller component count and lower cost instrumentations, you can use two op-amp for the same functions: a high impedance and good CMRR for sensor pre-amplification and buffering. The figure is show the circuit.


The circuit is a simplest form of the op-amps instrumentation amplifier circuit. A two-op-amp instrumentation amplifier can also be used to make a high-input impedance DC differential amplifier. As in the two-op-amp circuit, this instrumentation amplifier requires precise resistor matching for good CMRR. R4 should equal to R1 and R3 should equal R2.

Simple Thermometer Circuit Using LM335

This circuit in below is show the simple form of the thermometer circuit. The circuit is based on LM335 as sensing the temperature. This is the figure of the circuit.


The circuit consists of two parts: The LM335 and its adjustment. The output of the LM335 is 10 milli volts per degree C, with 25 degrees C corresponding to 2.982 VDC. A reference circuit provides a zero reference voltage. It is adjusted to (2.982 volts - (25 degrees x 10 milli volts/degree) = 2.832 volts. To read the temperature of the LM335 directly in degrees C, connect the + lead from a high impedance DVM to the output pin and the end of the DVM to the 2.732 volt pin. The factor of 10 milli volts per degree C is equivalent to 10 milli volts per degree K, since a change of one degree C is equal to a change of one degree K. The difference in the two scales is only their offsets. The melting point of water ice is 0 degrees C and 273.15 degrees K. The boiling point of water is 100 degrees C and 373.15 degrees K.

Simple Setting Time Test Circuit

This is a circuit for testing the time. This circuit is built using JFET. This is the simple form. Here’s the schematic circuit diagram of the time test circuit.


Settling time is tested with the LF155/6 connected as unity gain inverter and LF357 connected for AV = −5. The FET used to isolate the probe capacitance. The output = 10V step. AV = −5 for LF357. [Schematic source: National Semiconductor, Inc].

Simple Precision Half Wave Peak Detector Circuit

This is a simple circuit for half peak detector that is using transistor. This circuit is very compact and low cost, but if the application require precise detection for signal under 100mV then that circuit is become useless. In the figure below, the circuit is become precise half-wave peak detector circuit using only a single op-amp with few passive components. This is the figure of the circuit.


This peak detector circuit can be implemented both for average peak detector or a fast peak detector by selecting R2 and R3 values as shown in the table. Note that the output impedance is set by R3, so make sure that the output of this peak detector circuit will be connected to a higher impedance input stage. [Schematic diagram source: National Semiconductor Application Note]

Simple Positive Feedback Circuit Using Op Amps

This is the circuit that is due to the high gain and wide bandwidth of comparators. This circuit is built by op amps LM111. This is the figure of the circuit.


The trim pins (pins 5 and 6) act as unwanted auxiliary inputs. If these pins are not connected to a trim-pot, they should be shorted together. If they are connected to a trim-pot, a 0.01 mF capacitor C1 between pins 5 and 6 will minimize the susceptibility to AC coupling. A smaller capacitor is used if pin 5 is used for positive feedback. When the signal source is applied through a resistive network, RS, it is usually advantageous to choose an RSÊ of substantially the same value, both for DC and for dynamic (AC) considerations. Carbon, tin-oxide, and metal-film resistors have all been used successfully in comparator input circuitry. Inductive wire wound resistors are not suitable. [Circuit source: National Semiconductor, Inc]

Simple LED Pilot Circuit

This is a circuit diagram Led pilot using FET. The FET (Field Effect transistor) is ECG312, NTE312 good for up to 30Vdc max. MPF102 that will also work with both, only the max voltage is 25V. This is the figure of the circuit.


Principles of circuit when this is the gate and source associated with, the current behaves as a regulator. In circuits that are on the constant is between 6 and 8 mA at 5 to 30Vdc. Because the set at this time is about 6mA older than the LED will glow a little in the answer. If the diode is added, such as 1N4148 or similar, can be connected to a source of AC-AC 5-20 V (NTE312).

