Here is a 4-20 mA In/Out Analog Mux with Cascade option. This is a simple circuit i designed to make a Automation System within a budget.
Mixed Circuits Analog with Digital
This takes 4-20mA from many Transmitters and gives out just one 4-20 mA output. The Mux is done with a digital byte or word. This is a slow scanner as process is slow, that way many analog inputs can be multiplexed and sent into one analog input of a D/A. In near real time systems a faster mux could be used or mux totally avoided. This was made in some numbers, so the pcb is better than others.
4-20mA Multiplexer Circuit – pdf
The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.
Dual Differential Amp – Interactive Simulation
The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating Outputs may be random or Oscillating, it is good to have a bias network of 10M resistors to a potential even zero or COM this enables Vout when input floats.
Vout = (Vp – Vn) * (Rf+Ri)/Ri
Precision Instrumentation Amplifiers
When i had put the near Obsolete digital circuits online in the late nineties. One person who works in a public institution in the usa, wanted a modification of one of my existing circuits. He had those parts the CD40 Series Logic Chips. He wanted to use only those that he had in his Stock.
I made some modifications and sent it to him, that helped him with his task. These things can be done very easily using the Arduino. One could make a programmable Arduino Timer/Counter with a matching Configuring Software without coding, for such people. Easy and Affordable.
Digital Circuits 2 from delabs
Circuit 1 – Digital Timer Clock With Preset using Thumbwheel switch.
A Thumbwheel Switch has to be used in place of DIP switch shown, just know that 1-2-4-8 nibble (4 bit) should be generated by Thumbwheel switch at preset or jam inputs of 4029.
Use CD4511 if 4513 is not available, but circuit has to be changed a bit around 4511
Circuit 2 – 1 Hz or 1 pps crystal clock using CD4060 and 32768 Hz Crystal.
They have not been tested much… The 4513 control pins 8-4-5-3 connections verify, as i did not get the datasheet.
The circuits will work as the concepts are right, but some tweaks in R C values may be required.
the R C values can only be corrected if you have problem in making it work.
The main problem in the R C values may be related to “the reset at 6 for the tens of seconds and the tens of minutes”.
This is the continuation of the earlier post. Part of 80C39 based Process Controller. In this schematic you can see the Watchdog and D/A Converter.
My first observation of a very complex watchdog in action was an Agilent(hp) Benchtop Multimeter based on this 8048 family of 1st generation microcontrollers that did not even have a UART among many things.
At that time CMOS was just making an entry and FLASH memory was unheard of. The UV Eprom was the way firmware was set on these systems. These consumed a lot of power. 80C39 was the CMOS one.
The 4040 counter derives a slow clock from the 7555 timer. The counter has to be reset by firmware by periodically sending a reset pulse on port pin P2.7 to say “Alls Well”.
If the firmware or uC “hangs” or due to EMI or Spikes the uC gets into an endless loop. Then the “Alls Well” pulses stop coming. The 4040 keeps counting till Q10 output goes high and resets the uC or can we say Wakes it up rudely.
The D/A converter was used to get the 1-5 V to obtain 4-20 mA control Signal to operate the Actuators like a Motor Drive or Heaters in a Industrial Process control System.