Meters

Dual-trace, triggered oscilloscope

This site contains public-domain schematics for a dual-trace, triggered oscilloscope, and some theory. Click on the topic in the table below to see the schematic for that block. I designed and built this oscilloscope in 1986 using the chassis of a non-working vacuum tube oscilloscope, keeping only the original high voltage vacuum tube supply and CRT. This construction project is only for the experienced designer, since you will want to change and adapt parts of the circuit to accommodate new semiconductors and devices. I recommend that you do not build an oscilloscope from these schematics, but rather, understand the design principles and come up with a more modern design. The bandwidth can easily be extended beyond 100 MHz by using inexpensive high speed drivers and op-amps. Delayed sweep, and other features, are easy to add...

Scope Probe

Working on live telephone circuits can be a problem. You just can't connect a normal scope probe to the circuit while it is tied to the phone line. Neather side of a phone line is ground. When a phone or other device is 'offhook', one side floats at about -22 volts and the other side about -30 volts. This probe allows connecting to such 'floating' circuits to a normal scope input. As seen in the circuit, it is based on using an optocoupler. The one shown in the diagram is a dual unit, but I had a pile of them here, so thats what I used. Because this device responds to both polarities, an external diode was added to prevent negative voltages from producting an output...

Simple analogue Tachometer

Basically, this is a frequency-current converter. It is used as frequency meter. Calibrating it can be used as tachometer. Please be aware this is an experimental project, I didn't use it connected on any vehicle but it should work fine after calibration. It can be connected directly to the voltage from the car. The input signal can be connected directly to any source of frequency. I believe it will work fine if it is connected directly to the spark plug line. Be sure not to use germanium diodes for this one. Any silicon diode should work fine. The diodes at the input reduces the voltage to a safe level ( 0.7 Volts). DO NOT CONNECT the circuit without doing the calibration first. ..

Sound amplitude meter

I made this project as a test to improve a technic to read analog values without analog-to-digital converter. (ADC) I ended with this "sound meter". It may not work perfectly, it needs some improvement but works anyway. It have the feature of "auto-calibration", so it detects the signal, amplify it and measures it. Because the "auto-calibration" feature, it doesn't need an amplifier or any aditional circuit to drive the signal from the electret microphone. The capacitor can be a 103k or a 220k. A poliester capacitor works better. The code is compatible con 16F84A or 16F628 so I can't define exactly what value should have. Anyway, If you find this projects interesting, you may be able to experiment with the value of the capacitor. ..

PIC16F628 as digital Tachometer

Pin 3 defines the Display type: Common Cathode or Common Anode. Connect this pin to GROUND if you are using common cathode 7-segment led displays. To use common anode displays, connect it to VCC (positive) Pin 4 is the signal input, you can use optical sensors (Fototransistors, fotoresistors, etc), magnetic sensors (Hall effect, reed switch, etc) or using a switch. Unfortunately, I have no idea how to connect this circuit to a vehicle. If you want to share details about how to attach this circuit to an automovile, I will post this information...

Capacitance Meter ( 68HC711D3FN )

The proposed capacitance is a very powerful assembly to measure in 7 ranges capacitors of 1 pF to 999 uF, avecune excellent precision of about 10 -3 . The range change is fully automatic. The display shows 3 digits with decimal point. Three LEDs indicate the unit: pF nF uF or. Power is normally with a battery or 9V battery with automatic shutoff after 2.5 minutes. A jack for external power supply is also planned...

Audible Logic Probe with LM339

When testing circuits with a logic probe, it is sometimes difficult to watch the LEDS on the probe to determine the logic state. With this probe the logic states are audible. This probe is designed for TTL circuits only but could be modified for CMOS. The way it works is as follows. The 5 volt power source will be the circuit under test. Clip the ground input of the probe to the ground of the circuit being tested. The other input lead is used to probe the different chips of the circuit being tested. Any input greater then 2 volts will be high and output a high tone through the speaker. Any input less then .8 volts will be low and produce a low tone through the speaker...

Simple Soil Moisture Meter with 741

Stick the metal probes into a freshly watered plant and adjust R5 for a mid-scale meter deflection. The meter will monitor the soil wetness and the meter will indicate whether it is to moist or to dry. This circuit uses a dual power supply which could be created by two 9 volt batteries...

Transistor Checker with 555

This simple circuit has helped me out on many occasions. It is able to check transistors, in the circuit, down to 40 ohms across the collector-base or base-emitter junctions. It can also check the output power transistors on amplifier circuits. Circuit operation is as follows. The 555 timer ( IC1 ) is set up as a 12hz multi vibrator. The output on pin 3 drives the 4027 flip-flop ( IC2). This flip-flop divides the input frequency by two and delivers complementary voltage outputs to pin 15 and 14. The outputs are connected to LED1 and LED2 through the current limiting resistor R3. The LED's are arranged so that when the polarity across the circuit is one way only one LED will light and when the polarity reverses the other LED will light, therefore when no transistor is connected to the tester the LED's will alternately flash...

