The Vichy VC99 is an inexpensive 6000 count multimeter with pretty decent performance compared to some others in the same price bracket, one really nice thing about it is that it is very easy to add an RS232 output as described here, just a case of soldering a wire to an unused pin on the processor. Of course it makes sense to isolate the meter from your PC, usually this is done with an opto coupler as in that link and I’ve also seen people using infrared LEDs but I thought I’d go one step further and make it a radio link using a Ciseco SRF in the meter and a SRF-Stick at the receive end so I can still record data without being physically near the PC.
It’s all very simple to set up thanks to the SRF transparent serial mode. First you need to temporarily connect the SRF to a PC via a serial adapter of some sort and use the command mode to set it to 2400 baud then it is just a case of soldering a wire from the serial output on the VC99s processor to the DIN pin on the SRF and connecting power. The PCB in my meter is slightly different to the one in the article above and VCC wasn’t available in the same place so I used the upper connection to the buzzer which is VCC only when the meter is on, for the ground I just connected where the battery connection goes in. I used one of my RFM12B breakout boards to make soldering to the SRF a little bit easier but it’s certainly not necessary.
Once the SRF is connected press and hold the REL button on the meter to turn RS232 on (you’ll see the icon on the display) and stick an SRF-Stick in your PC and you’ve got a wireless serial link from the meter. Open a terminal emulator on the PC end set at 9600 8N1 and you will see the hex data from the meter coming in:
000001e0: 34 31 00 40 80 80 0d 0a 2d 30 30 39 38 20 34 31 000001f0: 00 40 80 81 0d 0a 2d 30 30 39 32 20 34 31 00 40 00000200: 80 80 0d 0a 2d 30 30 38 35 20 34 31 00 40 80 80 00000210: 0d 0a 2d 30 30 37 30 20 34 31 00 40 80 80 0d 0a 00000220: 2d 30 30 35 36 20 34 31 00 40 80 80 0d 0a 2d 30 00000230: 30 35 30 20 34 31 00 40 80 80 0d 0a 2d 30 30 34 00000240: 30 20 34 31 00 40 80 80 0d 0a 2d 30 30 33 32 20
To interpret this I am using QtDMM on Linux (also available for OS X) which gives you a live reading as well as min and max values plus data recording with a nice graph over time and an export function. The recorder also features manual start, scheduled start and triggered automatic start when given thresholds are reached as well as the ability to start an external application when given thresholds are reached.
You need to configure it for 9600 baud, 8 bits, no parity, 1 stop bit (not 2400 baud as we are talking to the SRF-Stick and not to the VC99 directly) and select the “14 bytes ASCII/binary, continuous (QM1537)” protocol and 6000 digits option. I have to confess that one dirty hack was committed here as it was getting late, there wasn’t an option in QtDMM for /dev/ttyACM0 where the SRF-Stick resides so I just created a symbolic link to /dev/tty.usbserial0 (normally used on a Mac I think) and used that. I should really grab the latest source and do it properly when I have time but it works for now.
The wireless link also opens up other possibilities such as using an SRF/XRF with an Arduino to trigger an event on a certain value or you could even make a custom LCD display. If you wanted to use it on Raspberry Pi you could use the Slice of Radio which plugs into the Pi I/O header and is half the price of the USB SRF-Stick.
Overall a nice quick hack with a good result adding new functionality and looking at the meter you wouldn’t know anything was different as it’s all hidden inside.