Simple Greatly Expanded Scale Circuit (Bar Mode Only) Using LM3914

This is the circuit for bar mode using LM3914 IC for great expanded scale. This circuit is very simple design. This circuit is only consists of some resistors circuit. This is the figure of the circuit.


Placing the LM3914 internal resistor divider in parallel with a section (.230Ω) of a stable, low resistance divider greatly reduces voltage changes due to IC resistor value changes with temperature. Voltage V1 should be trimmed to 1.1V first by use of R2. Then the voltage V2 across the IC divider string can be adjusted to 200mV, using R5 without affecting V1. LED current will be approximately 10mA. [Schematic source: National Semiconductor, Inc].

Simple Driving Capacitive Loads Circuit

This is a simple circuit design for capacitive load driver. This circuit is built by LF155. LF155 is an op amps that is feature low input bias and offset currents/low offset voltage and offset voltage drift, coupled with offset adjust which does not degrade drift or common-mode rejection. This is the figure of the circuit.


This figure in above is the circuit for drive capacitive loads. For make this circuit, we need resistor 5K if we used this op amps. This is due to a unique output stage design, these amplifiers have the ability to drive large capacitive loads and still maintain stability. CL(MAX) 0.01μF. This circuit has overshoot ≤ 20%. [Circuit source: National Semiconductor, Inc]

Simple Delay Circuit for Surround Sound

This circuit in below uses the Mitsubishi M65830 Digital Delay chip. This has been around for a while now. This circuit is a simplest form for delaying circuit. The serial data required to obtain different delay settings is not easily obtained, and would add considerably to the complexity of the circuit. This is the figure of the circuit.


The circuit is (almost) a direct adaptation from the Mitsubishi data sheet, and as shown will give good performance over a wide frequency range. The filters are tuned to around 9.5 kHz, and although this could be reduced there does not seem to be any good reason to do so. This seems to be the optimum response for rear channel speakers, so should be left alone. The filter circuits use internal op amps, and only require the external components. The unit can be constructed as a module quite easily, requiring a 5 Volt supply, analogue and digital earth connections, and an input and output.

Simple AC Current Indicator Light

This is design circuit for AC indicator. The circuit will indicate the current from the circuit that is used as load of the circuit. The circuit is very simple design circuit. This is the figure of the circuit.


This circuit could be wired into a 120vac power line, which feeds power to any load, ranging from 40 watts to 250 watts. It will turn on a LED light whenever current is being drawn by a load. It is especially useful for remote lights, where you may not be able to see if the lamps are receiving power.

Adjusting Linearity of Several Stacked Dividers

The circuit in below is shows about linear adjustment in the dividers. The circuit is using LM3914 for control the circuit. This circuit is very simple form. This is the figure of the circuit.


Three internal voltage dividers are shown connected in series to provide a 30-step display. If the resulting analog meter is to be accurate and linear the voltage on each divider must be adjusted, preferably without affecting any other adjustments. To do this, adjust R2 first, so that the voltage across R5 is exactly 1V. Then the voltages across R3 and R4 can be independently adjusted by shunting each with selected resistors of 6kΩ or higher resistance. This is possible because the reference of LM3914 No. 3 is acting as a constant current source. The references associated with LM3914s No. 1 and No. 2 should have their Ref Adj pins (pin 8) wired to ground, and their Ref Outputs loaded by a 620Ω resistor to ground. This makes available similar 20mA current outputs to all the LEDs in the system. [Circuit source: National Semiconductor, Inc]

Simple Wideband Output Amplifier Circuit

This is a simple design for wideband output amplifier. The wideband amplifier is suitable for use as a 50Ω transmission line driver. This circuit is built by CA3140. This is the figure of the circuit.


This circuit, when used in conjunction with the function generator and sine wave shaper circuits shown in Figures 10 and 12 provides 18VP-P output open circuited, or 9VP-P output when terminated in 50Ω. The slew rate required of this amplifier is 28V/μs (18VP-P x π x 0.5MHz). [Schematic diagram source: Intersil Corporation].

