This was a small circuit made for driving an Impact counter. The heart being ICL8038. It must have been a Motor driving a Conveyor, the motor has a feedback attachment called Tachogenerator.

Voltage to Frequency Converter ICL8038

Only part of the circuit is shown here. See the image of product here Tacho Counter. The configuration is derived from the Application Notes of Intersil. The Voltage from Tachogenerator is Measured on a DPM-DVM and also fed to this circuit after attenuation and filtering. The square pulses of 8038 is used to derive a Logic pulse train for a CD4040. The CD4040 works of 0 and 12V. The above circuit is on +12 and -12. That is the reason R10-R15-Q1 are used. The pulses are 0-12V pulses. The Zero is Virtual like the Virtual Ground in low current power supplies.

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.

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.

Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm’s Law the Voltage across Resistor is proportional to Resistance value.

Precision Current Source for Resistance Measurement

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded – Interactive Java Resistors Tutorial.

Measurement of resistor values in circuit configurations are required to be made often, as these might have changed in value due to various tolerance ranges, and hence could be the cause of faults. Likewise the resistance of components used in a circuit, may need to be known. In such cases the measurement of resistance is a must.

Simple Resistance Measurement

The circuit used for measurement of voltage can be modified to measure the value of the unknown resistance. The principle followed is the measurement of voltage drop across the resistance when a constant current flows through it. In the voltage measuring circuit, the unknown resistance is connected to the same input terminals and the switch SR is operated. Then a constant d.c. current from the collector of transistor T I is passed through resistor R16 to the unknown resistance which is grounded. The voltage drop across the unknown resistor is proportional to the value of the resistance as current is maintained constant. This d.c. voltage drop is measured after proper calibration.

For the constant current source a high gain, low leakage, pnp silicon transistor (T1) is required. The range selector switch Rs, which connects the positive voltage to the constant current source enables measurement of resistances in 5 decades i.e. 200 ohms, 2 kilo-ohms, 20 kilo-ohms, 200 kilo-ohms and 2 mega-ohms.

According to the range of resistance being measured the switch Rs also selects the decimal point of the displays in the DPM. A resistor R limits the current to the decimal point of the LED displays. Transistor T I is biased by resistor R17 and variable present VR5. As this preset sets the value of current in transistor T1, it has to be adjusted for calibrating the resistance range. Once the calibration is over, the resistance value is directly read on the DPM.

(This is scanned-ocr from my earlier file, some mistakes corrected – delabs)

Studying current measurement is a prerequisite for many of the measuring techniques. The current parameter mainly specifies the power consumption in a circuit, given the value of resistance. It is found convenient to measure current rather than voltage for knowing power output and determining efficiency. It may be required to measure leakages in circuits at certain times. Hence the measurement of current constitutes a priority.

Ammeter and Precision Rectifier

Measurement of DC Current –

The circuit diagram for the measurement of current (d.c. and a.c. modes) is shown aside. For measurement of current switch SI is operated. The switch S-ad is kept in d.c. mode. This enables the current to pass through a shunt circuit consisting of resistors R26, R27, R28, R29 and R 30. The current ranges are provided in 5 decades i.e. 200 micro-amps, 2 milli-amps, 20 milli-amps, 200 milli-amps and 2 amps. An additional current range that can be read upto 20 Amps is also provided. However, for measuring this high current the green terminal provided on the meter should be used. When a current to be measured is fed to the input terminals of the instrument appropriately, a voltage proportional to the current through the shunt resistor is fed into the DPM which measures the d.c. voltage which in turn indicates the d.c. current being fed.

Measurement of AC Current –

In case of a.c. measurement, the switch S-ad is kept in a.c. mode. The a.c. current path is similar to the d.c. current path in the shunt resistor. However the voltage tapped across the shunt resistor is fed into IC2 which is a buffer. The output of IC2 is fed to IC3 through capacitors C10 and C11. This IC is an operational amplifier acting as a precision rectifier. The output of IC3 is fed to the input of the DPM for measuring the a.c. current being fed to the input terminals. It can be seen that the current measurement is similar to the voltage measurement except that the attenuator chain is replaced by the shunt resistor circuit.

