This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.
U2A opamp LF353 is used here as a constant current source, R10-D13-D14 is for protection in case you measure voltage in the ohms range. U1 4052 helps digitally select four different currents, the currents pass thru the unknown resistor to be measured and an voltage developed across the resistor is measured.
U2B is a buffer which passes on the voltage measured to U5A for inversion of polarity as current source is a negative current (current sink). U5B amplifies to the level required for the LM3914 display circuit. U7 555 is used as a de-bouncing for switch SW1 so as to advance counter 4029 to change the range of resistance measurement.
Use any FET input dual opamp, TLO72 will work but LM358 will cause error as it is transistor input..
D1, D12 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/-3.5V only, with some FET it needs to be tweaked. LF353 is a Wide Bandwidth Dual JFET Input Operational Amplifier.
Build a DMM – LM3914 Analog display
U1A LF353 opamp turns on a buzzer when the voltage at Test + falls very low indicating a short between Test + and Test – or a resistor less than 5 ohms. The R9-R14 divider determines the minimum voltage that can be at Test + for buzzer to just turn on.
The Opamp here is a comparator and R10 10 Meg gives a very small hysteresis so that there is no oscillation at threshold levels, that is when both inputs at same levels.The Zener is for protection and R6 to limit current.
U2A and U1B opamps form a precision rectifier, note that this is not a true RMS rectifier circuit for that see some devices from Analog Devices. When you need to rectify a 200mV AC signal you cannot use a diode like 1N4148 as the diode turns on at 700mV so this circuit is used.
AC conversion and continuity buzzer test
The AC signal measurement is best to do with a True RMS convertor. This Type is able to quantify the Heating power of a AC voltage, includes all harmonics and Noise too.
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.