Simple RF Probe Circuit

This is a design circuit for simple RF probe can be used to determine if the oscillator is working. It will not tell you the frequency (you need a good RF frequency counter for that), but at least you will know if it is oscillating or not. This is the figure of the circuit.


This RF probe is useful for any low level RF work, and simply connects to your multi meter. The voltage shown will not be accurate, since this is a rectifier probe, but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should. Connect it up to your multi meter, which can be used on any suitable voltage or current range, or you can use a micro-ammeter if you happen to have one lying about. For use with lower frequencies (a few MHz only), C1 can be increased in value, but I would not go above 100 pF. High voltage circuits must be treated with the utmost respect, and a 500V cap is recommended for C1 unless you know that you will never use it on a valve transmitter or receiver circuit.

Simple Oxygen Sensor Simulator

This is a simple design for oxygen sensor simulator. The cigarette lighter plug used for power source. The adjustment knob is at the left, and the switch is on the right. The red indicator LED is in the middle. This circuit is control by 555 timer IC. This is the figure of the circuit.


Closing the switch engages the simulator. Turning the knob clockwise simulates a lean condition, turns the LED off, and the car should start running rich to compensate. The big "V" is a digital voltmeter (not shown in the pictures). Using a smaller value for C1, perhaps 4.7 uF, will make the circuit oscillate faster and might be more like a real oxygen sensor (a new sensor switches more often than an old one). The adapter cable. Note the connector recycled from an old oxygen sensor. Hard to see under the black tape: 100K resistor.

Simple Magnetic Reed Switch Alarm Circuit

This is one design secure circuit that is made to be an alarm, both for home and handbags. If it is installed for home it will placed on door or windows, and if it is installed for bags or handbags it will placed in the bag. This circuit consists of a small magnet and a reed switch. This is a simple design circuit. This is the figure of the circuit.


If the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound. So this circuit suitable for use as an anti-theft alarm. This circuit uses a complementary transistor-pair which is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply enable a very compact construction. The loudspeaker’s dimensions are limited only by the box that is contains it. An on-off switch is unnecessary because the stand-by current drawing is less than 20µA. If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. Do not supply this circuit with voltages exceeding 4.5V: it will not work and Q2 could be damaged.

Simple Electrostatic Charge Detector Circuit

This is one of design for detect electrostatic charge. This circuit is called as electroscope is a device that is used to detect electrostatic charges. This is a simplest form of the detector. This is the figure of the circuit.


To reduce AC noise this circuit uses C1, but it will lower the sensitivity a bit. As a voltage divider we use the MPF102 and R1. The divider’s output will be about 4.5V diving a half-scale reading on M1, a 200-uA meter, when the FET’s gate is earth grounded. A negatively charged object such as a plastic comb will give a negative meter deflection. A positively charged object such as cotton-rubbed glass will give a positive deflection from half scale. The whole circuit should be in a metal enclosure and short piece of bare wire makes a fine charge collector.

Simple AM-FM-SW Active Antenna

This is a design circuit for active antenna that can be used for AM, FM, and shortwave (SW). This is a simple design circuit. This is the figure of the circuit.


On the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna. This circuit is designed to be used on receivers that use un-tuned wire antennas, such as inexpensive units and car radios. L1 can be selected for the application. A 470uH coil works on lower frequencies (AM). For the short wave, try a 20uH coil. The unit can be powered by a 9 volt battery. If a power supply is used, bypass the power supply with a .04uF capacitor to prevent noise pickup. The antenna used on this circuit is a standard 18" telescoping type. Output is taken from jack J1 and run to the input on the receiver.

Simple TL Light Circuit Using Battery

This is circuit that using for TL light using battery. This circuit is based on the transistor. This is the figure of the circuit.


The 2N3055 transistor circuit core and several components that work as a series of oscillator with feedback through L1. Oscillator frequency operation is influenced by the value of R, C and L. When the output from oscillating collector, will show a high potential difference at the end of L3.

Simple Puff to Off Circuit

This is a simple design circuit which is the LED burning can be activated only by a gust OFF. This circuit is built using low-cost components. This is the figure of the circuit.


A condenser microphone (M1) is used to feel your breath. When the S1 button is pressed, transistors Q2 and Q3 as a couple wires attached will be activated and drives LED shines. LED remains in this condition. When you puff on a condenser microphone, the sound pressure converted into a voltage signal at the output. This voltage signal is amplified by the transistor Q1.Since collector of Q1 coupled to the emitter of the pair attached, the couple will stop doing as there ever was a signal from a condenser microphone for panting and the LED will go OFF. Push button switch S1 should be pressed again to turn the LED ON.

Simple Digital Volume Control Circuit

This is volume control that cab be used for replacing your manual volume control in a stereo amplifier. This circuit is types for digital controller. This circuit is control by DS1669. Here’s the figure of the circuit.


In this circuit, push-to-on switch S1 controls the forward (volume increase) operation of both channels while a similar switch S2 controls reverse (volume decrease) operation of both channels. This circuit has low-cost alternative to mechanical control. Wide differential input voltage range between 4.5 and 8 volts. The circuit is extremely simple and compact requiring very few external components. The power supply can vary from 4.5V to 8V. Applications include volume, tone, contrast, brightness, and dimmer control.

Simple AM/FM/SW Active Antenna Circuit

This circuit is design for an active antenna that can be used for AM, FM, and shortwave (SW). This circuit is work with based on MOSFET. Here’s the project circuit of the antenna.


