When Instruments are designed a analog front end is essential and also as most equipment have digital or microcontroller interface the analog circuit needs to have digital access. The Circuits DACT0008 and DACT0009 are both useful in building instruments which have digital control.
This circuit DACT0009 is similar to DACT0008 but gains of upto 100 can be realized in this configuration, this is useful for signal conditioning of low mV outputs of transducers. The gain selection resistors R3 to R6 can be selected by the user and can be anywhere from 1K to 1M and can also be trimpots for obtaining gains as required by user, the resistor values shown are for decade gains e.g. for an auto ranging DPM.

Precision Amplifier with Digital Control

R1 and C1 reduce ripple in input and also snubs transients, ZD1 and ZD2 Zeners clamp input to +/- 4.7V the input current is limited by R1 lastly C1 and C2 are decoupling capacitors. The OpAmp U3 is used to increase the input impedance so that very low mV inputs are not loaded on measurement, the user can terminate the inputs with a resistor of his choice like 10M or 1M to avoid floating of the inputs when no measurement is being made. U5 is used as an Inverting buffer to restore polarity of the input and U4 is used as a buffer on the output of 4052 because loading it by resistance of value less than 1M will cause an error. An alternative is use R7 = R8 =1M and remove U4 but this may not be ideal. Gains of greeter than 100 may not be practical because at 100 gain itself a 100uV offset will be around 10mV at the output (100uV*100) this can be trimmed using the offset null option in the OP07, connect a trimpot between 1 and 8 and connect wiper to +5.

Precision Amplifier with Digital Control

For better performance use ICL7650 ( not pin compatible ) instead of OP07 and use +/- 7.5V instead of +/-5V supply.

Eight steps for gain or attenuation can be added by using two 4051 and by using Pin 6 Inhibit on 4051/52 limitless steps can be added by cascading many 4051,52,53 as Pin 6 works like a chip select.

Some extended applications of this circuits are……. Error correction in Transducer amplifiers by correcting gain. Auto ranging in DMM. Sensor selection or Input type selection in Process control. Digitally Preset power supplies or electronic loads. Programmable Precision mV or mA sources. PC or uC or uP based instruments. Data loggers and Scanners.

his Circuit converts a voltage control output from a Process Controller to be converted into a Current Control if the AC-Drive or Valve needs a Current Control Signal.

Significance of Current Loop 4 to 20 mA Standard

Voltage to Current Convertor using LM723

This is a three wire voltage to current loop converter. The 1-5 V DC is attenuated and fed to pin 5 LM723 opamp section which tries to maintain the same voltage at pin 10 across the 10 E, thereby producing a open collector constant current sink proportional to the 1-5V input. By trimming the attenuator you can scale-calibrate 1-5V input to 4-20mA output for looping many instruments in series, like a controller, recorder or PLC. With a supply voltage upto 24V, three instruments can be looped. The connection to pin 6 is required to convert 0-1 input to 4-20mA.

All the transmitter circuits can be seen here. Industrial Process Control Circuits

This circuit was designed by me in the eighties, the 555 was for negative supply, The whole thing went into the anodized cast aluminuim head of a sensor.

How 4-20mA Works

This circuit is used to detect objects by reflected infrared light. It can be built into a cylindrical enclosure just like an inductive proximity switch.

Part of – InfraRed Detector for Proximity Switch

This is also useful as a level detector for colored liquids like oil. This has some immunity to ambient sunlight as it detects ac pulses.

Infrared Optical Proximity Switch

IC 555 is used as an astable oscillator and it flashes the Infra red LED D1 at a high speed, The object close to this LED reflects the light along with the ambient light which may also be sunlight.

Infra Red LED 555 Flasher

IR Led’s and Diodes

The types available are various and polarity hard to detect even photo IR transistors can be used. The IR Led can be tested in diode mode of a DMM (battery should be in good condition) it should give around 1.1V drop in proper polarity.

Se a Related circuit here Optical Obstacle Switch.

An IR detector diode or photo diode can be tested in the same way the drop will be 0.5V at 1 feet from a 60W lamp (no sunlight), closing the IR photo diode with your hand will be an over range on DMM this will happen on proper polarity. the photo diode shows around 10k ohm resistance in daylight and in Mega ohms when covered also the photo diode detects light on reverse bias and used like that.

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.

Battery Level Indicator

T1 and T2 make a differential amplifier. T3, T4 and T5 driving the LEDs are comparators.  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

Fluid or Water Level with Reed Relays 

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.

This article will explain the way a simple transistor based current source is designed, this will give an idea on how some components can be used in a practical way to make the circuit do some function, the objective is not design but to become familiar with the basic ideas.

Design of a Constant Current Source

In the circuit the LED is used as a reference so to keep it cool a 2.2K is chosen. (20V – 1.6V) / 2.2K = 8.3mA on the high side and when voltage is 10V the current will be 3.8mA min.

Constant Current Source for LED

You should know that the LED forward drop can change with ambient light as it is photo sensitive and will vary with temperature.

The circuit can be improved by using a zener in place of the LED or better still a temperature compensated reference like LM336.

Operating Current of LM336 is 400uA to 10mA, 20V The max. voltage 20V / 3.3K = 6mA. so within limits. Then you can compute the rest, wire it up to see if your design works.

In the circuit, use only metal film resistors (MFR) of 1 per cent tolerance, as this is an instrumentation application. Power supply should be a stable +5V, -5V supply, for which one can use 7805 and 7905 regulators.

The inputs TC+ and TC- terminals should go to a 4-way barrier terminal block, the 2 extra terminals are used to mount TH1 Cu thermistor. This forms an isothermal block, which is good enough.

A simple way to make a TH1 Cu thermistor, is to take a 1 Meg-ohm 2W resistor as a former and wind 2 meters of 46 SWG enameled copper (Cu) wire (5.91 ohm/meter) over it. This gives a 12-ohm value. Terminate wire ends on resistor leads.

Thermocouple Temperature using DPM or DMM

Test and Calibration –

For calibration, you will need a DMM-DPM and a milli-volt source (as shown in the Fig.). First connect source to terminals TC+ and TC-, then set source to 0.00 mV (verify with DMM for zero). The output across +out and -out (use DMM) terminals must be mV representing the room temperature (RT). For example, if RT is 30° C (use a glass thermometer) then +out should be 30mV at 0mV input. Adjust VR1 till 30mV is read at +out terminal. This is ‘zero cal’.

This circuit is derived from an application note of L296, It is a Power Switching Regulator from ST. The advantage of using a switching regulator is that there is not much Heat Dissipation in this circuit.

Switching Battery Charger with L296 – del20031

Digital 5V Power Supply using L296

If you had to build the same with a series regulator, it would be very big due to external transistor and a huge heat sink. This circuit takes a small place on PCB, efficiency is high so power is saved and reliability of product improves, lastly the thermal gradients within the cabinet is avoided so that any form of drift or component specs variation can be avoided.

L296 and L296P are stepdown power switching regulators 4 A at a voltage variable from 5.1 V to 40 V.  External programmable limiting current. Soft start, remote inhibit, thermal protection, a reset output for microprocessors.

The Schottky rectifier BYW80 is used as it switches very fast 200V-20A-35nS. The Inductor and Capacitor is for the filter to get a ripple free DC from the Chopped DC output. There may be a small high frequency ripple riding on the DC signal of 5V in most SMPS circuits. So for very sensitive circuits use extra filters and shields.

The Current output is limited, and can be reduced further with a resistor from Pin 4 to ground. Also if the feedback to Pin 10 is thru a Voltage Divider then more voltage can be set at the output. See the datasheet and application notes for other design details and circuits.