Various Circuits

Oscilloscope AC Probe

The probe described here was designed to be used over the range of several tens of Hz to a couple MHz with currents from a few tens of milliamps to about 10 amps peak-to-peak. Formulae are give which will allow you to create a design suitable for your own applications. To use this probe, a wire is connected in series with the load that you want to see the current through, and that wire is passed through the hole in the probe. An oscilloscope is used to view and measure the current waveform, which is calibrated in terms of volts output per amp of input current. ..

Schottky Field Strength Meter

When an antenna is attached, (or even if not, in the presence of a strong RF field) the field strength meter has a moving coil meter to indicate relative field strength. The box holds a Schottky diode detector, a DC amplifier, and a meter display. If the parts used in construction are the same values shown in the schematic, the meter shows full scale ("+3db" on the surplus store VU meter movement) when the output of the Schottky diode detector is between 1.6 millivolts and 160 millivolts, depending up the setting of the gain control. Since opamps are used as amplifiers, the range of the gain control can be modified easily. The actual sensitivity in terms of field strength is primarily a function of the antenna system, and that is well beyond the scope of this web page...

Simple Digital Potentiometer with DS1869

The experimenter's pot is a solid state potentiometer using Dallas semiconductor?s DS1869 and National?s LM78L05 two electrolytic capacitor two small push button switch and two optional Molex connector. this project is very useful especially in finding the right value of resistor in your experiments, you can attach this circuit to your next project that uses a pot or trimmer cause there are three types of DS1869 these are: DS1869-10 which is 10K. DS1869-50 a 50K version. DS1869-100 the 100K version..

LED Crystal Tester

Transistor Q1, a 2N3563, and its associated components form an oscillator circuit that will oscillate if, and only if, a good crystal is connected to the test clips. The output from the oscillator is then rectified by the 1N4148 signal diode and filtered by C3, a 100pF capacitor. The positive voltage developed across the capacitor is applied to the base of Q2, another 2N3563, causing it to conduct. When that happens, current flows through Led1, causing it to glow. Since only a good crystal will oscillate, a glowing LED indicates that the crystal is indeed OK. You can use the NTE123AP, PN100, or the 2N3904, for the transistors, however, the circuit works better with the 2N3563's for crystals in the higher MHz. The NTE108 is a direct replacement for the 2N3563. R3, the 330 ohm resistor for the Led, can be lowered to 220/270 ohm if your application is for crystals in the high MHz, which makes the led glow dimly in some instances. ..

Simple Transistor Tester

This is basically a high gain amplifier with feedback that causes the LED to flash at a rate determined by the 10u and 330k resistor. Remove one of the transistors and insert the unknown transistor. When it is NPN with the pins as shown in the photo, the LED will flash. To turn the unit off, remove one of the transistors...

Inductance adapter for frequency meter

The following circuit enables me to measure inductance of the inductor labeled LX which is the inductance to be measured. The o/p of the circuit is a TTL square wave whose frequency relates to the inductance being measured. The heart of the circuit is the buffer colpitts oscillator(the first stage) which resonates with the unknown inductance to give a Sine wave of a particular frequency. The frequency of the sine wave is a function of the unknown inductance and the four 1000pF capacitors. The output sine wave is amplified by the second transistor and is then rectified by the capacitor and diode combination that Follows. The rectified sine wave now having only positive excursions is buffered by the third transistor and is then fed to the 74ls393 Counter ic which is configured as a divide by 256 counter. ..

MOSFET Tester circuit

This is a variation on the astable multivibrator. Circuit was recently developed to test for N-mosfets(the power kind e.g irf830). I don?t claim circuit can test all bad mosfets or all fault mosfet conditions. If mosfet is working it will operate in the astable multivibrator circuit causing the Led to flash. A bad mosfet will not cause the LED to flash. Below is the circuit diagram, the other half of the astable utilizes an npn transistor to make the circuit cheap. Almost any npn transistor will work in this circuit. The npn transistor to the right is used as a common emitter buffer that also drives the led as it receives pulses from the mosfet drain...

This circuit uses a 74HCT74, 74HCT00, and a LM311 to form a frequency comparator. The two pulse trains are fed to two D-type flip-flops (triggered by the leading edges). The flip-flops' outputs are compared in a NAND gate. If the output of the NAND gate becomes "0", the flip-flops are reset. If the frequency of F1 is higher than the frequency of F2, the signal "Q2" consists of needles and "Q1" is a pulse train. ..

A differential amp used by the SIM, and it is a completely conventional circuit. This is adjustable by using VR1 to null out any normal variations that the amp might show when there is no distortion or other nastiness in evidence. The second stage is a high gain amplifier, and will amplify the residual signal to a level suitable for the rectifier and indicator circuits...

The full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure depends on the function switch position as follows: (a) 5V dc on position 1 (b) 5V ac rms in position 2 (c) 5V peak ac in position 3 (d) 5V ac peak-to-peak in position 4 The circuit is basically a voltage-to-current converter. The design procedure is as follows: Calculate RI according to the application from one of the following equations: (a) dc voltmeter: RIA = full-scale EDC/IFS (b) rms ac voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS (c) Peak reading voltmeter (sine wave only): RIC = 0.636 full-scale EPK/IFS (d) Peak-to-peak ac voltmeter (sine wave only): RID = 0.318 full-scale EPK-TO-PK / IFS..

The circuit in Figure 1 tests these characteristics. It charges the capacitor under test through 100 k for approximately 1 msec and then discharges it through 10 for approximately 40 nsec. The cycle then repeats. The circuit uses a double-sided copper-clad pc board. All the components except the 10 resistor connect to one side of the board, so they can benefit from shielding by the cast-aluminum enclosure (Figure 2)...

The 2x trigger is a little involved, but here is a circuit that will do divide by two preserving pulse width. You can use a 74LS00 for the NAND gates and a 74LS74 for the flip-flop. I think both are available at Radio Shack. Enjoy!..

The circuit accepts an input clock of any duty cycle and generates any desired duty cycle at the output. You need to add only one flip-flop to the earlier design to generate an arbitrary-duty-cycle output. You can use the circuit to correct a non-50% input to a 50% output or to create a non-50% output from any arbitrary input duty cycle...

Using a standard PLL circuit, such as the CMOS 4046B with some passive components, is a well-known way to design a clock multiplier. Unfortunately, using a PLL in a digital circuit has two disadvantages: It takes a long time for the circuit to reach a stable output frequency, and the frequency-drift compensation has a complex design...