Friday, October 14, 2016

Precision Op-Amp Current Source

In this circuit we tackle the error indicated in the earlier Current Source. The LM336-2.5V eliminates the tiny error of the regulated supply and resistors. Thereby increasing Precision to a higher degree.
Precision Op-Amp Current Source
The opamp mirrors the stable 2.5V across P3 + R13. With P3 Bourns 10 Turn Trimpot you can trim the current for calibration. Q1 BC557B having a Beta - hfe of 200 is used. But a higher gain or a FET here may reduce error further, that may be needed if you are going for 16 Bit or more resolution. Then even opamp needs to change.

Mini RTD Pt-100 Three Wire Transmitter

Suppose you build it with the best Opamp, FET etc., but place it close to a Warm transformer, Regulator chip or even a Cooling Fan, you will see the lower digits of a 5-1/2 DMM spinning fast.

Tuesday, October 11, 2016

Resistance Measurement Analog LED Meter

This is a LED Analog Meter, This can be used as a Resistance Meter and Low Impedance Voltmeter for Battery Levels.

Resistance measurement Current Source

To measure battery voltage, the R5-R12-R17 etc. part of the Reference Resistor Divider Network can be modified to suit. Shown here is for 4 LEDs, Use Three LM324 for 12 or More LEDs and Cascade as shown.

Resistance Measurement Analog LED Meter

This cannot Measure Voltage levels from High Impedance Sources, will work for Battery Voltage Tests. To make it into a Continuity tester. R27 must be a short and R23 5 Ohms. The Black probe should have a Built in Resistance of 2 Ohms. If you want it to be a dedicated voltmeter, remove R3, The Probe has to be a 10X Attenuator with 10M Ohm and The Resistor Divider Steps in 100mV per Step. The R27, R23 etc. is 20K.

Simple Test Equipment

A Leakage Tester a Mains Voltage Monitor are other possibilities. Use LM3914 for a easier solution. A nice book for your Design Library - Measuring Circuits By Rudolf F. Graf

Tuesday, October 04, 2016

RS232 with Opto-Isolation

I had to once interface an high voltage circuit to PC, The uC had to communicate thru RS232--Comm port--Serial Port.

Part of the 80C31 8051 SBC

Even though i had isolation at the sensors and actuators to make doubly sure the PC also has been isolated. There are chips that are available for this purpose, The circuit above is built with discrete and passive components except for the opto 4N35. You can use MCT2E and CNY17-3 Optos too. For MCT2E some tweak may be needed as current transfer ratio is 20, for the other two CTR is 100 so above design will work.
RS232 with Opto-Isolation

The circuit derives power from PC but does not load the PC supply. Any voltage above 5V applied to the PC connectors may lead to damage of motherboard in PC. Old PCs were more vulnerable but PCs today maybe a bit rugged at the Ports. Due to internal current limits and clamping.

The VCC, VDD and Agnd are derived from PC no other power needs to be applied on PC side of opto. On uC side of opto the uC power supply lines +5 and gnd has to be used. There is no copper link between the two sides and depending on opto a 1KV isolation is possible if PCB is well designed. The PCB should show the visual isolation above and components should be laid on separate areas of PCB to prevent creepage.

The LEDs are to indicate the port activity Rx and Tx, they are not required once testing is over. The circuit can be simpler, but this worked for me and it is not tested at very-high buad rates.

The levels of RS232 are not TTL like 0-5 we have both polarities +10 and -10. The circuit has to change that to drive the Opto Leds.

RS232 software. Understanding RS232 Serial Port Communication.

Wednesday, August 17, 2016

Single Digit Voltmeter with LM311

I wanted to design a logic probe as a tutorial, but there were many good ones in the web so i have tried to design a single digit voltmeter. This circuit is a design, i am unable to test it now, later if i test it and find mistakes i will update this page. You can help me by pointing out the errors.

Single Digit Voltmeter with LM311
First bear it in mind that it is a single digit voltmeter which is 0-9 counts only on the positive side, that is it can measure +0 to +9V DC +/- 1V error. That may not be practical for the cost of the components above. It may be used as a toy logic probe. The reason for the circuit is not for usage, but to give design ideas. The methodology used is Gut Feel - Thumb Rule method.

