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
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.
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.
When the microcontroller hangs due to a spike, EMI or RFI etc. the 7555 will reset the uC, if proper power supply design is done above circuit can also give a clean power on reset, the above circuit you should modify to suit your design.
80C39-8749 MCS-48 Examples and code
Better still use a uC with watchdog built in like some atmel chips, or use the watchdog chips from maxim which can also do RAM battery management.
The circuit was developed over a old TI application note, 7555 i think fairchild may be making it, 7555 is CMOS version of 555 timer, advantage of 7555 is that it can go to higher frequency, low power consumption, the disadvantage is its output drive mA is not as good as 555. now why i put it here was that you can see how charge and discharge paths are separated with diodes.
See Larger Circuit. 555 Watchdog
Edit the circuit eagle cad file del00013.zip,
IC1 astable gives a fixed square wave at pin 3, C1 and R1 derive uS trigger pulses from IC1 and this will trigger IC2 monostable or single shot, the voltage at pin 5 of IC2 will change the pulse width output of IC2, to get it working all the three RC combinations have to be figured out.
Optical Obstacle Switch.
You can even build a small SMPS with this or even control the temperature of your soldering iron using the SSR solid state relay circuits in power section, then you need to think and design the cycle time of a soldering iron heat control system, it will be in seconds but then above circuit is running at audio frequencies, then you have to work that out yourself..