(This is scanned-ocr from my earlier file, some mistakes corrected – delabs)

Proportional Temperature Controller

This is a Proportional Controller where the setpoint is derived from a Thumbwheel switch.

Proportional Temperature Controller

The conversion of Thumb-wheel Digital Data to Analog mV is similar to R-2R Weighted Resistor Network. In this case it is a 1-2-4-8 Binary Weighted Resistor Network. It has no Digital Components.

You can see an example circuit below for digit weights you just use like 10K-100K-1M etc. There is a problem of procuring 8M Resistors, so use series parallel combinations, avoid open presets. Trimpots can be used but then it raises the BOM cost.

1-2-4-8 Kilo Ohms may load opamp for high output levels. 1-2-4-8 Mega Ohms may be ok in the lowest digit. Greater than 10M designs are possible only in lab, not in commercial or industrial domains.

Binary Weighted Resistor Network

Make such R networks, solder array on thumbwheel, in some thumbwheels remove diodes or other connections. Club all of them, thumbwheels. One opamp will do. Use -2.5 V for positive mV output. The resistors should be close to 0.3% at least.

This Binary Resistor Network can also work with Digital CMOS Chips like CD4029. Use these chips on a separate supply, which is just a LM336 – 5V device. A digital thumb-wheel also can be used.

In this controller you can see a sensor open indication. When the sensor breaks, the temperature controller may continue to turn on the actuators or heaters, It may even Oscillate. So when a high impedance is detected in the sensor input terminals, the output relay is shut off and a LED is turned on to simplify operator’s diagnosis.

Mount the controller a distance away from heaters, ac-drives and vibrating parts. Avoid direct sunlight on controller, fix controller in a sealed control panel. Earth the point where the thermocouple senses heat. Some heaters leak. The machine has to be earthed.

The proximity switch can work for a wide range of power, from 8v to 18v DC, D3 protects reverse power supply connections, and U1 regulates the supply to +5v , -5v is derived from U2 555 oscillator which serves dual purpose.

Circuit Operation

Part of – InfraRed LED Flasher for Optical Switch

The infra red diode D2 detector gets the reflected light from LED and some ambient light, The forward voltage drop of D2 will vary with the amount of light falling on it. Ambient light causes a DC component and the pulsing light from D1 causes an AC component.

Proximity Switch - Driver Supply

The capacitor C6 blocks DC and only transfers AC pulses if any to opamp amplifier U3A whose gain is set by R18, D9 rectifies the pulses to DC and this DC voltage is used by opamp comparator U3B which drives Q1 through Q2 for an open collector output for relays. LED D7 turns on when relay Output is high.

R14 and R13 can be replaced with potentiometer for threshold adjustment if required.

Connect 12v DC supply to +V and GND Ports, Connect a relay coil Between OUT and GND Ports, you can use the relay contacts as you require to turn on a lamp, heater, fan or motor.

If all connections are ok and ICs are working you should see a +5V at U3 pin8 VCC and around -4 to -5 at U3 pin4 VDD.

See also – Mixed and Interface Circuits

Optical Proximity Switch - Detector


The Optic switch can be used for both reflecting detection (retro reflective) or obstacle detection. The mechanical construction will decide this, for obstacle detection the diodes D1 and D2 could be put in two different tubes and can be kept far apart 2mts+ and both should be exactly opposite each other, any obstacle like a passing person will be detected.

To make a retro reflective proximity switch this circuit is ideal, it can be housed in a cylindrical 30mm by 70mm metal unit with m30 threads and nuts for mounting, both D1 and D2 have to be fitted in the front of this tube on a plastic plug optically insulated from each other yet beside each other.

A Center tap 50Hz Step Down Transformer with two diodes is used to get a train of Positive Sine Pulses at 100Hz which is applied to Q6 base via R51. 2N2646 is a unijunction transistor(UJT) in a TO-18 metal package. The control voltage or voltage proportional to error is fed to R56-Q3, The UJT drives a pulse transformer which provides isolation from the Load which the SCR bridge is controlling. This phase angle control gives a near Linear closed loop control for a SCR bridge which may be used in Electroplating or a Preregulator of a big power supply.

2N2646 based Voltage controlled SCR Pulser