In this part I will explain the DMM mode selection schematics and will implement the base logic in a breadboard.
The schematic is given below:
There are 4 push buttons for each mode, with debounce circuit connected to Shmitt inverters. Output of the inverters are connected to one of the input of the OR logic gate. The second input is connected to GPIO pins of the microcontroller. By this way the DMM modes can be controlled via push buttons or via remote interface like GPIO/RS232/USB etc. Additional logic gates can be put here to disable/enable manual switch or remote control.
Every output of the OR gates are connected to own D flip-flop and to 4-input OR gate. The output of the 4 input OR gates is connected to CLK pin of every D flip-flop.
When one of the push button is connected, the clock signal is generated from the 4 input OR gate output. This clock signal will force every D flip-flip to "remember" his input until the next clock edge. By this way every time when one push button is pressed, only one of the D flip-flops output becomes logical 1 and the rest are in logical 0 state. This is what we needed according to the switch table in the part 1 : every mode connect one switch, except for 2W/4W resistance where the voltmeter switch must be closed as well:
This additional requirement can be realized with additional 2-input OR gate connected to the output of the 2W/4W D flip-flops and the output connected to another 2-input OR gate connected to Voltmeter D flip-flip. By this way, every time when the 2W/4W resistance is selected, the VM switch will be closed as well.
The prototype which I made, contain 3 push buttons and output of the every D flip-flop is connected to LED. There is one more LED for CLK signal and one more push button for reset the state of all D flip-flop to 0:
In the bread board the following IC are used:
- 74174 : 4 D flip-flops.
- 7414 : 6 Inverters with Shmitt inputs.
- 744075 : 3 x 3 input OR logic gate
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