Simple Video Line Driver Circuit

This is a simple design schematic for video line driver. This circuit is based on IC LT1206 that is suitable for driving multiple video cables since it has 60MHz bandwidth, 250mA output current capability, and low output impedance. Please keep in mind about un-terminated line effect when driving multiple cables. A reflected wave would propagate back to the driver cause a crosstalk to other lines since the driver output is non-zero impedance. This is the figure of the circuit.


The schematic diagram shows the LT1206 connected as a distribution amplifier. To minimize the effect of reflection, each line is separately terminated. Differential gain and phase performance are also important and have been considered in the internal design of the device, especially useful if the signal is video composite. Note that at 5MHz the output impedance of LT1206 is only 0.6W, much smaller than the terminating resistor separators. [Schematic diagram source: Linear Technology Application Notes]

Simple Symmetric Power Supply without CT Transformer

This is a kind design of power supply. It is like a center tapped transformer with a bridge diode is common solution to provide symmetric supply, but we can actually do it with simple solution, using a voltage double - rectifier diodes. This circuit is very simple design. This is the figure of the circuit.


The capacitor need to be large because the rectifying is half wave, so you can avoid power line hum. With the component values shown, the power supply is capable supplying 10mA at ripple voltage about 0.2V (peak to peak). The voltage ripple can be approximated by Vripple = (20 x I)/C, where I is the current drawn by the load in mA, and C is the capacitance in uF (micro farad).

Simple Intercom Circuit Using LM380

This is a circuit for manual telecommunication that anyone can put together and get to work. This circuit is very simple. This circuit is based using LM380. This chip is able to put out 2 watts of power if it is heat sink properly. This is the figure of the circuit.


The following pins should be grounded and attached to a foil to dissipate the heat. Pins 3, 4, 5, 10, 11, 12 should all be grounded. The circuit works as follows. Switch 1 is a double pole double throw switch. In one position is the talk position and in the other is the listen position. In the diagram shown the switch is in the talk position for the speaker on the left. Talking into the speaker inputs a signal to the IC chip through the matching transformer T1. The output from the IC chip goes to the speaker on the right.

Simple Adjustable Current Limiter Circuit

This circuit is design for provide automatic current limiting up to 8.4A. Unlike current limiter that uses only a resistor, this current limiting circuit doesn’t drop the voltage, or at least keep the voltage drop at minimum, until a certain current amount is exceeded. This current amount limit is adjustable from 1.4 A to 8.4A by a potentiometer. You can modify the component value to give different current limiting range. This is the simple figure of the circuit.


This circuit is very simple form. How is the circuit work? First, the resistor R1 is there to sense the current. At R2 potentiometer at minimum resistance (the center tap connected to R1), if the current drawn by the load reach 1.2A then the voltage across R1 reach 0.6V and Q2 begin conducting, thus shorting the base voltage of Q4 to ground. This shorting action to reduces the base current and therefore reduce the output voltage sensed by the load, and prevent the current to flow further. If you need the current limiter to limit at lower threshold range, you can change the R1 to 1R and you’ll get about 0.7A to 4.2A adjustment range. Because of the power dissipation capability of 2N3055 transistor, at the worst case that the load is shorted to ground (zero resistance), when you limit the current to 8.4 A then the circuit can handle maximum source voltage of 14V, while limiting the current at 4.2A can handle up to 27V source voltage. The maximum voltage that can be handled the circuit is 60 volt, but you can only safely set the current limit at 1.9A in the extreme condition, when the load is shorted to ground. The Q1 transistor must use sufficient heat sink. [Schematic project source: Hasan Murod].

Simple Slave Flash Light Control Circuit

This is a design circuit that is used to give auto trigger for your secondary flash light. This secondary flash will be automati8cally triggered when it receive the light signal from the primary flash. This circuit is a simplest form of the flasher light. This is the figure of the circuit.


How the circuits work? Look in this explain. The inductor 68mH is connected in parallel with the solar cell to prevent the false trigger by slowly changing environment light. Bright daylight won’t trigger your slave flash lamp because the output will be shorted by the inductor. Only fast luminance change caused by your primary flash lighting will trigger your secondary flash light. The solar cell should be 100mm2 at minimum (about 1cm2), and you can easily find on many electronics part store, and sure you can use a larger one for better sensitivity. For the inductor, because it is relatively huge, you can use a radio frequency choke (RFC) that usually used in AM radio transmitter circuit, but any inductor with low enough resistance could be used.