First i explain the simple part, D1 a seven segment common cathode LED display is chosen as CD4511 is a sourcing driver. 4511 can be latched so it has been used here, it decodes binary 4 bit decade info to seven segment output. The four bits are derived from CD4029 up-down clock pulse counter. LM311 is a analog comparator with single supply capability which is the A-D interface.

To avoid resistors for each of the LEDs the LEDs are turned ON-OFF at 10KHz 50% duty cycle. The Nand Schmitt Trigger CD4093 is used as in IC4D as a 10KHz Clock which drives T1 transistor with a resistor R5. On turn on IC4D one input is high which is pin 12 pulled up to +5 and another Pin 13 is Low as C4 is in discharged condition in NAND gate both inputs high, gives a low output, the other combinations the output is high. So the output goes high, this starts charging the cap C4 which soon makes both inputs high, which in turn makes output low starting the discharge of C4. This is now evidently a endless loop, hence it is a oscillator. R*C = T .... 0.01uF * 10K = 100uS or 10KHz as F=1/T approx or better still multiply by 1.1 ?. I am not good at formulae but i manage with a calculator.

The supply and ground pins of CMOS chips have not been shown, see datasheet or earlier circuits.

IC4A is also a oscillator but slower which is good enough, it is slow so that a measurement can be made nearly every second. The IC4A slow clock is read by 4029 which produces a count-down binary nibble at Q1....Q4, This is converted to crude analog with R1...R4. The voltage generated is compared with the voltage you are measuring by LM311 which generates a Latch pulse to 4511 to freeze the reading where both voltages match.

Method of Operation :
IC2 is a Counter in Decade-Down mode and IC3 is a BCD to Seven Segment Decoder which Drives the Display D1. The Circuit is wired in such a way as to keep counting the Pulses from the Clock IC4A. The IC4A which is wired as Schmitt Nand Oscillator Clocks the Counter. Now to understand how this Counter and display works see this Interactive Tutorial Simple Digital Counter. For every pulse at Pin-15 of 4029 the Counter Counts down from 9-8-7-6.... and so on. But the Display is Latched by IC4C, So the Display is static even when counter is running. So while testing counter you can remove IC4C and keep LE Pin-5 of 4511 low to ground. For testing this Circuit you can use the manual clock with a pushbutton (single step) or a slow clock rate 1 Hz as in the Tutorial Simple Digital Counter.

The Transistor T1 and Oscillator IC4D is to chop the power to display at a fast rate, this avoids the adding of seven resistors. This is not required, but it saves power and reduces parts count. IC4B is is like a ON indicator, it is a spare gate.

The BCD value at output of 4029 Q1-Q4, four bits, a nibble, is converted to an analog mV value across R6. The resistors R1-R4 which are connected to Q1-Q4 have weighted resistor values for the BCD 1-2-4-8. By ohms law you can understand that the analog value across R6 is approximately proportional to the BCD value. This circuit is just a single digit A-D converter, not even as good as 4 bit converter. Which means approximate value of analog at R6 will do.

Now lastly LM311 is a Comparator, it compares the Analog BCD reference at Pin-3 and the Attenuated Input signal at Pin-2. Output Pin-7 goes high when Pin-3 voltage becomes less than Pin-2. This is made to a narrow latch pulse by C2-R12-IC4C. The latch pulse freezes the BCD data to display till the next latch pulse. R8-R9 attenuate the 0-9 V DC input to a 1/100 value. The zener Diode Z1 is for protection.

I guess the LM311 circuit should work off a single supply, but a dual supply may be required as voltage levels may be near zero. You must be able to see a staircase waveform or ramp across R6. Narrow Latch pulses at Pin-5 4511 on every ramp cycle.

Monday, August 15, 2016

LED Voltage Level Indicator

This circuit is derived from a Siemens Application Note 1974. This circuit uses common components of today.

The circuit is here as it is of high educational value. I have not tested it. You can 'simulate and test' or 'wire it up and try' and let me know how it worked. The Circuit is also a simple analog to digital converter. You can use optos in place of LEDs.

T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators. Now to learn more on how they work you have to study circuits at 4QD-TEC.