On the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna. This circuit is designed to be used on receivers that use un-tuned wire antennas, such as inexpensive units and car radios. L1 can be selected for the application. A 470uH coil works on lower frequencies (AM ). For shortwave, try a 20uH coil. The unit can be powered by a 9 volt battery. If a power supply is used, bypass the power supply with a .04uF capacitor to prevent noise pickup. The antenna used on this circuit is a standard 18" telescoping type. Output is taken from jack J1 and run to the input on the receiver.
Source: "Popular Electronics" Magazine, Jul,89 issue. (C)Copyright Gernsback Publications,Inc., 1989 - Please read their disclaimers before implementing this circuit.

Simple Voltage Comparator Circuit

This circuit is design for indicates when the input voltage differs from two defined limits, V1 and V2. This is a simple design circuit. This circuit is work with based on op amp using the CA3140 MOSFET. Here’s the schematic circuit diagram.


The supply voltage, Vcc must be higher than the highest input voltage by at least 2 volts. One application here is to monitor a 12V car battery. V1 can be set to 14V and V2 to 11V thus giving an indication of more than the cost or weak batteries. They are used to advantage because they have less output offset voltage and can switch to 0volts close. If any other use op-amps like the LF351 or CA741, it will need to have an offset null control. This is just a 10k preset reached between pins 1 and 5, the wiper connected to the negative supply op-amps or 4 pins. With this circuit the op-amp will turn on the LED if the input voltage out of limits, the two 1N4148 diodes to form an “AND”-gate at the output. Input voltage to be monitored are fed through a series of 10k resistors on the input of both op-amps. If the input voltage is greater than the limit set by V1 it will CA3140 output swing to almost full supply voltage and LED lights. Similarly, if the input voltage is less than the limit set by V2 the op-amp will swing to the Vcc and the LED light.

Simple Electromagnetic Sensor

This is a design circuit for sensing the electromagnetic field. This sensing circuit is controlled by the op amp 741 IC. This is a simplest form. Here’s the figure of the circuit diagram.


The circuit diagram is working 1mH inductor current sensing is used for the electric field. The small electric field will cause the voltage on the inductor and the induced voltage sensor is amplified by op amp. Headphones are connected to the output of op amp audio will indicate of the electrical field. For example, the electric field around the main transformer can be heard as 50 Hz hum. R4 POT that can be used to adjust the gain of the amplifier. By maintaining the sensor inductor close to the phone, you can even hear the phone conversation. All electrolytic capacitors should be rated at least 15V. The switch S1 can be a slide type ON / OFF switch. POT R4 it can be used to adjust the gain. Better to have a radial type inductor for L1.

Simple Divider Down Current Circuit

This is a design circuit divides down the high common mode current shunt voltage, theoretically permitting the 5V powered amplifier to extract the current measurement over a 20V to 90V APD bias range. This is the figure of the circuit.


This circuit is based on LT1789 single op amp. This circuit is arrangement introduces prohibitive errors, primarily because the desired signal is also divided down. The current measurement information is buried in the divider resistor’s tolerance, even with 0.01% components. The desired 1% accuracy over a 100mA to 1nA range cannot be achieved. Finally, although the amplifier operates from a single 5V supply, it cannot swing all the way to zero. [Circuit’s source: Linear Technology Notes].

Simple Light Activated Relay Circuit

This is a simple design circuit for activated lighting relay with the addition of a photo resistor to trigger the flip flop instead of a push button. This circuit is a control by transistor. This is the figure of the circuit.


The bias resistor in series with photo resistor was chosen so that sufficient voltage is present at the base of the 2N3904 to supply current to the circuit in ambient lighting conditions. The circuit should toggle when the photo resistor is hit by a flashlight beam or other fast changing light source. Slow changes in light intensity will have no effect unless the light gets too bright to maintain sufficient bias for the 2N3904.

Simple Flashing Neon Circuit

This is design circuit that is used for one, two or three neon indicator bulbs can be made to flash in sequence at rates determined by the R and C values. This is the figure of the circuit.


In the single stage circuit, using one lamp, the capacitor charges through the resistor until the ionization potential of the neon is reached (about 70 volts) and then discharges quickly through the lamp until the voltage falls below what is needed to sustain the lamp which is approximately 45 volts. The cycle then repeats at a rate of about 3 Hz for values shown. Smaller R or C values increase frequency, larger values decrease frequency. All capacitors should be the non-polarized variety with a 100 volt or more rating. For more than 3 stages, the lamps may need to be matched for similar turn-on voltages.

Simple Basic Solid State Relays

This is the design for basic relays. This is a simplest form circuit. This is a solid state relays that could be used to replace mechanical types in many model railroad circuits. This is the figure of the circuit.


This circuit is design to so that with a controlling current of 10 Milliamps flowing through the LED the voltage drop across the output will be less than 0.3 volts at the circuits rated current. The control and load supply voltages for the examples is 12 Volts, other voltages can be used with a corresponding adjustment to resistors in the circuit. For example if the voltages are doubled then the value of the 1K and/or 470 Ohm resistors should also be doubled in order to keep the currents through the LED and base of Q1 at the same level. If the 2N3906 transistor is replaced with Darlington type, larger currents can be handled, 1 Amp and more, but the voltage drop across the relay increases and the cost of each unit rises. The diode D1, at the output of FIGURE 2 is a 1N4001. This is an anti-ringing diode and is only really needed if there is an inductive load such as a solenoid being controlled.