Simple Self Powered Current Sensor

This is a very simple design circuit of current sensing circuit that is sense the current at the high side (between positive supply and load). This circuit is control by op amp LT1494. This circuit is take the benefit of the microampere supply current and Rail-to-Rail input of the LT1494 op-amp. This is the figure of the circuit.


The inverting input is connected directly to the output, and this negative feedback will keep the inverting input to be in the same level with the non-inverting input. This mans that the voltage across RA will be kept equal to the voltage across Rsense. This voltage is make the current flows from the power source to the output pin of the op-amp through RA, and finally return to ground via RB. Almost all of the current at RB comes from RA since the internal circuit draws only very small amount of current through Vcc pin, so the output at RB is valid.
[Schematic diagram source: Linear Technology Application Notes]

Simple Ring Detector Circuit

This is a simple design for ring detector circuit. This is a small circuit that can changes the tone signal to 5 DC pulses that directly can feed by microcontroler pin. This is the figure of the circuit.


This circuit is explanation the call is received or call is calling. You can monitoring the tone signal with off-hook signal detector that can you read in the related site link in project-microcontroler.irone.org. In this circuit output pulse for each ring is delaying between the two pulses must be monitoring and if it is exceeds 4 seconds.

Simple Low Noise Audio Line Driver Circuit

This is a design for audio line driver. This circuit is based on 2 IC as controller the operation. This is a simple circuit and low noise. The LT1206 has been naturally selected as the components for video application since it has suitable bandwidth and output derive capability. This high performance features is not only suitable for video application, but also make it definitely suitable for audio application. This is the figure of the circuit.


The schematic diagram shows the LT1206 combined with the LT1115 low distortion, very low noise audio buffer with a gain of 10. With a 5VRMS and 32 Ohm load (780mW), the THD + noise for the circuit is only 0.0009% at 1 KHz, rising to 0.004% at 20 KHz. The frequency response is flat to 0.1dB from DC to 600 KHz, with a – 3dB bandwidth of 4MHz. The circuit is stable with capacitive loads of 250pF or less.
[Schematic diagram source: Linear Technology Application Notes]

Simple Low Current Amplifier

This is a design for amplifier, but in this circuit, it is a low current. This is a simple design circuit for amplifier. The circuit is built by CA3140 as op amp the signal. The low input terminal current needed to drive the CA3140 makes it ideal for use in current amplifier applications like the figure in below.


In this circuit, low current is supplied at the input potential as the power supply to load resistor RL. This load current is increased by the multiplication factor R2/R1, when the load current is monitored by the power supply meter M. Thus, if the load current is 100nA, with values shown, the load current presented to the supply will be 100μA; a much easier current to measure in many systems. [Project Schematic source: Intersil Corporation].

Simple Led Pulse Circuit

This circuit is called LED Pulse. This circuit is a simple design circuit for application the LED. In principle, the operation mode is obtained by the triangular wave generator formed by two op-amps in a very cheap 8 pin IC Dil cases. Q1 ensures current buffering, in order to get a better load drive. This is the figure of the circuit.


R4 & C1 are the components, the circuit where this total time about 4 seconds. If the value of R4 to adopt is under 220K, the pulsing effect of the peculiar effect of flash. Do not supply the circuit with the voltage exceeds 6V. At 6V supply, increase the value of R6 to 100 Ohm.

Part :
R1, R2 = 4K7
R3 = 22K
R4 = 2M2
R5 = 10K
R6 = 47R
C1 = 1µF
D1 = LED
IC1 = LM358
Q1 = BC337

Simple Fahrenheit Thermometer Using LM35

This is a design for thermometer. This is a ones simple circuit. If you have a digital voltmeter, or any voltmeter with milli volt resolution and high input impedance, then you can use this temperature-to-voltage adapter circuit to measure room temperature. This circuit is based on LM35 as temperature sensor. This is the figure of the circuit.