Simple RF field meter with LM358

This project will explain the function of a simple RF field meter. The unit will be in great help to tune transmitters for best performances. At the bottom left corner you will see a voltage divider. This divider is to produce a virtual ground of 4.5VDC. Above you will find the dipole antenna. The dipole antenna will pick up some radiated energy and the diode will rectify the RF signal to a DC voltage at VRF. This voltage is still quit low and needs to be amplified before it can control the panel meter. The signal then enter the OP which amplifies the voltage to suitable level set by the "Gain" potentiometers". The second OP acts as a voltage follower and set the offset (zero) for the panel meter. The panel meter is connected to the board via two wires (5meter long). To prevent any RF signal to be induced in this long wire I have added 2 ferrite block which will act as high impedance units. You can use any ferrite block or large inductor (10uH)...

LC inductance / capacitance meter

LC-meter will work equally good with or without your knowledge about the theory. The LC meter is actually an LC oscillator based around the familiar comparator circuit LM 311. I have not changed the LC oscillator much from others projects, since this is an excellent oscillator. The main part of this project is a LCD, PIC16F870 and an oscillator LM311. The LCD is a standard 2 line 16 chars display connected in 4-wire mode to the PIC. At the bottom you will find a 4.000 MHz crystal to run the PIC. The heart of this construction is the LC oscillator based around IC2. The frequency pops out at pin 7 and then enters the PIC at pin 11. Two relays have been added to make the measurement more automatic, and they will also prevent stray inductance/capacitance...

This project will explain how I build my new wattmeter. This watt meter will be able to measure power from 300nW to 30W @ (0-500MHz). This wattmeter is based up on a dummy load of 50 ohm which can handle 50W. The measurement will be displayed in Watt on a 2x16 Char display. The unit is very accurate, fast and easy to build. This watt meter project is very similar to my last wattmeter project. The main reason I made a new project is becasue I needed a unit which could handle higher power than 1W. I found a 50 ohm dummy load which could take 50W of power. Of course I could use attenuates for my 1W meter, but I prefered to build a new unit. The new thing with this project is that it will only display the power in Watt on the LCD display. This means that I will not need so many EEPROM to store display data (more about this later)...

RF Digital Wattmeter

The power is measured by the circuit AD8307 over a 50 ohm dummy load. An A/D converter of 12 bit convert the analog output from the AD8307 to a digital number. Since the AD is a12 bit A/D you will have 4096 combinations. The output values will be from 0 to 4095. To calculate the power into dBm you need some mathematical calculations. I have solved this in a more simpel way, since it is a bit difficult to implement mathematic into the PIC. I use a look up table. It is memory bank containing all the display text. I have pre-calculated all the display text for each A/D value. The output values from the AD (0 to 4095) points to a direction in the memory where the actual display text value is stored. The text will be fetched and presented on the display. I used a new HP 100MHz oscilloscope and I measured the amplitude over the 50 ohm dummy load. From the amplitude I then calculated the power. For month I was very dissapointed not to be able to generate more than 100mW output power. One day I burned up the dummy load totally. How is this possible with 100mW?..

Impedance meter for Headphones

This article has two parts. Part I describes a way to determine the effective average impedance of a pair of mono headphones or a speaker using a device called a FILVORA. This is the optimum resistance with which to drive the headphones or speaker to obtain the maximum possible volume. Part II shows how to compare the sensitivity of two speakers or two mono headphones using a dual FILVORA, even if the headphones differ greatly in impedance. Both parts also discuss how the use a FILVORA to measure the impedance and sensitivity of hi-fi stereo headphones...

Mini PC Digital Oscilloscope

This is a portable oscilloscope adapter that it can be held in breast pocket. Its operation is only sampling and sending to host PC. The most of functions of the oscilloscope are processed by host PC. Therefore, oscilloscope adapter can be simplified. This oscilloscope adapter using a TMP47P242VN 4bit microcontroller for acquisition control. But it is difficult to obtain and to programm personaly, so that I recommend to replace the microcontroller with any other microcontroller, such as PIC and AVR, when you want to follow this project. The details of the acquisition controller is as follows. Communication format between host PC and oscilloscope adapter is N81 (no parity, 8 data bits, one stop bit). Data transfer rate between oscilloscope adapter and host PC is 115.2kbps. ..