When input voltage is increased T1 is turned on which leads to more base current for T3 which Lights LED1. When input voltage is less T2 turns on as it gets a better base current from P3 which turns on LED2 via T4. When both LEDs are off T5 gets biased as no drop across R5 which lights the LED3 thru T5 hopefully.

LED Voltage Level Indicator

What you need to know is a small current Ib thru the base-emitter path in the direction of the emitter arrow will lead to a large Current Ic thru the emitter-collector path in direction of arrow. Ic = B * Ib where B - beta is the DC current gain, it could be 100-400

Beta is different in each transistor you buy and varies with the test conditions and even with temperature and age. The LED1 and LED2 will indicate above or below Limits set by P2 and P1. The Limit Threshold itself is set at P3 i think. LED3 will light when Hi LED and Lo LED both are off.

The applications of this circuit are FM tuning indicator, Stereo Balance Indicator (Wire T2 like T1 then we get two channel inputs) and battery level indicator.

Circuits by Application

Analog Circuits

  1. Battery Level Indicator
  2. Simple Sample and Hold
  3. Sample and Hold Standby
  4. Voltmeter Attenuator
  5. Precision Current Source
  6. Opamp Supply Virtual Ground

SCR and Triac

  1. Solid State Relay
  2. Normally Closed AC SSR
  3. AC-AC-SSR
  4. DC-DC SSR
  5. 2N2646 based Pulser
  6. Drive SCR thyristor

Mains Power

  1. Flashing Neon Lamp
  2. Dimmer power control
  3. Edison Bulb Life Extend
  4. Mains Current LED
  5. Mains Voltage LED

Digital Circuits

  1. Simple Digital Counter
  2. Running Lights
  3. Frequency Divider
  4. Crystal Oscillator
  5. Simple High speed switch
  6. Differential TTL converter

Measureall DMM

  1. Ohmmeter Measure Resistance
  2. Precision Digital Attenuator
  3. Precision Amplifier

Mixed Circuits

  1. Monostable Multivibrator
  2. Digital to Analog
  3. LM311 Oscillator
  4. PLL using 4046
  5. VCO with LM331
  6. BCD Thumbwheel to Analog
  7. V to F Converter ICL8038
555 Circuits
  1. OR gate with two 555
  2. fixed frequency duty cycle
  3. Pulse width modulation
  4. Astable Multivibrator
  5. uC Reset Generator
  6. LM555 Voltage Doubler
  7. 555 Power Oscillator
Discrete Circuits
  1. Isolated dual supply
  2. Sound to light converter
  3. Water operated relay
  4. Telephone Indicator
  5. Passive volume control
  6. RS232 Opto-Isolation
  7. Voltage Level Indicator
  8. Relay Driver
  9. Constant Current LED
  10. Voltage Doubler
  11. FET Current Source
Opamp Circuits
  1. Three Opamp Differential
  2. Two Opamp Differential
  3. Buffer Opamps
  4. Differential Op-Amp
  5. Inverting Opamp
  6. Non Inverting Opamp
  7. Digital gain control
  8. Square Triangle Oscillator
  9. Dual Polarity Output Amps
  10. Ammeter Precision Rectifier
  11. Voltage / Current 4-20 mA
  12. Current Source for RTD
Power Electronics
  1. Dual Power Supply
  2. Single Power Supply
  3. Battery Backup Supply
  4. 5V 1A Supply LM2575
  5. 5V Power Supply L296
  6. Dual Power Supply
  7. Tubelight Electronic Choke
  8. Voltage Doublers Multipliers
  9. White LED Lamp on Ni-Cd

uC and uP

  1. PC RS232 with MAX232A
  2. Battery Backup SRAM
  3. watchdog uC uP systems
Instrumentation Circuits
  1. Mains monitor LM3914
  2. Simple Mains monitor
  3. single digit voltmeter
  4. High Resistance Meter
  5. Diode Thermometer
  6. Function Generator
  7. Diode Leakage Tester
  8. Analog LED Ohm Meter
  9. Millivolt Source Current Loop
Process Control
  1. AD590 - temperature
  2. Thermocouple Amplifier
  3. Linearizing Thermocouple
  4. Thermocouple Amplifier
  5. 0-1V to 4-20 mA
  6. 1-5V to 4-20 mA
  7. InfraRed - Optical Switch
  8. InfraRed Detector