Note that the voltage output of this circuit is floating, not referenced to ground. You have to use separate supply if your voltmeter has single ended (referenced to ground) input. Differential input adapter described here is suitable to give a differential input for your single ended meter, remember that the input impedance must be high. You can set your voltmeter to 200 mV range to give temperature reading directly in Fahrenheit degree. [Schematic source: National Semiconductor Application Notes]

Simple Audio Graphic Equalizer Circuit

This is a design circuit for audio graphic equalizers, that are very common as commercial products but circuits for them are very rarely published. This circuit is a simple design circuit. The circuit is needs an op-amp for amplifying the input signal. This is the figure of the circuit.


Only one gyrator stage is shown: all 7 gyrators are the same circuit, only the capacitors change, as shown in the chart. I have shown three of the seven faders to show where they go. A gyrator is a circuit using active devices and transistors to simulate an inductor. In this case the gyrator is the transistor acting with R1, R3 and C2. It could just as easily be a unity gain op-amp. The circuit includes three formulae: one which gives f, the the centre frequency of the band. The second shows how the Q is related to the capacitor ratio. The third shows the impedance presented by the circuit. Note that this includes 3 terms, the first purely resistive, the second is the capacitive contribution from C1 and the third is an inductive term from the gyrator.

Simple AM FM Antenna Booster Circuit

This is design for antenna booster circuit. This circuit can be used to amplify the weak signal received by the antenna. This circuit is a simplest form of circuit. Antenna for AM/FM is usually not tuned for the optimal dimension of 1/4 wavelength, since we prefer small portable size. This un-tuned antenna has very low gain, so the antenna booster circuit here is very helpful in getting better signal reception. This is the figure of the circuit.


Use around 470uH coil for L1 if you use for AM frequency (700 KHz - 1.5 MHz) and use around 20uH for SW or FM receiver. For short wave performance, using this antenna booster, you’ll get a strong signal as we get from a 20-30 feet antenna, with only a standard 18″ telescopic antenna and this booster circuit. The power supply should be bypassed by a 47nF capacitor to ground, at a point that should be chosen as close as possible to L1.

Simple Accelerometer Amplifier Circuit

This is a circuit for accelerometer amplifier. This is a simple circuit. Precision accelerometer needs inverting mode amplifier since they are usually charge-output devices. This amplifier is convert charges into voltage output. The circuit below is an example of accelerometer with DC servo.


The charge from the transducer is converted to a voltage by C1, which should equal the transducer capacitance plus the input capacitance of the op amp. The low frequency bandwidth of the amplifier will depend on the value of R1 • C1 (or R1 (1 + R2/R3) for a Tee network). The noise gain will be 1 + C1/CT. The time constant of the servo (1/R5C5) should be larger than the time constant of the amplifier (1/R1C1).
[Schematic source: Linear Technology Application Notes]

Simple Audio Level Meter Using LM3915

This is a simple design of audio level meter. This circuit uses just one IC and a very few number of external components. This circuit is based on LM3915 as controller in the operation of the audio level meter circuit. It displays the audio level in terms of 10 LEDs. This is the figure of the circuit.


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.

A Simple Phase Control Circuit

This is a simple design schematic circuit for phase control circuit. The circuit can be used to control the power delivered to an AC load. The phase control circuit can control the AC waveform, cutting the cycle to give full cycle, half cycle, zero cycle, or somewhere in between. You can say this circuit is similar to a dimmer circuit, but the switching is synchronized with the zero crossing of the waveform. This circuit is works using based on IC U208B. This is the figure of the circuit.


The benefit of switching the power in zero crossing condition is that the triacs doesn’t suffer power dissipation, thus increasing the overall efficiency. This phase control circuit is suitable for brushed AC motor, heater filament, or incandescent lamps. The IC U208B is designed as a phase control circuit in bipolar technology with internal supply-voltage monitoring. As the voltage is built up, uncontrolled output pulses are avoided by internal monitoring. Furthermore, it has internal-current and voltage synchronization. It is recommended as a low cost open-loop control.
[Circuit's schematic diagram source: TEMIC TELEFUNKEN Microelectronic Application Notes]

Simple Open Loop Fast Peak Detector Circuit

This is a simple design schematic for fast peak detector similar but faster than previous peak detector, can be implemented using open loop configuration. This circuit is based on LT1190 IC for operation. This is the figure of the circuit.


In this circuit, operation is begun from D1 is the detector diode and D2 is a level shifting or compensating diode. A load resistor RL is connected to – 5V and an identical bias resistor, RB, is used to bias the compensating diode. This equal value resistor is RL and RB makes sure that the diode drops are equal. Low values of RB and RL (1k to 10k) yield in fast response, at the expense of poor low frequency accuracy.

High values of RB and RL provide good low frequency accuracy but cause the amplifier to slew rate limit, resulting in poor high frequency accuracy. A solution can be made by adding a feedback capacitor CFB, which improve the negative slew rate on the (–) input. We can expect is under 15% amplitude error for 2Vpp-6Vpp input at 20MHz, much faster than closed loop design. [Schematic source: Linear Technology Application Notes].

Simple Half Wave Precision Rectifiers Circuit

There are several different types of precision rectifier, but it is necessary to explain what a precision rectifier actually is. In its simplest form of circuit, a half wave precision rectifier is implemented using an op amp, and includes the diode in the feedback loop. This is the figure of the circuit.


For a low frequency positive input signal, 100% negative feedback is applied when the diode conducts. The forward voltage is effectively removed by the feedback, and the inverting input follows the positive half of the input signal almost perfectly. When the input signal becomes negative, the op amp has no feedback at all, so the output pin of the op amp swings negative as far as it can. Assuming 15V supplies, that means perhaps -14V on the op amp output.

Simple Fast Pulse Detector Circuit

This circuit is a schematic diagram for a fast pulse detector. At 100 ns or wider pulse width, the detection should be error free. At 60 ns pulse width, the detection error should be under 10%. This circuit is a simple design. This is the figure of the circuit.


This circuit should be used to detect at least 100 ns wide pulse to ensure an error free performance. This circuit is based on LT1190 for the operation.[Schematic diagram source: Linear Technology Application Notes]

Simple Current Loop Transmitter for Temperature Sensor

This is a simple design circuit for current loop transmitter circuit. Current loop interface has been widely used in industrial environment because it is robustness. Noise resistance and fail detection capability made it suitable for long distant transmission in harsh environment. This circuit provide current loop transmitter for temperature sensor. This is the schematic figure.


In the temperature measurement is done by LM35 temperature sensor chip. You can use general silicone diode such as 1N4001. The current controller function is done by LM317 current/voltage regulator. This circuit will draw a consistent current proportional to the temperature being measured, regardless the supply voltage variation caused by noise or long wire’s temperature-dependent resistance variation.

Simple 500 mW Audio Amplifier Using 3 Transistors

This is a simple design schematic for audio amplifier. This circuit is a small 3 transistors amplifier circuit will be suitable for small battery powered devices, and the circuit is really simple. This tiny power amplifier use same typology as the more complicated version of discrete amplifier in our previous circuit, but with here is in the simplest form. This is the figure of the circuit.


This circuit is a very good start as a DIY amplifier because its simplicity. Since this audio amplifier process only audio signal (not a dc amplifier), both the input and the output are dc-blocked using capacitors. You can see 3.3 ohm resistors connected to the emitter of PNP-NPN transistor couple, and the purpose of this resistor is to stabilize the transistor gain, so the temperature change won’t affect the performance much.

Any PNP-NPN transistor couple with identical performance that capable of handling 100mA collector current should be suitable to replace the final transistors. The voltage swing of the output will be 2 Volt at maximum (at 9V DC supply), so the current at 8 ohm speaker will be 0.25A, and the maximum power would be the 2V*0.25A=0.5Watts. [Source: Bill Bowden's Circuits Collection]

Simple 10 Watt Audio Power Amplifier Based On Transistors

This is a simple design circuit for audio power amplifier. In this design circuit is use discrete component available on many electronic part stores. This circuit is based on transistor for operation the circuit. This is the figure of the circuit.


The transistor that is using in the circuit is 2N3055 and MJE2055. The MJE2055 is an NPN transistor, but the configuration with TIP42A make the whole combination of TIP42A and MJE2055 are seen as PNP transistor. This audio amplifier circuit is suitable for moderate power application in home sound system.

Simple Sun Up Alarm Circuit

This circuit can be used to provide a audible alarm for when the sun comes up or it can be used in a dark area and detect when a light comes on. It can also be used to detect a light beam, headlights etc. This circuit is a simple design circuit for alarm. This is the figure of the circuit.


The circuit works as follows. The phototransistor is very sensitive to light. (Any phototransistor will work fine) The sun shining on this device will provide a high to one of the NAND gates. This will cause another NAND gate to oscillate which will drive another gate to output a 100 Hz tone. The transistor provides drive for the speaker.

Simple LED Photo Sensor Circuit

This is a simple design for sensor circuit. This circuit is using LED for sensor a light. But, for control operation and amplifier the output is using 1458 IC. This is an op-amp. This is the figure of the circuit.


A circuit that is takes advantage of the photo-voltaic voltage of an ordinary LED. The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain of about 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100K potentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts in bright light.

Simple AC Line Current Detector Circuit

This is a simple design for current detector circuit. This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines through an inductive pickup (L1) made with a 1 inch diameter U-bolt wound with 800 turns of #30 - #35 magnet wire. The pickup could be made from other iron type rings or transformer cores that allows enough space to pass one of the AC lines through the center. This output will amplifier using op-amp. This is the figure of the circuit.


The magnetic pickup (U-bolt) produces about 4 mV peaks for a AC line current of 250 mA, or AC load of around 30 watts. The signal from the pickup is raised about 200 times at the output of the op-amp pin 1 which is then peak detected by the capacitor and diode connected to pin 1. The second op-amp is used as a comparator which detects a voltage rise greater than the diode drop. The output 1458 op-amp will only swing within a couple volts of ground so a voltage divider (1K/470) is used to reduce the no-signal voltage to about 0.7 volts. An additional diode is added in series with the transistor base to ensure it turns off when the op-amp voltage is 2 volts.

Simple 3 Transistor Audio Amplifier Circuit for 80 mW

This circuit is for amplifier 80 mW uses positive feedback to get a little more amplitude to the speaker. This is a simple design for a small amplifier. This has a disadvantage in that as the output moves positive, the drop across the 470 ohm resistor decreases which reduces the base current to the top NPN transistor. Thus the output cannot move all the way to the + supply because there wouldn't be any voltage across the 470 resistor and no base current to the NPN transistor.


In this circuit, the 1K load resistor is tied to the speaker so that as the output moves negative, the voltage on the 1K resistor is reduced, which aids in turning off the top NPN transistor. When the output moves positive, the charge on the 470uF capacitor aids in turning on the top NPN transistor. The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion. The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume.

The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load. The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.

Simple 3 Transistor Audio Amp Circuit for 50 mW

This is a design for audio amplifier. This circuit is a little audio amplifier similar to what you might find in a small transistor radio. The input stage is biased so that the supply voltage is divided equally across the two complimentary output transistors which are slightly biased in conduction by the diodes between the bases. This is the figure of the circuit.


A 3.3 ohm resistor is used in series with the emitters of the output transistors to stabilize the bias current so it does not change much with temperature or with different transistors and diodes. As the bias current increases, the voltage between the emitter and base decreases, thus reducing the conduction. Input impedance is about 500 ohms and voltage gain is about 5 with an 8 ohm speaker attached. The voltage swing on the speaker is about 2 volts without distorting and power output is in the 50 milliwatt range. A higher supply voltage and the addition of heat sinks to the output transistors would provide more power. Circuit draws about 30 milliamps from a 9 volt supply.